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LOOP: DOING IT OVER AND OVER LOOP AS A LABOR FORCE One of the strengths of the computer is its ability to repeat a process over and over without getting tired or threatening to strike. The program that you have written to perform a single task can now be repeated to perform the same task as many times as you wish. This process of going over and over is known as repetition and is called a loop. Can you think of a loop? Of course! Our everyday activities, including getting up, eating, working, studying, and sleeping are examples of a loop. Can you identify other loops? With a little thought, I know you can. COMPUTERS IN THE LOOP Did you know that your computer is in a constant cycle of input, process, and output? This constant cycle of input, processing, and output means that your computer is in a loop. It waits for you to enter data, takes your input, analyzes it, performs the required task, and finally displays the result. We can generalize and say that every automated system, from the bank ATM machine, supermarket pricing, through the search engine of the Internet all use the loop. WHAT DO YOU NEED TO MAKE A LOOP? It is very simple to create a loop. Just follow these three steps: 1) Use a reserved word (construct) from the C/C++ language to tell the program to loop. 2) Set a condition as to whether to continue or to terminate the loop. 3) Set aside the body of a loop to determine what you want to loop. C/C++ LOOP CONSTRUCTS There are various ways to loop in a program, and for each way there is a construct (reserve word) from C/C++ that instructs the program to loop. These loop constructs are: 1) while 2) for 3) do while 4) goto label Each of the above four loop constructs can do the same job. However, some are better suited in some situations and there are others that are rarely used, such as the goto. For simplicity we will concentrate on the while command, and later the others will be introduced. TO CONTINUE THE LOOP OR TO TERMINATE The condition part of the loop is based on true or false values. The true value is like a green light for the loop, and the false value is like a red light that will stop it. The condition part can be shown as the comparison of two values or the testing of one value as true or false. The action of the loop will depend on the resulting value of the loop's condition. As long as the value of the condition is true then the loop will go on, but if the condition of the loop becomes false, the loop will terminate. COMPARISON OPERATORS To compare two values, comparison (relational) operators are used. The comparison operators are situated between two values (operands) that will result in either a true (1) or false (0) value. The less than operator is shown as <, the greater than operator is shown as>, and the equal to operator is shown as= = (double equal sign). The operator <=stands for less than or equal to and, similarly, the operator >= stands for greater than or equal to. The not equal to operator is shown as !=. The ! (exclamation mark) alone stands for the not operation, which means to negate. Negation makes a true value become false, and a false value become true. The above relational operators can be used as loop conditions. TRUE OR FALSE VALUE OF AN OPERATOR Depending on the relational operator and its operands, a true or a false value will be set. What would be the result of each of the following comparison (relational) operators?
Table 3.1 - Examples of relational operators and true vs. false evaluations. What would be the result of m > n? Answer: It all depends on the current values of variable m and n at the time of comparison. C/C++ PHILOSOPHY OF WHAT IS TRUE AND WHAT IS FALSE Surprisingly, in C/C++ every value is true except zero. For example, 2 is true, -5 is true, but 0 is false (C/C++ philosophy). Additionally, zero is the ASCII value of NULL, which means nil (nothing) and can be used as a terminator. COMMON SYNTAX OF THE while LOOP The while loop has the following common syntax (form): The word while is followed by an open"("and a close ")" parenthesis. Within the parenthesis there will be a test (condition) resulting in either true or false, such as a relational operation. An open brace "{" with a close "}" determine the beginning and the end of the loop, where the body of the loop (statements) is placed within these braces. However, if the loop only contains one statement, there is no need for braces. Figure 3.1 shows the format of the while loop.
while (condition tests for true or false value){
// Body of the loop…
}//MAIN
Figure 3.1 - The format of the while loop.
Be careful not to place a semicolon after the closed parenthesis because this may cause the body of the loop not to be iterated. To ensure this, place the open brace right after the close parenthesis. Note that if the loop has only one statement, then there is no need for the braces because the next statement, by default, will automatically belong to the loop. However, for clarity, we suggest that you always use braces. RUNNING FOREVER (INFINITE LOOP) Figure 3.2 illustrates a loop that never terminates. The reason is that the condition for the loop is always true.
1. #include <iostream>
2. using namespace std;
3. main(){
4. while( 1 ){
5. cout << " I WILL LIKE YOU FOREVER ";
6. }//WHILE
7. return 0;
8. }//MAIN
Figure 3.2 - C++ program illustrating an infinite loop.
The output of the program illustrated above will repeat, and forever display the message: I WILL LIKE YOU FOREVER. One way to terminate the infinite loop is to externally interrupt the execution through the operating system (for example, in the DOS environment, pressing Ctrl + C at the same time will interrupt program execution.) The applications that are required to work around the clock can do so by the inclusion of this infinite loop. The traffic light system, credit card transactions, and the Internet information retrieval system are just a few examples of loops with the infinite concept. IT'S AS IF IT NEVER HAPPENED (NOWHERE LOOP) Figure 3.3 illustrates a loop that never runs. The reason is that the condition for the loop is always false.
1. #include <iostream>
2. using namespace std;
3. main(){
4. while ( 0 ){
5. cout << " I WILL NEVER HATE YOU ";
6. }//WHILE
7. return 0;
8. }//MAIN
Figure 3.3 - The Nowhere loop. The never executed loop.
