Reading a text file with data into a linked list - c

Question

I am working on a project where I have to read the .txt file where I have data of four elements for each component, ex:
5 2 7 0.99 (which are the components id, inside node, outside node and reliability)
7 3 5 0.95
...
I want to read and write data into a linked list, over which I will later on be able to search for values and sign them to a new linked list. It's about a method of minimal paths in mechanical engineering used for calculating reliability of systems.
To test the code I just want to print out all the components that were put into the linked list. I get the right value for number of lines, but for components I just get out one random not all of them. Any kind of help will be much appreciated :)
Here is the code:
#include <stdlib.h>
#include <stdio.h>
struct Components_element {
int id;
int in_node;
int out_node;
float reliability;
struct Components_element *next_c;
};
struct Components_element *head_c = NULL;
int count_lines(char *filename)
{
int counter = 0;
char c;
FILE *ptr_sistem;
ptr_sistem = fopen(filename, "r");
if(ptr_sistem == NULL)
return 0;
while((c = fgetc(ptr_sistem)) != EOF)
if(c == '\n')
counter++;
fclose(ptr_sistem);
if(c != '\n')
counter++;
return counter;
}
struct Components_element *add_comp(struct Components_element *head_c, int id, int in_node, int out_node, float reliability)
{
struct Components_element *new;
struct Components_element *tail_c;
new = (struct Components_element*) malloc(sizeof(struct Components_element));
new->id = id;
new->in_node = in_node;
new->out_node = out_node;
new->reliability = reliability;
if(head_c == NULL)
return(new);
tail_c = head_c;
while(tail_c->next_c != NULL)
tail_c = tail_c->next_c;
tail_c->next_c = new;
return(head_c);
}
void write_out(struct Components_element *p)
{
while(p != NULL) {
printf("%d %d %d %f", p->id, p->in_node, p->out_node, p->reliability);
p = p->next_c;
}
printf("\n");
}
struct Components_element *empty(struct Components_element *p)
{
struct Components_element *tail_c;
while(p != NULL) {
tail_c = p;
p = p->next_c;
free(tail_c);
}
return(p);
}
main()
{
int i, id, in_node, out_node;
int n_lines;
float reliability;
struct Components_element *components;
FILE *ptr_file;
ptr_file = fopen("system.txt", "r");
if(ptr_file == NULL) {
printf("Cannot open file.\n");
return 0;
} else {
n_lines = count_lines("system.txt");
for(i = 0; i < n_lines; i++) {
fscanf(ptr_file, "%d %d %d %f", &id, &in_node, &out_node, &reliability);
components = add_comp(head_c, id, in_node, out_node, reliability);
++i;
}
}
printf("Number of lines: %d.\n", n_lines);
write_out(components);
empty(components);
fclose(ptr_file);
}

Answer 1

Writing the code is only a small fraction of what programming is about. The really hard part, is getting it to work as required.
To do this, we use various tools to aid with testing the code to make sure it works the way the programmer intended.
The most useful tool is the debugger. Learn how to use one and you can find your problem in a couple of minutes.
Sometimes, however, debuggers aren't always available and this is where things can get quite tricky and requires logging information (printf / fwrite / etc) and a lot of deductive reasoning.
But here, running the code through a debugger will show you your problem. I'll leave it as an exercise for you as you'd learn a lot more that way rather than having the answer spoon fed to you.