Creating a graph from a txt file in C

Question

I have a txt file, which contains groups of 3 elements in each line (2 strings for vertices and 1 integer for edge), which i want to use as input to create an adjacency list for a graph.

The txt file is as so: "string1 string2 34" and the elements are separated by tabs.

I've tried for starters trying to read from the file, using fgets , and managed to print the elements, but i'm stuck as to how i can parse them and store them. Any ideas?

Answer 1

A complete solution can be found at https://www.thecrazyprogrammer.com/2017/06/bellman-ford-algorithm-in-c-and-c.html

The Bellman-Ford algorithm in C is also discussed in https://stackoverflow.com/a/36831569/18980756.

Nevertheless, some code based on your input to put the elements into a dynamically allocated linked list:

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

typedef struct FORM {
    char *vertice_one;
    char *vertice_two;
    int weight;
    struct FORM *next;
} FORM;

int main() {
    int err = 0; // error indicator

    // helper to store temporarily store read values
    char s1[256], s2[256]; 
    int w;

    FORM *firstEntry = NULL; // root element for the list

    FILE *file = fopen("forms.txt", "r");
    if (file == NULL) goto cleanup;

    // read three line elements into variables
    while (fscanf(file, "%s%s%d\n", &s1[0], &s2[0], &w) == 3) {
        // print read elements for debugging purposes
        printf("read: %s-%s-%d\n", s1, s2, w);

        // dynamically allocate memory to store the read values
        FORM *newEntry = malloc(sizeof *newEntry);
        if (newEntry == NULL) {
            fprintf(stderr, "Error - Not enough memory!");
            err = 1;
            goto cleanup;
        }

        char *vert1 = malloc(strlen(&s1[0]) + 1);
        char *vert2 = malloc(strlen(&s2[0]) + 1);
        if (vert1 == NULL || vert2 == NULL) {
            fprintf(stderr, "Error - Not enough memory!");
            err = 1;
            goto cleanup;
        }
        memcpy(vert1, &s1[0], strlen(&s1[0]) + 1);
        memcpy(vert2, &s2[0], strlen(&s2[0]) + 1);

        newEntry->vertice_one = vert1;
        newEntry->vertice_two = vert2;
        newEntry->weight = w;

        // put new entry to front a linked list
        newEntry->next = firstEntry;
        firstEntry = newEntry;
    }

    FORM *p = firstEntry;
    while(p != NULL) {
        printf("%s\t%s\t%d\n", p->vertice_one, p->vertice_two, p->weight);
        p = p->next;
    }

    FORM *q;
cleanup:
    q = firstEntry;
    while(q != NULL) {
        FORM *entry = q;
        q = q->next;
        if (entry->vertice_one != NULL) free(entry->vertice_one);
        if (entry->vertice_two != NULL) free(entry->vertice_two);
        free(entry);
    }

    if (file != NULL) {
        fclose(file);
    }

    return err;
}

$ gcc -Wall -Wextra graph.c
$ ./a.out                  
read: vert1-vert2-1
read: vert3-vert4-2
vert3   vert4   2
vert1   vert2   1
$ 

Now you are in C land and can start thinking about how you want to work with the data read.

I guess

• FORM should be named EDGE
• vertex_one should be named source
• vertex_two should be named destination
• weight remains weight

You could avoid the dynamic allocation if you define arrays which are large enough to hold the data from the file.