Since the condition of the loop is false, the body of the loop will never be executed. Therefore, the message I WILL NEVER HATE YOU will not be displayed. One reason to set the condition of the loop to zero is to trace and track down errors in a program. LOOP WITH A CONTROL VARIABLE Once you know how to go forever or never to loop, it is desirable to learn how to control the loop. It is possible to loop a program a number of times, or loop it until some designated value has been reached. One way to accomplish that is to use a variable, instead of just a number, for the condition part of the loop. By using a variable, a loop can be controlled. Control depends on the value of the variable or the value of the variable in a comparison. For instance, while the value of a variable is true (non-zero) the loop will continue, but as soon as the control variable tested is false (zero) the loop will stop. LOOP WITH A CONTROL VARIABLE: AN EXAMPLE In order to use a control variable in a loop, the following steps must be taken into consideration:
1. #include <iostream>
2. using namespace std;
3. main(){
4. int counter;
5. counter = 5;
6. while ( counter ){
7. cout<<" I LIKE YOU ";
8. cout<<endl;
9. counter = counter - 1;
10. }//WHILE
11. return 0;
12. }//MAIN
Figure 3.4a - Decrementing a counter to print a message five times.
1. #include <iostream>
2. using namespace std;
3. main(){
4. int counter;
5. counter = 5;
6. while ( counter > 0 ){
7. cout<<" I LIKE YOU ";
8. cout<<endl;
9. counter = counter - 1;
10. }//WHILE
11. return 0;
12. }//MAIN
Figure 3.4b - Decrementing a counter to print a message five times.
1. #include <iostream>
2. using namespace std;
3. main(){
4. int counter;
5. counter = 0;
6. while ( counter < 5 ){
7. cout<<" I LIKE YOU ";
8. cout<<endl;
9. counter = counter + 1;
10. }//WHILE
11. return 0;
12. }//MAIN
Figure 3.4c - Incrementing a counter to print a message five times.
I LIKE YOU I LIKE YOU I LIKE YOU I LIKE YOU I LIKE YOUFigure 3.4d - Illustrates the output of Figures 3.4a, 3.4b, and 3.4c. All three programs produce the same output. WHAT IS WRONG IN THESE THREE PROGRAMS? Each of the following programs is supposed to loop five times, and display the message I LIKE YOU. However, each program has logical errors that will not allow this to happen. For example, the first program (figure 3.5a) will never display the message, the second program (figure 3.5b) will display the message repeatedly, and the third program (figure 3.5c) will be off by one. Can you identify these problems and fix them? The first program's initialization is set wrong, since it should be set to 5. In the second program you should increment the counter rather than decrement it. In the third program you must either change the condition test to <= or set the initialization to 0.
1. #include <iostream>
2. using namespace std;
3. main(){
4. int counter;
5. counter = 0;
6. while ( counter ){
7. cout<<" I LIKE YOU ";
8. cout<<endl;
9. counter = counter - 1;
10. }//WHILE
11. return 0;
12. }//MAIN
Figure 3.5a - The while loop will never run as counter is evaluated to false from start.
1. #include <iostream>
2. using namespace std;
3. main(){
4. int counter;
5. counter = 0;
6. while ( counter < 5 ) {
7. cout<<" I LIKE YOU ";
8. cout<<endl;
9. counter = counter - 1;
10. }//WHILE
11. return 0;
12. }//MAIN
Figure 3.5b - Repeatedly displays message as counter goes backwards from condition.
1. #include <iostream>
2. using namespace std;
3. main(){
4. int counter;
5. counter = 1;
6. while ( counter < 5 ) {
7. cout<<" I LIKE YOU ";
8. cout<<endl;
9. counter = counter + 1;
10. }//WHILE
11. return 0;
12. }//MAIN
Figure 3.5c - Displays message only four times as counter has the values of 1,2,3, and 4.
Figure 3.5d - No output from Figure 3.5a. I LIKE YOU I LIKE YOU I LIKE YOU . . .Figure 3.5e - The while loop from Figure 3.5b runs infinitely. I LIKE YOU I LIKE YOU I LIKE YOU I LIKE YOUFigure 3.5f - The while loop from Figure 3.5c only runs four times. MAKE YOUR LOOP VISIBLE It is important to make the start, the body, and the end of the loop visible for the purpose of clarity and to avoid errors. One way to accomplish this is by the use of indentation and comments. If the body of the loop is only one statement then there is no need for braces, although in the case of beginners, it is recommended to use them. Another recommendation is to leave a blank line before and after the loop.
1. #include <iostream>
2. using namespace std;
3. main(){
4. int counter = 0;
5.
6. while( counter < 5 ){
7. cout << " Hello World " << endl;
8. counter = counter +1;
9. }//WHILE
10.
