Iterate through a vector with indices in C++

This post will discuss how to iterate through a vector with indices in C++.

1. Iterator-based for-loop

The idea is to traverse the vector using iterators. To get the required index, we can either use the std::distance function or apply the pointer arithmetic. This would translate to the code below:

#include <iostream>

#include <vector>

int main()

{

std::vector<int> vec = {5, 3, 4, 1, 7};

for (auto it = vec.begin(); it != vec.end(); it++) {

int index = std::distance(vec.begin(), it);

std::cout << "Element " << *it << " found at index " << index << std::endl;

}

return 0;

}

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2. Index-based for-loop

We can simplify the above code with an index-based for-loop, as shown below:

#include <iostream>

#include <vector>

int main()

{

std::vector<int> vec = {5, 3, 4, 1, 7};

for (int i = 0; i < vec.size(); i++) {

std::cout << "Element " << vec[i] << " found at index " << i << std::endl;

}

return 0;

}

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3. Using Range-based for-loop

Alternatively, we can use the range-based for-loop and apply the pointer arithmetic to get the index of each element.

#include <iostream>

#include <vector>

#include <algorithm>

int main()

{

std::vector<int> vec = {5, 3, 4, 1, 7};

for (int &i: vec) {

int index = &i - &vec[0];

std::cout << "Element " << i << " found at index " << index << std::endl;

}

return 0;

}

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This is equivalent to the following version using the std::for_each algorithm with lambda expressions, introduced in C++11.

#include <iostream>

#include <vector>

#include <algorithm>

int main()

{

std::vector<int> vec = {5, 3, 4, 1, 7};

std::for_each(vec.begin(), vec.end(), [&vec](int const &i) {

auto index = &i - &vec[0];

std::cout << "Element " << i << " found at index " << index << std::endl;

});

return 0;

}

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That’s all about iterating through a vector with indices in C++.

Iterate through a vector with indices in C++

1. Iterator-based for-loop

2. Index-based for-loop

3. Using Range-based for-loop

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