Question

I’m doing my final project for my algorithms course in C. For the project, we have to take an input text file that contains lines like:

P|A|0

or

E|0|1|2

The former indicates a vertex to be added to the graph we’re using in the program, the 2nd token being the name of the vertex, and the last token being its index in the vertices[] array of the graph struct.

I’ve got a while loop going through this program line by line, it takes the first token to decide whether to make a vertex or an edge, and then proceeds accordingly.

When I finish the file traversal, I call my show_vertices function, which is just a for-loop that prints each name (g->vertices[i].name) sequentially.

The problem is that where the name should go in the output (%s), I keep getting the last “token1” I collected. In the case of the particular input file I’m using it happens to be the source node of the last edge in the list…which is odd because there are two other values passed through the strtok() function afterward. The line in the file looks like:

E|6|7|1

which creates an edge from indexes 6 to 7 with a weight of 1. The edge comes up fine. But when I call any printf with a %s, it comes up “6”. Regardless.

This is the file traversal.

fgets(currLn, sizeof(currLn), infile);
maxv = atoi(currLn);
if(maxv = 0)
{
    //file not formatted correctly, print error message
    return;
}

t_graph *g = new_graph(maxv, TRUE);

while((fgets(currLn, sizeof(currLn), infile)) != NULL)
{
    token1 = strtok(currLn, "|");
    key = token1[0];

    if(key == 'P' || key == 'p')
    {
        token1 = strtok(NULL, "|");
        if(!add_vertex(g, token1))
        {
            //file integration fail, throw error!
            return;
        }
        //***If I print the name here, it works fine and gives me the right name!****
        continue;
    }
    if(key == 'E' || key == 'e')
    {
        token1 = strtok(NULL, "|");
        token2 = strtok(NULL, "|");
        token3 = strtok(NULL, "|");
        src = atoi(token1);
        dst = atoi(token2);
        w = atoi(token3);

        if(!add_edge(g, src, dst, w))
        {
            //file integration fail, throw error
            return;
        }
        continue;
    }
    else
    {
        //epic error message because user doesn't know what they're doing.
        return;
    }
}

If I run show_vertices here, I get:

0. 6  
1. 6  
2. 6  
etc...

Answer 1

You aren’t copying the name. So you end up with a pointer (returned by strtok) to single static array in which you read each line. Since the name is always at offset 2, it that pointer will always be currLn+2. When you traverse and print, that will be the last name you read.

You need to strdup(token1) before passing it to (or in) add_vertex.

No there isn’t enough information to be certain this is the answer. But I’ll bet money this is it.