11. return 0;
12. } //MAIN
Figure 3.6a - Hello World Program
Hello World Hello World Hello World Hello World Hello WorldFigure 3.6b - Output of Hello World Program of Figure 3.6a. A common error of a beginning programmer is to place a ; right after the closed parenthesis of a loop. By placing an open brace right after the closed parenthesis in a loop, an error can be avoided. LOOPING THE PAYROLL PROGRAM TO COVER MORE EMPLOYEES After you finish writing a program for one employee, you will soon realize how much easier it is to make it for more than one. The example of this would be a payroll program, in which a loop is included to cover more than one employee, or where a loop is placed around the payroll program. The condition of the loop will determine how many times the loop should be repeated, such as the number of employees. The loop should be placed after the declaration of the loop control variable, and the closing of the loop should be placed before return 0; of the main program. See figure 3.7a for the complete program. LOOPING THE INVOICE PROGRAM You can simply loop the Invoice Program by placing the loop construct whileafter the declaration of the program, and terminate the loop right before the return 0; of the main program. The loop can be 5, 50, 5000, or more times. In any case, only the condition test value will change to 5, 50, 5000, or any other number. See figure 3.8 for the program. ASSIGNMENT STATEMENT A variable represents a memory location, where a value can be stored and retrieved. However, each memory location can hold only one value at a time. An assignment can be used to initialize or change the value of a memory location. An equal sign = represents an assignment where the result value of the right-hand side of the equal sign is placed in the memory location of the variable in the left-hand side of the equal sign. An example of an assignment statement would be counter = 0;counter = counter + 1; where 0 is assigned to the variable counter in the first statement, and in the second statement the value of 1 is added to the old value of counter to produce a new value of 1 for counter. The process of assigning a value to a variable for the first time is called initialization. Remember that when a new value is assigned to a variable, the old value will be replaced. What do you do if you want to keep adding to a variable or subtracting from a variable? Simply add the value to the variable, and assign the result into the same variable. Similarly, subtract the new value from the variable.
1. #include <iostream>
2. using namespace std;
3. main(){
4. int numberofemployees;
5. int employeeid, hoursworked;
6. float hourlyrate, grosspay, taxamount, netpay;
7. const float TAXRATE = 0.20;
8. numberofemployees = 0;
9. while( numberofemployees < 5 ){
10. cout << " ENTER THE EMPLOYEE ID: ";
11. cin >> employeeid;
12. cout << " ENTER THE HOURS WORKED: ";
13. cin >> hoursworked;
14. cout << " ENTER THE HOURLY RATE: ";
15. cin >> hourlyrate;
16. grosspay = hoursworked * hourlyrate;
17. taxamount = grosspay * TAXRATE;
18. netpay = grosspay - taxamount;
19. cout << " EMPLOYEE ID IS " << employeeid << endl;
20. cout << " YOUR GROSSPAY IS " << grosspay << endl;
21. cout << " YOUR TAX AMOUNT IS " << taxamount << endl;
22. cout << " YOUR NETPAY IS " << netpay << endl<<endl;
23. numberofemployees = numberofemployees + 1 ;
24. }//WHILE
25. return 0;
26. }//MAIN
Figure 3.7a - The Payroll program with five while repetitions.
ENTER THE EMPLOYEE ID: 3456 ENTER THE HOURS WORKED: 30 ENTER THE HOURLY RATE: 15.25 EMPLOYEE ID IS 3456 YOUR GROSSPAY IS 457.5 YOUR TAX AMOUNT IS 91.5 YOUR NETPAY IS 366 ENTER THE EMPLOYEE ID: 6758 ENTER THE HOURS WORKED: 35 ENTER THE HOURLY RATE: 35.25 EMPLOYEE ID IS 6758 YOUR GROSSPAY IS 1233.75 YOUR TAX AMOUNT IS 246.75 YOUR NETPAY IS 987 ENTER THE EMPLOYEE ID: 3452 ENTER THE HOURS WORKED: 22 ENTER THE HOURLY RATE: 23.40 EMPLOYEE ID IS 3452 YOUR GROSSPAY IS 514.8 YOUR TAX AMOUNT IS 102.96 YOUR NETPAY IS 411.84 ENTER THE EMPLOYEE ID: 8749 ENTER THE HOURS WORKED: 32 ENTER THE HOURLY RATE: 15.10 EMPLOYEE ID IS 8749 YOUR GROSSPAY IS 483.2 YOUR TAX AMOUNT IS 96.64 YOUR NETPAY IS 386.56 ENTER THE EMPLOYEE ID: 5364 ENTER THE HOURS WORKED: 25 ENTER THE HOURLY RATE: 12.50 EMPLOYEE ID IS 5364 YOUR GROSSPAY IS 312.5 YOUR TAX AMOUNT IS 62.5 YOUR NETPAY IS 250Figure 3.7b - Sample output of the Payroll Program.
1. #include <iostream>
2. using namespace std;
3. main(){
4. int itemid, quantity, counter;
5. float unitprice, subtotal, taxamount, totalprice;
6. const float TAXRATE= 0.08; //Sale’s tax
7. counter = 1;
8. while( counter <= 5 ){
9. cout << ”Please Enter the Item ID: ";
10. cin >> itemid;
11. cout << ”Please Enter the Quantity: “;
12. cin >> quantity;
13. cout << ”Please Enter the Unitprice: ";
14. cin >> unitprice;
15. subtotal = quantity * unitprice;
16. taxamount = subtotal * TAXRATE;
17. totalprice = subtotal + taxamount;
18. cout << “The Item’s Id is " << itemid << endl;
19. cout << “The Quantity is " << quantity << endl;
20. cout << ”The Unit Price is " << unitprice << endl;
21. cout << ”The SubTotal is " << subtotal << endl;
22. cout << “The Tax Amount is “ << taxamount << endl;
23. cout << ”The Total Price is " << totalprice<< endl;
24. cout << endl;
25. counter = counter + 1;
26. }//WHILE
27. return 0;
28. }//MAIN
Figure 3.8a - Invoice Program with five while repetitions.
