array::operator[ ] in C++ STL


Array classes are generally more efficient, light-weight and reliable than C-style arrays. The introduction of array class from C++11 has offered a better alternative for C-style arrays.

array::operator[]

This operator is used to reference the element present at position given inside the operator. It is similar to the at() function, the only difference is that the at() function throws an out-of-range exception when the position is not in the bounds of the size of array, while this operator causes undefined behaviour.

Syntax :

arrayname[position]
Parameters :
Position of the element to be fetched.
Returns :
Direct reference to the element at the given position.

Examples:

Input :  myarray = {1, 2, 3, 4, 5}
         myarray[2]
Output : 3

Input :  myarray = {1, 2, 3, 4, 5}
         myarray[4]
Output : 5

Errors and Exceptions

1. If the position is not present in the array, it shows undefined behaviour.
2. It has a no exception throw guarantee otherwise.

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// CPP program to illustrate
// Implementation of [] operator
#include <iostream>
#include <array>
using namespace std;
  
int main()
{
    array<int,5> myarray{ 1, 2, 3, 4, 5 };
    cout << myarray[4];
    return 0;
}

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Output:

5

Application
Given an array of integers, print all the integers present at even positions.

Input  :1, 2, 3, 4, 5
Output :1 3 5
Explanation - 1, 3 and 5 are at position 0,2,4 which are even

Algorithm
1. Run a loop till the size of the array.
2. Check if the position is divisible by 2, if yes, print the element at that position.

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// CPP program to illustrate
// Application of [] operator
#include <iostream>
#include <array>
using namespace std;
  
int main()
{
    array<int,5> myarray{ 1, 2, 3, 4, 5 };
    for(int i=0; i<myarray.size(); ++i)
    {
        if(i%2==0){
            cout<<myarray[i];
            cout<<" ";
        }
    }
    return 0;
}

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Output:

1 3 5


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