Find index of an element in an array in C++

This post provides an overview of available methods to find an index of the first occurrence of an element in the array in C++.

1. Naive solution

A simple solution is to write our own custom routine for finding the index of the first occurrence of an element. The idea is to perform a linear search on the given array for determining the index. This approach is demonstrated below:

#include <iostream>

using namespace std;

int main()

{

int arr[] = { 6, 3, 5, 2, 8 };

int n = sizeof(arr)/sizeof(arr[0]);

int elem = 2;

int i = 0;

while (i < n)

{

if (arr[i] == elem) {

break;

}

i++;

}

if (i < n)

{

cout << "Element " << elem << " is present at index " << i

<< " in the given array";

}

else {

cout << "Element is not present in the given array";

}

return 0;

}

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Output:

Element 2 is present at index 3 in the given array

2. Using `std::find` algorithm

We can also use the std::find algorithm, which returns an iterator that points to the target value. It is defined in the <algorithm> header. To get the required index, apply pointer arithmetic, or make a call to std::distance.

#include <iostream>

#include <algorithm>

using namespace std;

int main()

{

int arr[] = { 6, 3, 5, 2, 8 };

int n = sizeof(arr)/sizeof(arr[0]);

int elem = 2;

auto itr = find(arr, arr + n, elem);

if (itr != end(arr))

{

cout << "Element " << elem << " is present at index "

<< distance(arr, itr) << " in the given array";

}

else {

cout << "Element is not present in the given array";

}

return 0;

}

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Output:

Element 2 is present at index 3 in the given array

3. Using `std::find_if` algorithm

Sometimes it is desired to search for an element that meets certain conditions in the array. For instance, find the index of the first 2-digit number in the array. The recommended approach is to use the std::find_if algorithm, which accepts a predicate to handle such cases.

#include <iostream>

#include <algorithm>

using namespace std;

struct comp

{

int elem;

comp(int const &i): elem(i) {}

bool operator()(int const &i) {

return (i == elem);

}

};

int main()

{

int arr[] = { 6, 3, 5, 2, 8 };

int n = sizeof(arr)/sizeof(arr[0]);

int elem = 2;

auto itr = find_if(arr, arr + n, comp(elem));

if (itr != end(arr))

{

cout << "Element " << elem << " is present at index "

<< distance(arr, itr) << " in the given array";

}

else {

cout << "Element is not present in the given array";

}

return 0;

}

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Output:

Element 2 is present at index 3 in the given array

That’s all about finding the index of an element in an array in C++.

Find index of an element in an array in C++

1. Naive solution

2. Using std::find algorithm

3. Using std::find_if algorithm

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2. Using `std::find` algorithm

3. Using `std::find_if` algorithm