Please Enter the Item ID: 245 Please Enter the Quantity: 20 Please Enter the Unitprice: 2.15 The Item's Id is 245 The Quantity is 20 The Unit Price is 2.15 The SubTotal is 43 The Tax Amount is 3.44 The Total Price is 46.44 Please Enter the Item ID: 547 Please Enter the Quantity: 12 Please Enter the Unitprice: 12.45 The Item's Id is 547 The Quantity is 12 The Unit Price is 12.45 The SubTotal is 149.4 The Tax Amount is 11.952 The Total Price is 161.352 Please Enter the Item ID: 903 Please Enter the Quantity: 44 Please Enter the Unitprice: 15.22 The Item's Id is 903 The Quantity is 44 The Unit Price is 15.22 The SubTotal is 669.68 The Tax Amount is 53.5744 The Total Price is 723.254 Please Enter the Item ID: 278 Please Enter the Quantity: 34 Please Enter the Unitprice: 17.25 The Item's Id is 278 The Quantity is 34 The Unit Price is 17.25 The SubTotal is 586.5 The Tax Amount is 46.92 The Total Price is 633.42 Please Enter the Item ID: 641 Please Enter the Quantity: 12 Please Enter the Unitprice: 40.05 The Item's Id is 641 The Quantity is 12 The Unit Price is 40.05 The SubTotal is 480.6 The Tax Amount is 38.448 The Total Price is 519.048Figure 3.8b - Sample output of the Invoice Program. ASSIGNMENT: NOT A MATHEMATICAL STATEMENT By Looking at the assignment statement x = x + 1, you may wonder how x becomes x+1 mathematically. Regardless of the value you select for x, you would not satisfy the above statement. The problem arises because, historically, the = sign has been used instead of an arrow <= sign. An assignment statement may involve several steps that take place at different times, and in which the result value would be placed in the memory at the final step. INCREMENT OR DECREMENT BY ONE: VARIATIONS The following examples in Table 3.2 illustrate different ways to increment or decrement a variable by 1.
COMMON ASSIGNMENT STATEMENT: EXAMPLES Table 3.3 contains common assignment statements used by programmers, as well as sample initialization and the update of assignments.
Table 3.3 - Sample initialization statements and assignments. C/C++ AS A COMPACT LANGUAGE C/C++ is known as a compact language, meaning that most of the time there is a shortcut for typing a statement or an expression. For example, the shortcut for counter=counter+1 would be counter++ or counter+=1 and even ++counter. Similarly, a shortcut for sum = sum + price would be sum+=price. WHAT DOES C++ MEAN? C++ is a shortcut for C = C+1, and means "add one to C". Now you know why the language is called C++: Add 1 to Language C, that is, a better and improved C. COUNTING-DOWN LOOP There are many ways to loop 10 times. Start the control variable with 1 and loop as long as it is less than or equal to 10, and increment the variable by 1. Similarly, you can start from 0 and then continue, as long as it is less than 10. You can start from 100 and stop by 110. Another way would be to start from 10 and stop once 0 is reached. An example would be the space shuttle countdown program below.
1. #include <iostream>
2. using namespace std;
3. main(){
4. int count;
5. count = 10;
6. while( count > 0 ){
7. cout << count << endl;
8. count--;
9. }//WHILE
10. cout << "Blast off " << endl;
11. return 0;
12. }//MAIN
Figure 3.9a - Simple Space Shuttle program illustrating decrementing.
10 9 8 7 6 5 4 3 2 1 Blast offFigure 3.9b - Output of Figure 3.9a. TEST YOUR COMPUTER: WHEN WILL THE LOOP STOP?
1. #include <iostream>
2. using namespace std;
3. main(){
4. int counter = 1;
5. while( counter ){
6. cout << " I LIKE YOU " << counter << endl;
7. counter = counter +1;
8. }//WHILE
9. return 0;
10. }//MAIN
Figure 3.10b - Output of Figure 3.10a.
I LIKE YOU 1 I LIKE YOU 2 . . . I LIKE YOU 32767 I LIKE YOU -32768 I LIKE YOU -32767 . . . I LIKE YOU -1Figure 3.10a - Testing your C++ knowledge of the integer data type. The above loop goes through 1, 2, 3,. 32767, -32768, -32767,..,-1. Once the number reaches the largest positive number (32768), it then turns to the smallest number (-32768). Eventually the number will reach -1 and finally 0, where the loop will stop. Depending on the compiler the maximum range of integer varies. Therefore, wait until the maximum positive number is reached, or add the reserve word short before the reserved word int. E.g. change line 4 of above program to short int. STOP THE LOOP WITH THE KEYBOARD: THE ESC KEY A loop can be stopped interactively by pressing a keyboard key such as the Esc key. In this case the ASCII value of a key is used as a terminator value. The following program computes the subtotal and total price of items and conclude when the Esc key is pressed. The ASCII value of the Esc key is the value 27. As long as the variable more is not equal to 27, the program will continue.
