Thread: f scanf to dynamic arrays in C

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What is 'buffer' initialised to prior to calling malloc?

What stops 'o' from running past the end of allocated space?

Since you've already read through the whole file to determine the number of lines, you need to reset it back to the beginning to read the numbers. You can close it and re-open it, use fseek, or use the convenient rewind function. You should also use a struct to hold the information in a single array.

Code:

#include <stdio.h>
#include <stdlib.h>
 
#define INPUT_FILE "data.txt"
 
typedef struct Record
{
    int no, kod, puan, offset;
} Record;
 
int main()
{
    FILE *fin = fopen(INPUT_FILE, "r");
    if (!fin)
    {
        printf("Error: cannot open input file %s\n", INPUT_FILE);
        exit(EXIT_FAILURE);
    }
 
    int lines = 0;
    for (int a, b, c, d; fscanf(fin, "%d%d%d%d", &a, &b, &c, &d) == 4; ++lines)
        ;
 
    Record *recs = malloc(lines * sizeof *recs);
 
    rewind(fin);
 
    for (int i = 0; i < lines && fscanf(fin, "%d%d%d%d", &recs[i].no, &recs[i].kod, &recs[i].puan, &recs[i].offset) == 4; ++i)
        ;
 
    fclose(fin);
 
    for (int i = 0; i < lines; ++i)
         printf("%d %d %d %d\n", recs[i].no, recs[i].kod, recs[i].puan, recs[i].offset);
 
    free(recs);
 
    return 0;
}

No problem. Note that you didn't use the fgets followed by *scanf idiom correctly. The *scanf shouldn't have been an fscanf but an sscanf, taking input from the line you read with fgets. One good thing about this technique is that you can ensure the lines have the correct number of values, something like the following code, which will consider any of these lines as an error:
1 2 3
1 2 3 4 5
1 2 x 3 4
1 2 3 4 x

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
#define INPUT_FILE "data.txt"
 
typedef struct Record {
    int no, kod, puan, offset;
} Record;
 
int main() {
    FILE *fin = fopen(INPUT_FILE, "r");
    if (!fin) {
        printf("Error: cannot open input file %s\n", INPUT_FILE);
        exit(EXIT_FAILURE);
    }
 
    int rec_count = 0;
 
    char line[1024];
    for (int line_no = 0; fgets(line, sizeof line, fin); ++line_no) {
        if (line[strspn(line, " \t\n")] == '\0')
            continue; // skip blank lines
        // ensure line has exactly 4 integers
        int a, b, c, d;
        char ch;
        if (sscanf(line, "%d%d%d%d %c", &a, &b, &c, &d, &ch) != 4)
        {
            printf("Error: bad line #%d:\n%s\n", line_no + 1, line);
            exit(EXIT_FAILURE);
        }
        ++rec_count;
    }
 
    Record *recs = malloc(rec_count * sizeof *recs);
 
    rewind(fin);
 
    for (int i = 0;
         i < rec_count && fscanf(fin, "%d%d%d%d", &recs[i].no,
                                                  &recs[i].kod,
                                                  &recs[i].puan,
                                                  &recs[i].offset) == 4;
         ++i)
        ;
 
    fclose(fin);
 
    for (int i = 0; i < rec_count; ++i)
         printf("%d %d %d %d\n", recs[i].no,
                                 recs[i].kod,
                                 recs[i].puan,
                                 recs[i].offset);
 
    free(recs);
 
    return 0;
}