File to dynamic array in c

Question

DISCLAIMER: I'm new to C.

What is the best way to convert every line in a .txt file (can be other file types too) to a dinamic calloc() array, and the other way around?

In my file I have to following:

1 Hello
2 18
3 World
4 15
etc...

I want something like this in the array:

[0] Hello
[1] 18
[2] World
[3] 15
etc...

The code i have now:

FILE *file;
file = fopen("test.txt", "r");
int i = 0;

//make dynamic calloc array
//while line!= EOF
    //put line[i] into array
    //i++
    //realloc array, size +1

fclose(file);

Is this way of doing this a good one or is there a better one? If someone could help me a little bit with the code I would be really thankful.

Answer 1

You are close to the right solution, but here you are reallocing the dynamic array every time there is a new line, what you could do, is to allocate N byte in advance in the array and realloc each time with this size, this will avoid frequent memory movement of the array and sys call:

FILE *file;
file = fopen("test.txt", "r");
int i = 0;

int max = 256;
int resize = 512;
char **arr = malloc(max * sizeof(*arr));


//make dynamic calloc array
//while line!= EOF
    //put line[i] into array
    //i++

    if(i == max)
      realloc array, size + reisze;

fclose(file);

Answer 2

i would get the number of lines first and then allocate memory for an array to avoid realloc() call (possibly allocates a new memory block and copies memory area). but i'm not sure that this way is more efficient. here's my example code:

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

static int get_nlines(FILE* fp)
{
    int nlines = 0;

    int ch;
    while (!feof(fp)) {
        ch = fgetc(fp);
        if (ch == '\n') {
            nlines++;
        }
    }

    fseek(fp, 0, SEEK_SET);
    return nlines;
}

static char* get_value(char* s)
{
    char* pch;
    pch = strtok(s, " ");
    if (pch != NULL) {
        pch = strtok(NULL, " ");
        if (pch != NULL) {
            return strdup(pch); // need to free() after use
        } else {
            return NULL;
        }
    } else {
        return NULL;
    }
}

int main()
{
    FILE* fp = fopen("test.txt", "r");
    if (fp != NULL) {
        int nlines = get_nlines(fp);

        printf("nlines: %d\n", nlines);

        // make values..
        char** values = calloc(nlines + 1, sizeof(char*)); // +1 for a sentinel
        if (values != NULL) {
            char line[1024];
            int idx = 0;
            while (fgets(line, sizeof(line), fp) != NULL) {
                values[idx] = get_value(line);
                idx++;
            }
        }

        // use values..
        char** p = &values[0];
        while (*p != NULL) {
            printf("%s\n", *p);
            p++;
        }

        // clean values..
        p = &values[0];
        while (*p != NULL) {
            free(*p);
            p++;
        }

        fclose(fp);
    } else {
        perror("test.txt");
    }
}

the result:

$ ./a.out
nlines: 4
Hello

18

World

15