Why should you be careful not to place a statement in the body of a for loop that changes the value of the loop's counter variable?

Answer 1

By changing the loop counter variable within the loop can cause the loop to exit early or to loop more times than the limit.

The is nothing that says that you can not change the loop counter variable within the loop, you may indeed want to do that. Just be careful and be sure of what will happen.

Answer 2

Counted loops are best implemented as for loops because this allows us to make the control variable local to the loop:

for (int x=0; x<10; ++x) {

// ...

}

x = 42; // error: x is no longer in scope...

We can also declare the loop using a while or do loop, however this means the control variable must be declared outside the loop, widening the scope of the variable:

int y=0;

while (y++<10) {

...

}

y = 42; // ok

int z=0;

do {

} while (z++<10);

z = 42; // ok

Regardless of which version we use, the control variable is always within the scope of the loop, which means that any statement in the loop body can change its value. However, if the control statement also changes the value, as it does in all three examples using the increment operator (++), we'd be changing the value twice on each iteration, which would naturally change the number of iterations:

for (int x=0; x<10; ++x) {

// ...

++x; // really?

}

Here, the loop will execute just 5 times because at the start of each successful iteration x would be 0, 2, 4, 6 and 8. On the next iteration x is 10, thus x<10 is false and the loop terminates. This is probably not what we wanted because the forstatement clearly tells us that we expect this loop to execute exactly 10 times.

Note that it is not unusual for loop bodies to modify a control variable, hence it has to be accessible from within the loop body, but with counted loops we should avoid doing this as it could adversely affect the count. If we need to modify a counter variable (without affecting the loop count), then we should copy it and modify the copy instead:

for (int x=0; x<10; ++x) {

int y {x}; // copy x

++y; // modify the copy

// use y...

}