Sorting Array of Pointers in C++

algorithm, arrays, c++, pointers, sorting

Solution

I figured someone might actually need to sort an array of pointers in a sane way:

#include <iostream>
#include <array>
#include <algorithm>

int main() {
    std::array<int, 8> arr { 3, 5, 4, 1, 2, 7, 6, 8 };
    std::array<int*, 8> p_arr;

    for (unsigned i = 0; i < 8; ++i) {
        p_arr[i] = &arr[i];
    }

    std::sort(p_arr.begin(), p_arr.end(), [](int* a, int* b) { return *a < *b; });

    for (auto i : p_arr) 
        std::cout << *i;
}

The ugly middle loop is totally replace'able by range for over `zip`pped range, but I don't have my own implementation with reference semantics right now, and I am too lazy to check the Boost one.1

Here's a live sample on Coliru.

Also, because I think we should repeat this over and over until newbies understand it:

- Don't reinvent the sorting wheel (unless it's a toy implementation)

- Try to avoid using pointers in C++ if reasonably possible.

1This is actually important in order to make sure both ranges (in this case two arrays) have the same length. Different zipping conventions either require the ranges to be of the same length (crashing or throwing otherwise) or fill in the empty data should one of the ranges be too short. While seemingly obvious in such a simple program, be careful in real-world code.

Problem

Hoping I can get a little advice on a sorting method I made. The purpose of this code is to create a int pointer array and sort the pointers in that array by the contents of regular int array. Then assign values for a different variable based on the location of the original int array. The strangeness I am experiencing with this code is that the test code which shouldn't effect anything as far as I know... IS actually effecting the contents of my pointers. Perhaps the values aren't changing but the way I'm writing the test code is causing errors. ``` //create array int c[8] = {3,1,5,7,8,2,6,4}; //create pointer array int *newptr[8]; for(int k = 0; k<8; k++) { newptr[k] = &c[k]; } //sort pointer array for(int j = 0; j<8; j++) { for(; j > -1 && *newptr[j] < *newptr[j+1]; j--) { int *temp = newptr[j+1]; newptr[j+1] = newptr[j]; newptr[j] = temp; } } //set lookuplocation int lookuplocation; for(int i = 0; i<8; i++) { cout << *newptr[i]; if(newptr[i] == &c[0]) { cout << *newptr[i] << endl; //If I use endl or \n to test the pointers values I end up with only //a part of the correct data. cout << "\nSuccess!\n"; lookuplocation = 0; } } //Also for my last test sometimes the first element gets messed up as well //test arrays for(int k = 0; k<8; k++) { cout << "Element " << k << ": " << *newptr[k] << endl; cout << "Element " << k << ": " << newptr[k] << endl; } ```

Original source