Sorting array of strings in cpp using std:sort

Question

I am trying to sort arrays (yes, not vectors) of strings, following some suggestions from sites such as this one. This is the code I wrote so far, but it always prints out this long error (which has no results on google). How can this be fixed?

#include <algorithm>
#include <cstring>
#include <iostream>
//qsort(names, n, 15, (int (*)(const void *, const void *))strcmp);



int main()
{
    std::cout << "hi";
    char arr[3][6];
    strcpy(arr[0], "hello"), strcpy(arr[1], "hillo"), strcpy(arr[2], "hallo");
    std::sort(arr, arr + 3, [](char const *lhs,
                           char const *rhs) { return strcmp(lhs, rhs) < 0; });
    //qsort(arr, 3, 20, (int (*)(const void *, const void *))strcmp);
    for (int i = 0; i < 3; ++i)
        std::cout << arr[i] << '\n';
}

Consider the element type of the array you're sorting. Arrays are not assignable, and therefore the stock swap-mechanics for exchanging rows in your array of arrays (each element is, in fact, an array of char) isn't going to work. — WhozCraig
– WhozCraig, Commented Sep 27, 2020 at 10:12
@WhozCraig what you're trying to say is that I have an array of arrays (strings are arrays) and since arrays are not assignable we can't swap items which is what "sort" does? then how do people on other SO questions manage to do it? — Ariel Yael
– Ariel Yael, Commented Sep 27, 2020 at 10:17
Pretty much. Throw out your decl of arr, and the strcpy calls, and replace them with const char *arr[] = { "hello", "hillo", "hallo" }; and try again. Then consider what changed and why it now works. — WhozCraig
– WhozCraig, Commented Sep 27, 2020 at 10:18
@WhozCraig will using the type "string" work here? (instead of doing a 2d array doing a 1d array with each element having string type) — Ariel Yael
– Ariel Yael, Commented Sep 27, 2020 at 10:18

eerorika · Accepted Answer · 2020-09-27 10:29:08Z

1

This is the relevant part of the error message:

error: array must be initialized with a brace-enclosed initializer

Which is not very clear unfortunately, but ot comes from within the sorting implementation. So, let us revise the preconditions of the algorithm:

std::sort is implemented by swapping elements. As such, it requires that the elements are swappable. The elements of your array are arrays. Arrays are not swappable. Therefore an array of arrays is not sortable.

will using the type "string" work here?

Using std::string will work. It is swappable. You won't even need a custom comparison function because it is even comparable.

what makes arrays unswappable and other types swappable

Swap is inplemented by move initialisation (and assignment). Language rules say that array is not move initialisable (nor is it assignable). Knowing this, see if the error messsge makes some sense.

edited Sep 27, 2020 at 10:29

answered Sep 27, 2020 at 10:18

eerorika

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2 Comments

Ariel Yael Over a year ago

Thank you. what makes arrays unswappable and other types swappable?

Ariel Yael Over a year ago

Very interesting, I will continue exploring the subject for a deeper understanding. thank you. I will mark your answer as correct and will metion that the comments of @WhozCraig are very explanatory.

T A · Accepted Answer · 2021-01-07 20:19:28Z

0

You cannot sort array of arrays using sort inbuilt function. You can traverse through the array of strings and sort each string(str) using sort(str.begin(), str.end()).

edited Jan 7, 2021 at 20:19

T A

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answered Jan 7, 2021 at 11:46

Pavan Kumar R

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