The DO Statement

Last time, we embarked on a journey of loops. So let's continue from where we left.

The while statement discussed is an entry-controlled loop structure, in which the loop will not be executed if the test condition comes out to be false. But in some situations it might be necessary to execute the loop, even if the test-condition fails. So this kind of loops are called the exit-controlled loop structure. This takes the form:

   body of the loop
while (test condition);


   ch = getchar();
   printf("The character entered is %c",ch);
while (ch != 'n')

ch is assumed to be a variable of type character. Now the loop is executed first under no test and the input by the user is outputted. Then the entered value would be tested by the condition for further advancement.

The FOR Statement:

The for loop is an entry-controlled loop that provides a more concise loop control structure. The general form of the for loop is:

for (initialization; test-condition ; increment)
  body of the loop

The execution of the for statement is as follows:

  1. Initialization of the control variables is done first, using assignment statements 
    such as i = 1 and count = 0.  The variables i and count are known as the loop control variables.
  2. The value of the control variable is tested using the test-condition. The test-condition is a relational expression, such as i<10 that determines when the loop will exit. If the condition is true, the body of the loop is executed; otherwise the loop is terminated and the execution continues with the statement that immediately follows the loop.
  3. When the body of the loop is executed, the control is transferred  back to the for statement after evaluating the last statement in the loop. Now , the control variable is incremented using an assignment statement such as i = i+1 and the new value of the control variable is again tested to see whether it satisfies the loop condition. If the condition is satisfied, the body of the loop is again executed. This process continues till the value of the control variable fails to satisfy the test-condition.    


for( i = 0; i<10; i = i+1)

The for loop will be executed 10 times. Each time a value equal to the current value of i will be printed out. 

Additional Features of for Loop

1. More than one variable can be initialized at a time in the for statement. For example:

for (p=1,n=0; n<18; ++n)  

2. Like the initialization section, the increment section may also have more than one part. 
    For example,

for (a=2,b=30; n <= m; n=n+1,m=m-1)                   
p = b/a;
printf("%d ",p);

3. It is also permissible to use expressions in the assignment statements of initialization and increment sections. For example 

for (w = (a+b); w>0; w=w/2) is valid.

4. One or more sections in the for statement can be omitted, if necessary. For example

e = 2;
for (; e != 10 ;)
   e = e + 2;

Both the initialization and increment sections are omitted in the for statement. In such cases, the sections are left blank. However, the semicolons separating the sections must remain. If the test-condition is not present, the for statement sets up an infinite loop.   


More by :  Sachin Mehta

Top | Computing

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