Question

With your help I could take the input from a text file (input.txt) where lines consisted of city1 city2 distance … and write the names of cities in a matrix without repetition. According to this matrix I wrote a piece of code to add their distances in a adjacency matrix. But the output looks weird I mean, it is not correct. I guess in my code below should be something missing or wrong. Any little help is highly appreciated.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
int main(int argc, char *argv[]){
int i=1,j, state=0, k, dist,x=0,y=0;

   int** myMat;
   char *city1, *city2, **matnames;
   FILE* p;
    city1 = (char*) malloc(sizeof(char));
    city2 = (char*) malloc(sizeof(char));
    matnames = (char**) malloc(sizeof(char*));
    myMat = (int**) malloc(sizeof(int*)*4);             
p = fopen(argv[1],"r");

/************************************************************/
    matnames[0] = (char*) malloc(sizeof(char));
    matnames[1] = (char*) malloc(sizeof(char));
    matnames[2] = (char*) malloc(sizeof(char));
    matnames[2] = NULL;
    fscanf(p, "%s %s %d", city1, city2, &dist);
        strcpy(matnames[0],city1);                  
        strcpy(matnames[1],city2);
/************************************************************/  
    for(i=0;i<3;i++){
        myMat[i] = (int*) malloc(sizeof(int));
    }
    myMat[1][2] = dist;         /* the first two distances placed at matnames */
    myMat[2][1] = dist;

/************************************************************/  
while( fscanf(p,"%s %s %d",city1,city2, &dist) != EOF){                 
        for(j=0; matnames[j]!=NULL; j++){                   
            if( strcmp(matnames[j], city1) != 0){
                 state++;
                }               
        }       
        if(state  == j){
            matnames = realloc(matnames, sizeof(char*)*(j+3));
            matnames[j] = (char*) malloc(sizeof(char));
            strcpy(matnames[j], city1);
            matnames[j+1] = (char*) malloc(sizeof(char));
            matnames[j+1] = NULL;   
            }
            state = 0;
        for(j=0; strcmp(matnames[j], city1) != 0;j++){ 
                x++;                   /* "x" finds the city1 indeks from matnames*/
        }

        for(k=0; matnames[k] != NULL;k++){                  
                if( strcmp(matnames[k], city2) != 0){
                    state++;
                }               
        }
        if(state == k){
            matnames = realloc(matnames, sizeof(char*)*(k+4));
            matnames[k] = (char*) malloc(sizeof(char));
            matnames[k+1] = (char*) malloc(sizeof(char));
            strcpy(matnames[k], city2); 
            matnames[k+1] = NULL;   
            }
        state = 0;
    /* till to here the names of cities are placed in matnames without repetion*/
        for(j=0; strcmp(matnames[j], city2) != 0;j++){ 
                y++;             /* "y" finds the city2 indeks from matnames*/
        }
        /****** the problem should be in this part */
        myMat = realloc(myMat,sizeof(int*)*(k+3));          
        for(i=2;i<k+2;i++){
            myMat[i] = (int*) malloc(sizeof(int));
        }
        myMat[x][y] = dist;
        myMat[y][x] = dist;
        x=0; y=0;
}   
return 0;
}

Answer 1

you allocate 1 char to city1 and city2 (malloc with sizeof(char)). Unless your cities all have just one character, the fscanf will go out of bounds.

Try again with city1 = (char*) malloc(1024) so a city can be a bit longer.

matnames is not very obvious what you want it to be, but you allocate it 1 sizeof(pointer), i.e. 4 bytes. Then you do matnames[0] = …, matnames[1] = …
matnames[1] is already overflowing the memory you allocated. So you’re writing in random places.

The C-language is very forgiving regarding memory allocations, but the results are completely unpredictable. Be sure your malloc enough space, and consider other languages when possible. 🙂