Why Does Comparing a Negative Integer to an Unsigned Integer in C Yield Unexpected Results?

Negative Comparison to Positive Unsigned Int?

In C , we often encounter situations where values of different types are compared. However, understanding how these comparisons behave can be tricky.

Consider this code:

#include <iostream>
#include <vector>

int main() {
    std::vector<int> a;
    std::cout << "vector size " << a.size() << std::endl;

    int b = -1;
    if (b < a.size())
        std::cout << "Less";
    else
        std::cout << "Greater";
}

The code prints "Greater," even though "-1" is clearly less than "0." Why is this happening?

The key lies in the different data types involved. a.size() returns an unsigned integer, while b is a negative signed integer. When comparing these two types, the negative signed integer is promoted to an unsigned integer. This results in a large unsigned value being compared to 0, which is obviously greater.

To illustrate this further, let's look at the following example:

#include <iostream>

int main() {
    std::cout << std::boolalpha;
    unsigned int a = 0;
    int b = -1;
    std::cout << (b < a) << "\n";  // prints "false"
}

In this case, the result is "false," as expected. This is because both a and b are promoted to unsigned integers, and "-1" is now a large unsigned value that is greater than 0.

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