The for statement implements an iterative loop. The syntax of the for statement is:

for ([init-expression]; [condition-expression]; [increment-expression]) statement

Before the first iteration of the loop, init-expression sets the starting variables for the loop. You cannot pass declarations in init-expression.

condition-expression is checked before the first entry into the block; statement is executed repeatedly until the value of condition-expression is false. After each iteration of the loop, increment-expression increments a loop counter. Consequently, i++ is functionally the same as ++i.

All expressions are optional. If condition-expression is left out, it is assumed to be always true. Thus, “empty” for statement is commonly used to create an endless loop in C:

for ( ; ; ) statement

The only way to break out of this loop is by means of the break statement.

Here is an example of calculating scalar product of two vectors, using the for statement:

for ( s = 0, i = 0; i < n; i++ ) s += a[i] * b[i];

There is another way to do this:

for ( s = 0, i = 0; i < n; s += a[i] * b[i], i++ );  /* valid, but ugly */

but it is considered a bad programming style. Although legal, calculating the sum should not be a part of the incrementing expression, because it is not in the service of loop routine. Note that null statement (;) is used for the loop body.

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