How Can I Preserve Original Indexes When Sorting Data in C ?

Preserving Original Indexes During C Sorting

Sorting data while retaining the original indexes is a common requirement in data manipulation tasks. In C , using the standard library, this task encompasses the following steps:

1. Vector Initialization: Create vectors to store the original data values and their corresponding indexes, initialized with the appropriate values.
2. Sorting with Index Preservation: Utilize standard algorithms like std::stable_sort to arrange data by values while maintaining the correlation between the original and sorted vector indexes.
3. Returning Sorted Indexes: Extract and return a vector containing the sorted indexes, which preserves the order of the original elements.
4. Example Application: Use the sorted indexes in subsequent iterations to retrieve data from the original vector in the modified order.

Implementation in C 11:

#include <iostream>
#include <vector>
#include <numeric>      // std::iota
#include <algorithm>    // std::sort, std::stable_sort

using namespace std;

template <typename T>
vector<size_t> sort_indexes(const vector<T> &v) {

  // Initialize original index locations
  vector<size_t> idx(v.size());
  iota(idx.begin(), idx.end(), 0);

  // Sort indexes based on comparing values in v
  // Using std::stable_sort instead of std::sort
  // to avoid unnecessary index re-orderings
  // when v contains elements of equal values 
  stable_sort(idx.begin(), idx.end(),
       [&v](size_t i1, size_t i2) {return v[i1] < v[i2];});

  return idx;
}

Example usage:

vector<double> values = {5, 2, 1, 4, 3};
vector<size_t> sorted_indexes = sort_indexes(values);

for (auto i : sorted_indexes) {
  cout << values[i] << endl;
}

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