1. #include <iostream>
2. using namespace std;
3. main(){
4. const float SALESTAX=0.08;
5. float itemprice, subtotal, taxamount, total;
6. char more;
7. subtotal = 0;
8. cout << " Hit any key to continue or ESC to stop. " << endl;
9. cin >> more;
10.
11. while( more != 27 ){
12. cout << "Enter the price of item: ";
13. cin >> itemprice;
14. subtotal = subtotal + itemprice;
15. cout << "Hit any key to continue or ESC to stop. " << endl;
12. cin >> more;
13. }//WHILE
14.
15. taxamount = subtotal * SALESTAX;
16. total = subtotal + taxamount;
17. cout << "The total price is " << total << endl;
18. return 0;
19. }//MAIN
Figure 3.11a - Using the Esc key to end a program.
Hit any key to continue or ESC to stop. s Enter the price of item: 35 Hit any key to continue or ESC to stop. s Enter the price of item: 35 Hit any key to continue or ESC to stop. s Enter the price of item: 30 Hit any key to continue or ESC to stop. The total price is 108Figure 3.11b - Output to Figure 3.11a, the use of the Esc key to end a program. CONTROLLING THE CONTROL VARIABLE: INTERACTIVE LOOP A user can control the number of times a loop needs to go by entering a number. Observe figure 3.12 below for specifics.
{
.
.
.
cout <<" Enter the number of times to loop ";
cin>>n;
counter = 1;
while(counter <= n){
.
.
.
counter ++;
}//WHILE
.
.
.
}
Figure 3.12 - Entering a number interactively to control a loop.
DO WHILE: DO IT ONCE AND THEN CHECK CONDITION Another variation of looping is the do while, where the condition is placed at the end of the loop. The codes within the do while will be executed at least one time before the condition is tested. The advantage of the do while is that it eliminates the input of data twice, such as was the case with the Esc key program above. As you can see from studying the program, there is user interaction before and within the while loop. The disadvantage of the do while loop is that the invalid data will be processed if additional checking is not performed. Similar to the whileloop, the do while stops when the condition of the loop becomes false. Figure 3.13 illustrates a menu program using the do while.
1. #include<iostream>
2. using namespace std;
3. main(){
4. int option;
5. do {
6. cout << " Ebrahimi Bank of New York”<<endl;
7. cout << ” Old Westbury, NY 11568”<<endl<<endl;
8. cout << ”\t 1. Deposit \t\t 2. Withdraw”<<endl;
9. cout << ”\t 3. Transfer \t\t 4. Loan”<<endl;
10. cout << ”\t 5. Balance \t\t 6. Help”<<endl;
11. cout << "\nCHOOSE ONE OF THE FOLLOWING OPTIONS.”;
12. cout << “\nPRESS 0 T0 EXIT: ";
13. cin >> option;
14. }while( option != 0 ); //DO WHILE
15. return 0;
16. }//MAIN
Figure 3-13 - Illustrates the same menu of Figure 2.5 of Chapter two using a dowhile loop. The menu will execute at least once. Pressing zero(0) will end the program. Pressing any other key will redisplay the menu.
FOR LOOP: A COMPACT LOOP Another version of the loop where the initialization, testing, and update of the loop are positioned one after the other (same line), is known as the for loop, and it is the quickest way to write a loop. The forloop is preferable when the number of repetitions is known. The form of the forloop is listed in figure 3.14.
for ( initialization ; testing condition ; counter update ){
// Body of the loop….
}//FOR
Figure 3.14 - Format of the for loop.
A simple example of a for loop is shown in Figure 3.15 below.
1. #include<iostream>
2. using namespace std;
3. main( ){
4.
5. for (int counter = 1 ; counter <= 5 ; counter++){
6. cout << "Hello"<<endl;
7. }//FOR
8. return 0;
9. }//MAIN
Figure 3.15a - Simple for loop implementation.
Hello Hello Hello Hello HelloFigure 3.15b - Output of for loop implementation of Figure 3.15a. NESTED LOOP: A TIME CLOCK When is a loop within a loop called a nested loop? How does a nested loop work? Start with an outer loop. Finish each inner loop until the outer loop is completely finished. When there is more than one loop nested, the innermost loop should be closed first. A time clock analogy can be a useful example in the sense that for every hour of a day there would be sixty minutes, and for every minute there would be sixty seconds. Figure 3.16 illustrates the analogy.
1. #include<iostream>
2. using namespace std;
3. main(){
4. int hours, minutes, seconds;
5. for( hours = 1 ; hours < 24 ; hours++ ){
6. for( minutes = 1 ; minutes < 60 ; minutes++ ){
7. for( seconds = 1 ; seconds < 60 ; seconds++ ){
8. }//FOR
9. }//FOR
10. }//FOR
11. return 0;
12. }//MAIN
Figure 3.16 - Time clock analogy.
NESTED LOOP: WHAT WOULD BE THE OUTPUT?
1. #include<iostream>
2. using namespace std;
3. main(){
4. int i = 0, j;
5. while( i < 5 ) {
6. cout<<" i is "<< i <<endl;
7. j=0;
8. while( j < 5 ){
9. cout << " j is "<< j <<endl;
10. j++;
11. }//INNER LOOP
12. i++;
13. }//OUTER LOOP
14. return 0;
15. }//MAIN
Figure 3.17 - What would be the output?
i is 0
j is 0
j is 1
j is 2
j is 3
j is 4
i is 1
j is 0
j is 1
j is 2
j is 3
j is 4
i is 2
j is 0
j is 1
j is 2
j is 3
j is 4
i is 3
j is 0
j is 1
j is 2
j is 3
j is 4
i is 4
j is 0
j is 1
j is 2
j is 3
j is 4
Figure 3.17b - Output of Figure 3.17a - What would be the output?
