When we iterate over a std::vector in C++, we need to choose the right type for the index variable to maintain both the correctness and clarity of our code. A common question arises: Should we use an unsigned or signed integer for the index variable?

In this article, we will learn the differences between using unsigned and signed index variables, and provide ideas on which to use in different scenarios.

Unsigned Index Variables

An unsigned index variable is typically defined as size_t, which is an unsigned integer type that is used to represent the size of any object in memory. We use size_t because it is guaranteed to be able to express the maximum size of any theoretically possible object on the target platform.

Example:

// C++ program to iterate through a vector using Unsigned
// Index Variables

// Include necessary header file
#include <iostream>
#include <vector>

using namespace std;

int main()
{
    // Initialize a vector with values 1 to 5
    vector<int> vec = { 1, 2, 3, 4, 5 };

    // Iterate through the vector and print each element
    for (size_t i = 0; i < vec.size(); ++i) {
        cout << vec[i] << " ";
    }
    // Print a newline character after the vector elements
    cout << endl;

    return 0;
}

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Signed Index Variable

A signed index variable is generally defined as int, which is a signed integer type. We use int for indexing when the possibility of negative values is necessary, such as in arithmetic operations involving indices. It handles arithmetic operations more intuitively, especially when the result can be negative.

Example:

// C++ program to iterate through a vector using signed
// index variable

// Include necessary header file
#include <iostream>
#include <vector>

using namespace std;

int main()
{
    // Initialize a vector with values 1 to 5
    vector<int> vec = { 1, 2, 3, 4, 5 };

    // Iterate through the vector and print each element
    for (int i = 0; i < static_cast<int>(vec.size()); ++i) {
        cout << vec[i] << " ";
    }

    // Print a newline character after the vector elements
    cout << endl;
    // Return 0 to indicate successful execution
    return 0;
}

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When to Use Which Index Variable?

• Use Unsigned for Consistency: When iterating over containers like std::vector, prefer using size_t or another unsigned type to match the return type of size().
• Use Signed for Arithmetic: If the index variable needs to be manipulated with arithmetic operations that can result in negative values, prefer using a signed integer type like int.
• Avoid Mixed Comparisons: Make sure that comparisons between index variables and container sizes are done with matching types to avoid warnings and potential bugs.

Difference Between Unsigned and Signed Index Variables in C++

The below table lists the major differences between using unsigned and signed index variables when iterating over a std::vector in C++: