Sorting an array of 2D characters using sort () in C ++

Question

I have a two-dimensional array of characters (I don't want to use the std :: string array). How to sort strings (char *) in ascending order according to string length using std :: sort ()?

I have tried the following. But that does not 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 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 ==========

Answer 1

If your data is in

char names[100][30];

then you cannot sort "pointers" because this data structure does not have pointers at all ... only 100 * 30 = 3000 characters one by one. Therefore, for sorting, you will need to actually navigate through 100 rows, all of their contents.

std::sort , , - ++ (, ).

Answer 2

std::sort , :

• ValueSwappable RandomAccessIterator.

:

• MoveAssignable MoveConstructible.

, (.. ). , std::sort .

, std::array<std::array<char, N>, M> :

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

@Jarod42

Answer 3

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

update - changed to use links to 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;
}

Answer 4

You can try the following:

#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]);
}