MULTIPLICATION TABLE: A NESTED LOOP EXAMPLE
1. #include <iostream>
2. using namespace std;
3.
4. int main(){
5. int row,column;
6. for(row=1;row<10;row++){
7. for(column=1;column<10;column++)
8. cout<<row*column<<"\t";
9. cout<<endl;}
10. return 0;
11. }//MAIN
Figure 3.18b - Multiplication Table output from Figure 3.18a
1 2 3 4 5 6 7 8 9 2 4 6 8 10 12 14 16 18 3 6 9 12 15 18 21 24 27 4 8 12 16 20 24 28 32 36 5 10 15 20 25 30 35 40 45 6 12 18 24 30 36 42 48 54 7 14 21 28 35 42 49 56 63 8 16 24 32 40 48 56 64 72 9 18 27 36 45 54 63 72 81Figure 3.18b - Multiplication Table output from Figure 3.18a LOOP WITH GOTO: AN OLD-WAY LOOP The least-used and oldest kind of loop is known as a goto loop. The use of goto was controversial for a period of time, and as a result its usage is not recommended. The goto keyword allows program flow to go back and forth anywhere within a program. Hence, following the logic of a program becomes difficult at best. Figure 3.19 shows the use of the goto loop.
1. #include<iostream>
2. using namespace std;
3. main(){
4. int counter = 1;
5. back:
6. counter = counter + 1;
7. goto back;
8.
9. return 0;
10. }//MAIN
Figure 3.19 - The infinite goto loop.
The previous loop is infinite. However, in the next chapter we will discuss the condition statement, which will allow the above code to terminate. BREAK AND CONTINUE LOOP: A POLITE WAY Inserting the keyword break can stop a loop. A loop can be made to continue by using the keyword continue. In either case, the break or continue keywords will be used in conjunction with a condition that we will discuss in the next chapter.
while( condition ){
.
.
.
break;
.
.
.
}//WHILE
Figure 3.20a - Use of the keyword break to end a while loop.
while( condition ){
.
.
.
continue;
.
.
.
}//WHILE
Figure 3.20b - Use of the keyword continue to continue within a while loop.
CONTINUE TILL I TELL YOU TO STOP A user can control whether a loop should be continued or stopped by responding to a loop control variable. Figures 3.21a and 3.21b show the use of a control variable using a while loop and a dowhile loop respectively.
1. #include<iostream>
2. using namespace std;
3. main(){
4. int response = 1;
5. while( response !=0 ){
6. .
7. .
8. .
9. cout << "want to continue- press 0 to stop";
10. cin >> response;
11. }//WHILE
12.
13. return 0;
14. }//MAIN
Figure 3.21a -The use of a loop control variable in a while loop.
1. #include<iostream>
2. using namespace std;
3. main(){
4. char response;
5. do{
6. .
7. .
8. .
9. cout << ”want to continue press n to stop”;
10. cin >> response;
11. }while( response != ’n’ ); //DO WHILE
12.
13. return 0;
14. }//MAIN
Figure 3.21b - The use of a loop control variable in a do...while loop.
LOOP WITH SENTINEL VALUE: DUMMY STOP A loop can be terminated by a "dummy' value that is not a valid input data. This dummy is called a sentinel value. An example of a sentinel value would be a dummy number such as 9999 or a dummy name such as zzzzzzzz. Historically, sentinel values were widely used before the introduction of the end of file. In the selection of a sentinel value you must be careful not to use a value that is within the range of valid data.
1. #include<iostream>
2. using namespace std;
3. main(){
4. int rainfall;
5. rainfall = 0;
6. while( rainfall != 9999 ){
7. cout<<”Enter a rainfall amount: “;
8. cin >> rainfall;
9. }//WHILE
10.
11. return 0;
12. }//MAIN
Figure 3.22a - Using a sentinel value to stop a while loop.
pre>
Enter a rainfall amount: 12
Enter a rainfall amount: 2
Enter a rainfall amount: 4
Enter a rainfall amount: 3
Enter a rainfall amount: 5
Enter a rainfall amount: 9999
Figure 3.22b - The output of the Sentinel program of Figure 3.22a. The program ends when the sentinel value of 9999 is entered by the user.
TESTING THE LOOP FOR AN EQUALITY: WATCH OUT There is a possibility that the condition of a loop will never be met and the loop will run infinitely. The following example demonstrates how a user intends to display the first ten odd numbers. However, the value of control variable odd never becomes 10. The variable odd increments by 2, and therefore contains the value of 9, then 11, and so on.
1. #include<iostream>
2. using namespace std;
3. int main(){
4. int odd=1;
5. while( odd !=10 ){
6. cout << odd << endl;
7. odd = odd +2;
8. }//WHILE
9. return 0;
10. }//MAIN
Figure 3.23a - Testing the loop condition for equality.
1
3
5
7
9
11
.
.
.
