4.3. Loops

4.3.1. While loop

We also discussed this in Topic 1.

while( expr ) statement;

while( expr ) {
   code block;
}

4.3.2. For loop

The while and the for loop are the two most frequently used looping constructs. The for loop is most often used when iterating through a sequence of a pre-defined length.

for( expr1; expr2; expr3 ) {
   code block;
}

Is equivalent to:

expr1;
while( expr2 ) {
   code block;
   expr3;
}

• expr1 is the initializer of the for loop.

• expr2 is the condition for continuing with the loop.

• expr3 is evaluated after each execution of the code block and is usually used to facilitate iterating through the loop.

  int i, x=0, y=0;
  
  for( i = 0; i < MAX; i++ ) {
     x += i * y*(i - 1);
     y = x*i;
  }
  printf( "x = %d, y = %d\n", x,y );
  

4.3.3. Variations on for loops

4.3.3.1. Using empty statements

An empty statement is a statement consisting of just a semicolon (;). It is useful when one statement is needed syntacticly, but no action needs to be performed.

For example …

/* Variable i is already set in the program */
for( ; i < MAX; i++ ) {
   block;
}

/* Some code in the body of the for loop alters the value of i */
for( i = 0; i < MAX; ) {
   block of code which updates i;
}

/* The code has a break statement */
for( i = 0; ; i++ ) {
   block;
   if( some_test ) break;
}

4.3.3.2. The comma operator

The comma operator allows multiple statements to be executed as part of either expr1 or expr3.

for( sum=0, i=1; i <= n; i++ )
   sum += i;

for( sum=0, i=1; i <= n; sum += i, i++)
   ;

Note: Although seldom used, the comma operator may also be used in a while loop construct or in the second expression of a for loop. The last expression determines if the body of the loop is executed.

while( i = 0, testvalue < 0 ) {
   statements;
}

4.3.4. The do loop

The do loop is similar to the while loop except the code block always executes at least one time. The test of whether to continue or not is at the end of the loop.

do {
   code block;
}while( expr );