parse ints from char array

Question

I'm trying to read in a graph with space separated edges. Having an issue parsing from the char* buffer to the ints.

sample graph:

1   2 
1   3
2   3
3   4
4   5
1   5
5   6 
1   6 

output:

first edge: 1
second edge: 2
first edge: 0
second edge: 0
first edge: 1
second edge: 3
first edge: 0
second edge: 0
first edge: 2
second edge: 3
first edge: 0
second edge: 0
first edge: 3
second edge: 4
first edge: 0
second edge: 0
first edge: 4
second edge: 5
first edge: 0
second edge: 0
first edge: 1
second edge: 5
first edge: 0
second edge: 0
first edge: 5
second edge: 6
first edge: 0
second edge: 0
first edge: 0
second edge: 0
first edge: 1
second edge: 6
first edge: 0
second edge: 0
first edge: 0
second edge: 0

obviously all those zeroes shouldn't be there.

code:

#include <stdio.h>
#include <math.h>
#include <ctype.h>

int main(int argc, char* argv[]){   

    FILE *fin; 
    fin = fopen(argv[1], "r");
    int n_e = 0;
    char* buffer = (char*)malloc(2048);
    char* n1 = (char*)malloc(256); 
    char* n2 = (char*)malloc(256);
    int v_1 = 0; 
    int v_2 = 0;
    int flag = 0; 

    while(!feof(fin)){
        int bytes_read = fread(buffer, 1, 2048, fin);
        for(int i = 0; i < bytes_read; i++){
            if(isdigit(buffer[i])){
                if(flag == 0){ 
                    n1[v_1++] = buffer[i];
                }
                else 
                    n2[v_2++] = buffer[i]; 
            }
            else if(buffer[i] == ' ')
                flag = 1; 
            else{ //end of the line??
                n_e++;
                flag = 0; 
                n1[v_1++] = '\0';
                n2[v_2++] = '\0';
                int first = atoi(n1);
                int second = atoi(n2);
                printf("first edge: %d\n", first);
                printf("second edge: %d\n", second);
                v_1 = 0;
                v_2 = 0;
            }
        }
    }

return 0;
}

Answer 1

You can achieve the nub of what you want in a single line: the fscanf line. The rest is devoted to the framework and error checking - that's an essential part of any program. Given your graph edges as a text file:

#include <stdio.h>

int main(int argc, char *argv[]) {
    FILE *fin; 
    int first, second;
    if(argc < 2) {                                      // check # arguments
        return 0;                                       // or other action
    }
    fin = fopen(argv[1], "rt");
    if(fin == NULL) {                                   // check the file opened
        return 0;                                       // or other action
    }
    while(fscanf(fin, "%d%d", &first, &second) == 2) {  // check the number of items read
        printf("first edge: %d, second edge: %d\n", first, second);
    }
    fclose(fin);
    return 0;
}

Program output:

first edge: 1, second edge: 2
first edge: 1, second edge: 3
first edge: 2, second edge: 3
first edge: 3, second edge: 4
first edge: 4, second edge: 5
first edge: 1, second edge: 5
first edge: 5, second edge: 6
first edge: 1, second edge: 6

Note too, that feof is not used the way you did, as commented above.

Answer 2

You can check the value returned by sscanf() function in order to determine if the sample was parsed successfully from the line.

I rewrote you code to demonstrate a more efficient way to solve your problem:

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


#define MAX_LINE_LEN            (32)
#define MAX_EDGES_SAMPLES       (256)


/* Represents an Edge Object */
typedef struct edge_s
{
    int first;
    int second;
} edge_t;


int main( int argc, char * argv[] )
{
    FILE * pf = NULL;
    char line[ MAX_LINE_LEN + 1 ] = {0};
    int count = 0;
    int i = 0;

    edge_t edges[ MAX_EDGES_SAMPLES ]; /* Edges array */


    /* Initialize Edges array */
    memset( &edges, 0, sizeof( edges ) );

    /* Open input text file for reading */
    pf = fopen( argv[1], "r" );

    if(!pf)
    {
        printf("Error openning file: %s\n", argv[1] );
        return 1;
    }

    /* For each line... */
    while( fgets( line, MAX_LINE_LEN, pf ) )
    {
        edge_t aux;

        /* Parse Line */
        int ret = sscanf( line,  "%d %d", &aux.first, &aux.second );

        /* Ignore malformed lines */
        if( ret != 2 )
            continue;

        /* Add values to the Edges array */
        edges[count].first = aux.first;
        edges[count].second = aux.second;

        /* Increment Edges Count */
        count++;

        if( count >= MAX_EDGES_SAMPLES )
            break;
    }

    /* Free resource */
    fclose(pf);

    /* Display Edges array data */
    for( i = 0; i < count; i++ )
        printf("Edge[%d]: first=%d second=%d\n", i, edges[i].first, edges[i].second );

    return 0;
}

/* eof */

Hope it Helps!