Return to Topic Menu | Computer Science Main Page | MathBits.com | Terms of Use | Resource CD

 

The for loop

The statements in the for loop repeat continuously for a specific number of times.  The while and do-while loops repeat until a certain condition is met.  The for loop repeats until a specific count is met.  Use a for loop when the number of repetition is know, or can be supplied by the user.  The coding format is:

for(startExpression; testExpression; countExpression)
{
    block of code;

}

The startExpression is evaluated before the loop begins.  It is acceptable to declare and assign in the startExpression (such as int x = 1;).  This startExpression is evaluated only once at the beginning of the loop.

The testExpression will evaluate to TRUE (nonzero) or FALSE (zero).  While TRUE, the body of the loop repeats.  When the testExpression becomes FALSE, the looping stops and the program continues with the statement immediately following the for loop body in the program code.

The countExpression executes after each trip through the loop.  The count may increase/decrease by an increment of 1 or of some other value. 

Braces are not required if the body of the for loop consists of only ONE statement.  Please indent the body of the loop for readability.

CAREFUL:  When a for loop terminates, the value stored in the computer's memory under the looping variable will be "beyond" the testExpression in the loop.  It must be sufficiently large (or small) to cause a false condition.  Consider:

for (x = 0; x <= 13; x++)
     cout<<"Melody";

When this loop is finished, the value 14 is stored in x.

 

The following program fragment prints the numbers 1 - 20. 


for:

int ctr;
for(ctr=1;ctr<=20; ctr++)
{
     cout<< ctr <<"\n";

 

Common Error:  If you wish the body of a for loop to be executed, DO NOT put a semicolon after the for's parentheses.  Doing so will cause the for statement to execute only the counting portion of the loop, creating the illusion of a timing loop.  The body of the loop will never be executed.

 

Semicolons are required inside the parentheses of the for loop.  The for loop is the only statement that requires such semicolon placement.
 

 

Return to Topic Menu | Computer Science Main Page | MathBits.com | Terms of Use | Resource CD