Figure 3.23b - The results of Figure 3.23a - an infinite loop.
TERMINATE THE LOOP WITH INPUT MISMATCH The input routines of C/C++ expect to receive entry values that are identical to the expected data type. A contradiction will result to a false value (0), which can be used to terminate the loop. For example, the employee's id data type of input routine cin is an integer. Any non-integer values entered by the user will result in false value, which can be used as the terminator of the loop. In the below program, you can type any word to stop the loop. LOOP WITH END OF DATA FILE
1. #include<iostream>
2. using namespace std;
3. main(){
4. int empid;
5. cout << ”Enter an Employee ID: “;
6. while( cin >> empid ){
7. cout << empid << endl;
8. cout <<”Enter an Employee ID: “; }//WHILE
9. return 0;
10. }//MAIN
Figure 3.24a - Implicit end of file.
Enter an Employee ID: 3423 3423 Enter an Employee ID: 4567 4567 Enter an Employee ID: 1209 1209 Enter an Employee ID: 3498 3498 Enter an Employee ID: finishFigure 3.24b - The while loop will terminate when the incorrect data type is read. There are situations when the size of the data is not known in advance, or the number of times the loop occurs is constantly changing. Thus, the use of a loop with a control variable will not be effective. The use of an end of file marker within the program will resolve this problem. At the end of each data file, there is a marker (character) that can be used as a terminator. The loop will cease when this indicator has been reached, meaning no more data is to be read. The following illustrates end of file usage as a loop terminator.
1. #include<iostream>
2. using namespace std;
3. main(){
4. int id,hw;
5. float hr,gp;
6.
7. cin >> id >> hw >> hr;
8. while( !cin.eof( ) ){
9. gp = hw * hr;
10. cout << ”THE ID IS “<<id <<” THE GROSS PAY IS “<<gp<<endl;
11. cin >> id >> hw >> hr;
12. }//WHILE
13.
14. return 0;
15. }//MAIN
Figure 3.25 - Payroll Program ( payroll.cpp ) with a data file.
WHY AND HOW TO CREATE A DATA FILE FOR A PROGRAM? When data becomes large, entering it interactively or retyping becomes costly and troublesome. Can you imagine entering supermarket data every time you need it? To avoid repetition, data is entered and saved in a file for future usage. The data file can be created the same way the program is created, except that the extension is not going to be a .cpp or .c. The file extensions, such as .in, .dat and .txt, are commonly used with the data file. The majority of C/C++ compilers come with an environment that allows editing a program that can be used to create the data file as well. However, the operating system text editors or word processors can be used to create a data file. In the case of a word processor it is safe to save the file as text. PROGRAM AND DATA FILE LINKAGE: THE OPERATING SYSTEM Operating systems provide utilities the means to execute a program with the data file. The linkage of data with a program can be similarly used with a DOS or Unix command, which is known as redirection "<". For example, for a payroll program "payroll.cpp' and its executable file "payroll.exe', the following can be used where "payroll.in' contains the input data: payroll < payroll.in Similarly, the output file can be created using ">" redirection: payroll < payroll.in > payroll.out Input data, instead of coming from the keyboard, comes from the input file (e.g. in this example from payroll.in). The output, instead of being shown on the screen, goes into a file (e.g., payroll.out). The operating system commanding "<" for the input file and ">" for the output file has nothing to do with C/C++ operator signs "<", ">", "<<", ">>". In Chapter 8, you will learn how to access and create a data file directly with your program without using the above commands. INPUT FILE AND OUTPUT FILE EXAMPLE: THE PAYROLL PROGRAM The numbers listed in Figure 3.26 are examples of an input data file that is saved as "payroll.in." Just remember that the order of data values written in the data file is important, since the program needs to read these values in a specific order. For clarity, each line is dedicated to an individual employee and contains the employee's ID, hours worked, and the hourly rate. Also, note that the line that reads the data file (cin>>id>>hw>>hr; from Figure 3.25 ) matches the format of the data file. The created data file must be in the same directory as the payroll program executable file in order for the payroll program to be able to read the data. 1645 10 9.99 8932 40 20.00 7104 20 15.99 2816 15 50.00 5387 30 6.99Figure 3.26 - Input file named payroll.in for Payroll Program in Figure 3.25. In order to use the operating system redirection operators, you must use the command line prompt, such as the MS-DOS prompt if your machine is running the Windows operating system. However, an executable file has to be built before you may use the command line. Building an executable file is dependent upon your compiler and you should consult the documentation of your compiler for specific information. For simplicity, we will use the Microsoft Visual C++ Version 6.0 compiler. Within the MS Visual C++ compiler, an executable file is created by first compiling the source code file by clicking on the menu item named Build and clicking Compile payroll.cpp ( or using the keyboard keys Ctrl + F7 ). Once the source code is compiled with no errors, the payroll.exe ( executable ) file can be created by clicking the menu item Build and then clicking Build payroll.exe ( or using the keyboard key F7 ). After the payroll executable file has been created, open up a MS-DOS prompt by selecting Startà ProgramsàMS-DOS prompt from the Windows task bar. Once the MS-DOS prompt window has been displayed, move to the directory that holds your executable file. Once again, for simplicity, we are using a directory named Cexample off of the root of C. Table 3.4 shows a list of simple DOS commands that will help you navigate through DOS based directories to reach the directory where your executable file is stored.
