Unsigned integers in C++ for loops

The problem is, your loop allows i to be as low as zero and only expects to exit the loop if i is less than 0. Since i is unsigned, it can never be less than 0. It rolls over to 2^32-1. That is greater than the size of your vector and so results in a segfault.

Whatever the value of unsigned int i it is always true that i >= 0 so your for loop never ends.

In other words, if at some point i is 0 and you decrement it, it still stays non-negative, because it contains then a huge number, probably 4294967295 (that is 2³²-1).

The problem here is that an unsigned integer is never negative.

Therefore, the loop-test:

i >= 0

will always be true. Thus you get an infinite loop.

When it drops below zero, it wraps around to the largest value unsigned value.
Thus, you will also be accessing x[i] out-of-bounds.

This is not a problem for signed integers because it will simply go negative and thus fail i >= 0.

Thus, if you want to use unsigned integers, you can try one of the following possibilities:

for (unsigned int i = 9; i-- != 0; )

and

for (unsigned int i = 9; i != -1; i--)

These two were suggested by GManNickG and AndreyT from the comments.

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And here's my original 3 versions:

for (unsigned int i = 9; i != (unsigned)0 - 1; i--)

or

for (unsigned int i = 9; i != ~(unsigned)0; i--)

or

for (unsigned int i = 9; i != UINT_MAX; i--)