I have made some research on Stackoverflow about reverse for loops in C++ that use an unsigned integer instead of a signed one. But I still do NOT understand why there is a problem (see Unsigned int reverse iteration with for loops). Why the following code will yield a segmentation fault?
#include <vector>
#include <iostream>
using namespace std;
int main(void)
{
vector<double> x(10);
for (unsigned int i = 9; i >= 0; i--)
{
cout << "i= " << i << endl;
x[i] = 1.0;
}
cout << "x0= " << x[0] << endl;
return 0;
}
I understand that the problem is when the index i will be equal to zero, because there is something like an overflow. But I think an unsigned integer is allowed to take the zero value, isn’t it? Now if I replace it with a signed integer, there is absolutely no problem.
Does somebody can explain me the mechanism behind that reverse loop with an unsigned integer?
Thank you very much!
The problem here is that an unsigned integer is never negative.
Therefore, the loop-test:
will always be true. Thus you get an infinite loop.
When it drops below zero, it wraps around to the largest value
unsignedvalue.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:
and
These two were suggested by GManNickG and AndreyT from the comments.
And here’s my original 3 versions:
or
or