字符串数组打印错误的值

Question

I trying to make a program that calculates proper prefixes and proper suffixes , and then compare the sets , and then return the array containing values representing number of matched pairs , This could be used later in KMP algorithm . But the problem is prefixes and suffixes array give wrong values . Even after appending a new element at new index, it replaces the all value in the array with the new element .

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

int* lps(char *,int );

int main()
{

char *pat = "abababca";

int *ptr ;
ptr = lps(pat,strlen(pat));

printf("\n***LPS***\n");


}

int * lps (char *p,int s)
{


char *prefixes[s] ;
char *suffixes[s] ;
char tmp1[s] , tmp2[s];
int i , j , k , c1 , c2 , c;

for (i = 0 ; i < s ; i ++)
{
    printf("\n\n*** --- Creating Prefixes and Suffixes for i = %d --- ***",i);

    c1 = 0 ;
    //create prefixes
    for (j = 0 ; j < i; j++)
    {
        for (k =0 ; k <= j; k++)
        {
            tmp1[k]=*(p+k);
            printf("\n *(p+%d)= %c , tmp1[%d]=%c",k,*(p+k),k,tmp1[k]);
        }

        tmp1[k]='\0';
        printf("\nprefixes[0]:%s",prefixes[0]);
        prefixes[c1] = tmp1;
        //strcpy(prefixes[c1], tmp1);

        printf("\ncurrently added %s to prefixes at %d and prefixes[%d]= %s\n ",tmp1,c1,c1,prefixes[c1]);
        c1++;
    }

    //print prefixes
    for (k = 0; k<c1; k++)
    {
        printf("\tprefixes[%d] = %s",k,prefixes[k]);
    }
    printf("\n");



    //create suffixes
    c2 = 0;
    for (j = 1 ; j <= i; j++)
    {
        for (k = j ; k  <= i; k++)
        {
            tmp2[k-j] = *((p+k));
            printf("\n *(p+%d)= %c , tmp2[%d]=%c",k,*(p+k),k-j,tmp2[k-j]);
        }

         tmp2[k-j]='\0';
         suffixes[c2] = tmp2 ;
        // strcpy(suffixes[c2], tmp2);

         printf("\ncurrently added %s to suffixes at %d and suffixes[%d]= %s\n",tmp2,c2,c2,suffixes[c2]);
        c2++;
    }

     //prinf suffixes
    for (k = 0; k<c2; k++)
    {
        printf("\tsuffixes[%d] = %s",k,suffixes[k]);
    }
    printf("\n");
    //compare the prefixes and suffixes

    c = 0 ;
    for (j = 0; j < c1; j++)
    {
        for(k=0 ; k < c2 ; k++)
        {
            printf("\nprefixes[%d] = %s , suffixes[%d] = %s\n ",j,prefixes[j],k,suffixes[k]);

            if (strcmp(prefixes[j], suffixes[k])==0)
            {
                c = c + 1 ;
            }
        }
    }

  }
  }

OUTPUT (Some part of OUTPUT) :-

prefixes[0] = ab    prefixes[1] = ab   //it should be prefixes[0] = a   prefixes[1] = ab

Answer 1

The problem is that you aren't allocating any strings. The only strings that you have in lps are tmp1 and tmp2 . You then make assignments like this:

prefixes[c1] = tmp1;

That assigns a pointer but does not copy the contents of the string. You'll end up with every entry in prefixes pointing to the same string, tmp1 . And likewise for suffixes .

You'll need to use malloc and strcpy to make new string instances.

In the code you have commented out calls to strcpy . I suspect you tried these and encountered runtime errors. Those runtime errors were because you did not allocate any memory. The corrected code would look like:

prefixes[c1] = malloc(strlen(tmp1)+1);
strcpy(prefixes[c1], tmp1);

And similarly for suffixes .

In production quality code you'd include error checking. And you'd want to make sure that you call free() on any pointer returned by the calls malloc() , once you had finished with it.

I'd also question the use of C variable length arrays, VLAs. In your code, prefixes , suffixes , tmp1 and tmp2 are all VLAs. Using VLAs can readily lead to stack overflow if you have large array dimensions. My instincts say that heap allocation is what you need here.