# Keep track of previous indexes after sorting a vector in C++ STL

Prerequisite: vector, vector pair sorting

Given a vector, keep track of the present indexes corresponding to each element and after sorting print element with its previous respective indexes.

Examples:

Input: Arr[] = {2, 5, 3, 7, 1}
Output: {1, 4} {2, 0} {3, 2} {5, 1} {7, 3}
Explanation:
Before sorting [index(element)]: [0(2), 1(5), 2(3), 3(7), 4(1)]
After sorting [previous_index(element)]: [4(1), 0(2), 2(3), 1(5), 3(7)]

Input: Arr[] = {4, 5, 10, 8, 3, 11}
Output: {3, 4} {4, 0} {5, 1} {8, 3} {10, 2} {11, 5}

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

Approach: The idea is to store each element with its present index in a vector pair and then sort all the elements of the vector, Finally, print the elements with its index associated with it.

Below is the implementation of the above approach:

 `// C++ implementation to keep track ` `// of previous indexes ` `// after sorting a vector ` ` `  `#include ` `using` `namespace` `std; ` ` `  `void` `sortArr(``int` `arr[], ``int` `n) ` `{ ` ` `  `    ``// Vector to store element ` `    ``// with respective present index ` `    ``vector > vp; ` ` `  `    ``// Inserting element in pair vector ` `    ``// to keep track of previous indexes ` `    ``for` `(``int` `i = 0; i < n; ++i) { ` `        ``vp.push_back(make_pair(arr[i], i)); ` `    ``} ` ` `  `    ``// Sorting pair vector ` `    ``sort(vp.begin(), vp.end()); ` ` `  `    ``// Displaying sorted element ` `    ``// with previous indexes ` `    ``// corresponding to each element ` `    ``cout << ``"Element\t"` `         ``<< ``"index"` `<< endl; ` `    ``for` `(``int` `i = 0; i < vp.size(); i++) { ` `        ``cout << vp[i].first << ``"\t"` `             ``<< vp[i].second << endl; ` `    ``} ` `} ` ` `  `// Driver code ` `int` `main() ` `{ ` `    ``int` `arr[] = { 2, 5, 3, 7, 1 }; ` `    ``int` `n = ``sizeof``(arr) / ``sizeof``(arr); ` `    ``sortArr(arr, n); ` ` `  `    ``return` `0; ` `} `

Output:

```Element    index
1    4
2    0
3    2
5    1
7    3
```

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