For statement

for (<statement1> ; <expression> ;<statement2>) <statement3>

This works as follows. First statement1 is executed. Then the expression is evaluated. If it is true, statement 3 is executed; then statement 2 and then the expression is evaluated again. This continues until the expression is false.

This may sound complicated. You should interpret this as follows. The first statement initializes the for-loop. The expression tests whether the loop should be ended. Statement2 is the step statement that goes to the next loop evaluation.

The most common use is to have a counter run through some range.

Example
The following program initializes an array of length 10 with the values 1- 10.

{
  for (i=0; i<=9; i+=1) list[i] = i+1;
}