How to store integer values from a file into a 2D array using Malloc function-CodePudding

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

int main() {
    int row = 0, colu = 3, j, i;
    FILE *fp;
    fp = fopen("file.txt", "r");
    fscanf(fp, "%d", &row);
    int *arr = (int *)malloc(row * colu * sizeof(int));
  
    for (i = 0; i < row; i  ) {
        for (j = 0; j < col; j  ) {
            *(arr   i * col   j) = i   j;
            printf("%d ", *(arr   i * col   j));
        }

        printf("\n");
    }
}

So I was able to make a 2D array using malloc but now I want to get information from a file and store it into my 2D array.

The File:

4
34.1  54.2  54
23.4  19.5  53.2
43.2  54.2  54.2 
53.2  62.4  27.5

and so on......

In the first line is the number of rows I will use. I already figured out how to do that. But now I want to store the numbers below "4" into a 2D array. How could I do this. Sorry if I did not explain this too well, I'm new to coding.

CodePudding user response：

... to store the numbers below "4" into a 2D array.

Add a fscanf() to store into the 1D array pointed to by arr.

Consider adding error checking too.

  if (fscanf(fp, "%d ", arr   i * col   j) != 1) {
    fprintf(stderr, "Failed to read.\n");
    exit(-1);
  }
  printf("%d ", *(arr   i * col   j));

I was able to make a 2D array using malloc

Code did not make allocate a 2D array. Just a 1D array.
More worked needed for a 2D array.

CodePudding user response：

Function fscanf takes an address of the object to which it writes scanned data. It returns a number of successfully scanned elements whiat is going to be 1 in your case. That it why typical usage of scanf-like function is following.

int data;
if (scanf("%d", &data) != 1)
   ... error handling ...

... use data ...

Actually, you don't use 2D array in your code but rather emulate it using 1D array. To create a true dynamic 2D array you should use a pointer to a Variable-Length Array (VLA). Below you can find a updated version enhanced with error handling which is strongly recommended when working with files.

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

int main() {
    int rows = 0, cols = 3, j, i;
    FILE *fp;
    fp = fopen("file.txt", "r");
    if (!fp) {
         fprintf(stderr, "failed to open file.txt");
         exit(-1);
    }
    if (fscanf(fp, "%d", &rows) != 1) {
         fprintf(stderr, "failed to read number of rows");
         exit(-1);
    }
    // allocate memory for `row` elements of type `*arr` which is `int[cols]`
    int (*arr)[cols] = calloc(rows, sizeof *arr);
    if (!arr) {
         fprintf(stderr, "out of memory");
         exit(-1);
    }
  
    for (i = 0; i < rows; i  ) {
    for (j = 0; j < cols; j  ) {
        if (fscanf(fp, "%d", &arr[i][j]) != 1) {
           fprintf(stderr, "failed to parse entry at [%d,%d]", i, j);
           exit(-1);
        }
    }}

    for (i = 0; i < rows; i  ) {
        for (j = 0; j < cols; j  )
            printf("%d ", arr[i][j]);
        puts(""); // newline
    }
    free(arr);
}