使用这种方式时，C++ 中的 For 循环没有运行：for(int i=-1;i

Question

I have a particular use case where I have to initialize i value in for loop to -1 and write the exiting condition based on vector size. But the problem is the loop is not getting executed at all. This is the code.

#include <bits/stdc++.h>
using namespace std;

int main()
{
    vector<int> vec = { 1, 2, 3, 4, 5 };
    for(int i=-1;i<vec.size();i++) cout << i << " ";
    return 0;
}

But I am able to do like this.

#include <bits/stdc++.h>
using namespace std;

int main()
{
    vector<int> vec = { 1, 2, 3, 4, 5 };
    int size = vec.size();
    for(int i=-1;i<size;i++) cout << i << " ";
    return 0;
}

Can anyone explain this weird behavior or am I doing something wrong there?

Answer 1

i<size() in the first snippet compares a signed int i; to an unsigned size_t size; ( size_t is the typical typedef for vector::size_type )

When the compiler sees that comparison, it converts the int to an unsigned value. Typically something like 2^64 - 1 , for your first value of -1 . This value is much bigger than 5, so the loop doesn't run.

Take-away: Don't compare signed and unsigned value with less-than or greater-than.