Sort a 2D character Array using sort() in C++

Question

I have a 2D character array (I don't want to use array of std::string). How can I sort the strings (char*) in ascending order according to the length of the string using std::sort()?

I have tried the following. But it doesn't work.

char names[100][30];

bool comp(const char* a, const char* b){
    return strlen(a)<strlen(b);
}

int main(){
    ...
    //I want to sort the first n strings 
    sort(names,names+n,comp); //n<=100
    ...
}

I have found these errors:

1>e:\program files (x86) in e\microsoft visual studio 9.0\vc\include\algorithm(3128) : error C2075: '_Val' : array initialization needs curly braces
1>        e:\program files (x86) in e\microsoft visual studio 9.0\vc\include\algorithm(3150) : see reference to function template instantiation 'void std::_Insertion_sort1<_BidIt,bool(__cdecl *)(const char *,const char *),char[30]>(_BidIt,_BidIt,_Pr,_Ty (*))' being compiled
1>        with
1>        [
1>            _BidIt=char (*)[30],
1>            _Pr=bool (__cdecl *)(const char *,const char *),
1>            _Ty=char [30]
1>        ]
1>        e:\program files (x86) in e\microsoft visual studio 9.0\vc\include\algorithm(3270) : see reference to function template instantiation 'void std::_Insertion_sort<_RanIt,bool(__cdecl *)(const char *,const char *)>(_BidIt,_BidIt,_Pr)' being compiled
1>        with
1>        [
1>            _RanIt=char (*)[30],
1>            _BidIt=char (*)[30],
1>            _Pr=bool (__cdecl *)(const char *,const char *)
1>        ]
1>        e:\program files (x86) in e\microsoft visual studio 9.0\vc\include\algorithm(3279) : see reference to function template instantiation 'void std::_Sort<char(*)[30],int,bool(__cdecl *)(const char *,const char *)>(_RanIt,_RanIt,_Diff,_Pr)' being compiled
1>        with
1>        [
1>            _RanIt=char (*)[30],
1>            _Diff=int,
1>            _Pr=bool (__cdecl *)(const char *,const char *)
1>        ]
1>        e:\projects visual studio2008\sample\sample\sorting.cpp(51) : see reference to function template instantiation 'void std::sort<char(*)[30],bool(__cdecl *)(const char *,const char *)>(_RanIt,_RanIt,_Pr)' being compiled
1>        with
1>        [
1>            _RanIt=char (*)[30],
1>            _Pr=bool (__cdecl *)(const char *,const char *)
1>        ]
1>e:\program files (x86) in e\microsoft visual studio 9.0\vc\include\algorithm(3133) : error C2106: '=' : left operand must be l-value
1>e:\program files (x86) in e\microsoft visual studio 9.0\vc\include\algorithm(3140) : error C2106: '=' : left operand must be l-value
1>e:\program files (x86) in e\microsoft visual studio 9.0\vc\include\algorithm(3141) : error C2106: '=' : left operand must be l-value
1>Build log was saved at "file://e:\projects visual studio2008\sample\sample\Debug\BuildLog.htm"
1>sample - 4 error(s), 3 warning(s)
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========

Doesn't work isn't an error destription. That said, use std::vector or std::array and std::string, it would work then. — Ulrich Eckhardt
– Ulrich Eckhardt, Commented Oct 22, 2015 at 6:23
@UlrichEckhardt Unfortunately there isn't an equivalent of a 2D array using either of those. Of course one can always write a 2D array class with a single array/vector/whatever as storage. — juanchopanza
– juanchopanza, Commented Oct 22, 2015 at 6:27
FRR: You don't even mention an error message let alone provide a minimal example. @juanchopanza: That's just a 1-D array of fixed-length strings, if I understand it correctly given the limited info here. — Ulrich Eckhardt
– Ulrich Eckhardt, Commented Oct 22, 2015 at 6:29
sort will try to swap a and b using the result of comp. Not going to work well with char[30]. — Bo Persson
– Bo Persson, Commented Oct 22, 2015 at 6:35
The elements of names are not pointers (char*), they are arrays (char[30]). char* names[100] would work. — molbdnilo
– molbdnilo, Commented Oct 22, 2015 at 6:43

6502 · Accepted Answer · 2015-10-22 06:45:06Z

5

If your data is in

char names[100][30];

then you cannot sort "the pointers" because that data structure has no pointers at all... just 100*30 = 3000 characters one after another. Therefore to do the sorting you will need to actually move around the 100 rows will all their content.

std::sort cannot be used directly because the data structure is an array of arrays, and arrays are in C++ second-class citizens (for example you cannot assign an array to another).

answered Oct 22, 2015 at 6:45

6502

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Community · Accepted Answer · 2017-05-23 12:31:42Z

2

std::sort requires that its iterator type parameter must be:

ValueSwappable and RandomAccessIterator.

The type of dereferenced iterator type must meet the requirements of:

MoveAssignable and MoveConstructible.

Unfortunately, arrays are not swappable (i.e., you cannot assign one to the other). Consequently, you can't use std::sort with arrays.

What you can do is use std::array<std::array<char, N>, M> in the following manner:

template<std::size_t N, std::size_t M>
void custom_sort(std::array<std::array<char, M>, N> &arrs) {
  std::sort(std::begin(arrs), std::end(arrs), [](auto const &a, auto const &b){ return strnlen(a.data(), M) < strnlen(b.data(), M); });
}

LIVE DEMO

Impovements to code thanks to @Jarod42

edited May 23, 2017 at 12:31

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answered Oct 22, 2015 at 6:51

Dimitrios Bouzas

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3 Comments

Jarod42 Over a year ago

strnlen(a.data(), M) < strnlen(b.data(), M) to avoid possible out of bound accesses.

Jarod42 Over a year ago

And missing reference to avoid unneeded copy.

Dimitrios Bouzas Over a year ago

@Jarod42 Thanks I wasn't even aware of strnlen :)

rcgldr · Accepted Answer · 2015-10-22 10:46:55Z

1

As already mentioned, arrays can't be assigned. Structures can be assigned, so this might be close to what you want. The array of structures might be padded for alignment. In the case of Visual Studio 2015, the array of structures was not padded, so the memory layout was the same as a 2d array.

update - changed to using references for compare parameters and switched to strnlen as suggested by Jarod42.

#include <algorithm>
#include <cstring>

using namespace std;

typedef struct
{
    char name[30];
}name;

name names[4] = { { "12345678" },{ "123" },{ "12345" },{ "12" } };

bool comp(const name &a, const name &b)
{
    return strnlen(a.name,30) < strnlen(b.name,30);
}

int main(){
    sort(names, names+4, comp);
    return 0;
}

edited Oct 22, 2015 at 10:46

answered Oct 22, 2015 at 6:56

rcgldr

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Sakib Ahammed · Accepted Answer · 2015-10-22 06:50:57Z

0

You can try this:

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#define M 10000

int main()
{
    char names[M][15];
    int n, i;
    scanf("%d", &n);
    for (i = 0; i < n; i++)
        scanf("%s", names[i]);
    qsort(names, n, 15, (int (*)(const void *, const void *))strcmp);

    for(i = 0; i < n; i++)
        printf("%s\n", names[i]);
}

answered Oct 22, 2015 at 6:50

Sakib Ahammed

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