When using the Microsoft Visual C++ compiler, the executable file created will be in a subdirectory of the directory, in which your payroll.cpp file is located. For our example, the C++ source file named payroll.cpp is located in the directory C:\Cexample. Therefore, when the source code is built into an executable file, the file will be placed into the following directory: C:\Cexample\Debug. The Microsoft Visual C++ compiler automatically creates the Debug directory if it does not exist. The Debug directory is where the payroll.in file must be located so that the payroll.exe file may read the input data via help from the operating system indirection operators. Figure 3.27a and 3.27b show the output of the program of Figure 3.25. Figure 3.27a shows the MS-DOS prompt and how to travel to the correct directory as well as how the system indirection operators work. For example, the following system command produces output directed to the MS-DOS Window: C:\Cexample\Debug>payroll<payroll.in Figure 3.27b illustrates the output directed to a file that is created named payroll.out. The file is overwritten if it already exists. The following system command produces the new output file rather than displaying the information back to the MS-DOS Window. C:\Cexample\Debug>payroll<payroll.in>payroll.out The file named payroll.out can be opened from the MS-DOS Window from the MS-DOS prompt by typing the following command: EDIT payroll.out The file may also be opened using Windows applications, such as Notepad. Figure 3.27a - Illustrates the MS-DOS prompt and the commands needed to run the examples discussed in the text. Notice the first payroll execution brings the output back to the MS-DOS Window. The second execution returns the output back to a file named payroll.out. Either way, the payroll executable file uses system indirection to access the payroll.in file. THE ID IS 1645 THE GROSS PAY IS 99.9 THE ID IS 8932 THE GROSS PAY IS 800 THE ID IS 7104 THE GROSS PAY IS 319.8 THE ID IS 2816 THE GROSS PAY IS 750Figure 3.27b - Output located within the file named payroll.out created by the Payroll Program in Figure 3.25. The output is created using operating system indirection, as illustrated in Figure 3.27a. You might want to print or display these three files together for further need or development. WHO DOES DETECT THE END OF FILE? The input routines of a program are those responsible for getting the data from the data file. The input routines either detects the end of file character, acknowledges it by returning a value of -1, or, in the case of a mismatch, the input routines will return a value of 0. Tables 3.5 and 3.6 show the C Syntax and C++ Syntax respectively for detecting the end of file.
Table 3.5 - C Syntax for end of file.
END OF FILE: EXAMPLE Figure 3.28 illustrates an example of end of file using C where the program ECHOES the content of a file of employees' ids. Figure 3.29 illustrates a C++ version.
1. #include <stdio.h>
2.
3. void main(){
4. int empid;
5.
6. while( scanf( “%d”, &empid ) != EOF ){
7. printf( “%d\n”,empid );
8. }//WHILE
9. }//MAIN
Figure 3.28 - Example of scanning a file and echoing the read information to the screen. The program is written in C and behaves similarly to the previous payroll program.
Similarly, each of the following loops can be used as well:
while (scanf(“%d”,&empid){
printf(“%d\n”,empid);
}//WHILE
OR
while (!eof()){
scanf(“%d”,&empid);
printf(“d\n”,empid);
}//WHILE
</pre>
<pre>
1. #include <iostream>
2. using namespace std;
3. int main(){
4. long int empid;
5. while( cin >> empid ){
6. cout << empid << endl;
7. }//WHILE
8. }//MAIN
Figure 3.29 - Example of scanning a file and echoing the read information to the screen. The program is written in C++ and behaves similarly to the previous payroll program.
Similarly, the following can be used:
cin>>empid;
while (!cin.eof()){
cout <<empid <<endl;
cin>>empid;
}//WHILE
1254 2458 6215 3563Figure 3.30 - Output to Figure 3.28 and 3.29 respectively. An input file with the four above numbers was created and read into the programs of Figure 3.28 and 3.29 using the same technique as used by the payroll program of Figure 3.25. MAGIC OF END OF FILE What happens if you don't know the size of the input data, or your input keeps changing and becoming more or less? How many times do you loop? What happens if you do not want to type data over and over? The simple answer is to use a data file with your program. The combination of the end of file and looping is an amazing phenomenon, in which a program can visit a data file of any size without knowing in advance the number of cycles. Program repetition becomes independent of the data file; whether an employee is hired or fired the program remains the same. Every time the data changes we do not need to go to the program to make changes. The program continues until the end of file marker is reached. In fact end of file is a magical tool for the programmer. CLOSING REMARKS AND LOOKING AHEAD One unique feature of the computer is the fact that a task can be performed over and over as many times as is needed. A C++ loop, such as the while, for, or dowhile, makes this possible. A loop can be repeated for a specific time, until a condition is satisfied, or can be stopped by the user. Moreover a loop can work with an external data file allowing all information within the data file to be accessed. These attributes make the loop an extremely valuable tool. It is important not to forget that looping ten times or ten thousand times is just a mater of adding additional zeros to the test condition of the loop. With a human being, the processes of repetition described above would be a huge effort. Obtaining an early start with the concept of the data file in this chapter will give a better understanding of the work load a program is capable of accomplishing. The next chapter will introduce the concept of decision-making and selection, which is what makes programs intelligent. Selection allows a program to make decisions related to conditions within a program at a specific point in time. |
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