Question

here is code which prints longest string duplicated M times

#include <iostream>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>

using namespace std;
#define M 1
#define  MAXN  5000000
char c[MAXN],*a[MAXN];
int pstrcmp( char **p,char **q){  return strcmp(*p,*q) ;}
int comlen(char *p,char *q){

    int i=0;
     while  (*p &&(*p++==*q++)){
         i++;
     }


      return i;


}
int main(){

    int maxlen=-1;
     int maxi; 
      int ch,n=0; 
       while ((ch=getchar())!=EOF){
           a[n]=&c[n];
            c[n++]=ch;
       }
       c[n]=0;
       qsort(a,n,sizeof( char *),pstrcmp);
         for (int i=0;i<n-M;i++) 
               if (comlen(a[i],a[i+M])>maxlen){
                    maxlen=comlen(a[i],a[i+M]);
                    maxi=i;
               }
               printf("%.*s\n",maxlen,a[maxi]);



     return 0;
}

but here is mistake

1>c:\users\david\documents\visual studio 2010\projects\longest_repeted\longest_repeated.cpp(33): error C2664: 'qsort' : cannot convert parameter 4 from 'int (__cdecl *)(char **,char **)' to 'int (__cdecl *)(const void *,const void *)'
1>          None of the functions with this name in scope match the target type

Answer 1

Change

int pstrcmp( char **p,char **q){  return strcmp(*p,*q) ;}

to

int pstrcmp(const void *p, const void *q)
{  
  return strcmp(*reinterpret_cast<const char**>(p), *reinterpret_cast<const char**>(q));
}