You wouldn't modify an integer in place. At least it wouldn't be idiomatic. Quite a few languages, especially functional, have that constraint. You can always do range-based-for:

    for i in [0, 100]:
        // i is known to be in [0, 100]
        // the argument is known to be in [0, 200]
        do_something(i*2)

If you really want unbounded growth, you need a bignum. If you want to be bounded in size but not in time, you have to specify overflow behavior. Something like (ugly pseudocode):

    integer[modulo 256] a = 0;
    while some_condition:
        a += 1

or

    integer[0, 1000] b = 0;
    while some_condition:
        guard if b < 999:
            // b is now in [0, 999]
            b += 1

The whole point is, forcing you to make your mind up about overflow behavior (and not just using int32 or int all the time and hoping i is going to be "small").