Reading a grid from a text file and storing it within a two-dimensional array?

Question

Suppose you had a file called "input.txt" that looked like this:

A 5 x 5 grid above. And you wanted to store the 5 x 5 grid into a two-dimensional C array. My issue is reading the file into that grid.

This is my C file that reads in the data, stores it in a 2D integer array and outputs its contents

int main (int argc){

    FILE *fp;
    char ch;
    int **C; //Our 2D Array
    char filenamein[] = "input.txt";

    fp = fopen(filenamein,"r"); 

    N = (ch = fgetc(fp)) - 48; 

    //Initialize Grid, set all cells to 0
    C = malloc(N * sizeof(int *));
      for (i = 0; i < N; i++) {
             C[i] = malloc(N * sizeof(int));
      }
    for (i=0;i<N;i++) {
     for (j=0;j<N;j++) {
       C[i][j]=0;
     }
    }

   //Read array, store into array
    while ((ch = fgetc(fp) ) != EOF)
    {
    for (i=0;i<N;i++){
        for (j=0;j<N;j++){
        C[i][j] = ch - 48;
        }
    }
    }

    //Print 2D Array:

     for (i = 0; i < N; i++) {
             for (j = 0; j < N; j++)
               printf("%d ", C[i][j]);
             printf("\n");
           }

fclose(fp);
}

Upon calling this I get:

-38 -38 -38 -38 -38
-38 -38 -38 -38 -38
-38 -38 -38 -38 -38
-38 -38 -38 -38 -38
-38 -38 -38 -38 -38

The grid outputs the char-to-decimal ASCII code for Whitespace/NULL from what I deducted so my issue is reading in the text file into the array.

How can I read the text file to store the numbers into the array?

Why are you using `fgetc` instead of `fscanf` ? when you use fgetc you read whitespace character so you need to ignore them in your grid building part. — Parham Alvani, Feb 15 '15 at 22:20
There's very little to be gained by faffing about with `fgetc()` and converting ASCII values to numbers. Disk access is the bottleneck here, not character conversion. Just use `scanf()` to input the data as decimal numbers. (Also, `fgetc()` returns an integer, so `while ((ch = fgetc(fp) ) != EOF)` is liable to fail catastrophically).) — r3mainer, Feb 15 '15 at 22:20
Also there is a error in your grid initiating part, you fill all your grid with new read character every time. — Parham Alvani, Feb 15 '15 at 22:24
And not to pile on but when you subtract an ASCII value, let the compiler do the work. `C[i][j] = ch - 48;` should be `C[i][j] = ch - '0';` — indiv, Feb 15 '15 at 22:29
@ParhamAlvani If I swap fgetc with 'fscanf(ch,"%d")' and change ch to an integer, then remove the (-48) I get nothing for output — Jebathon, Feb 15 '15 at 22:31
@ParhamAlvani In regards to my error in the grid initialization I want my integer grid to have all 0's in the beginning — Jebathon, Feb 15 '15 at 22:32

score 1 · Accepted Answer · answered Feb 15 '15 at 22:35

This code work as expected. it's better to use fscanf instead of fgetc but I did not correct this in your code.

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

int main (int argc, char *argv[])
{
    FILE *fp;
    unsigned char ch;
    int **C; //Our 2D Array
    int N, i, j;
    char filenamein[] = "input.txt";

    fp = fopen(filenamein,"r");

    N = (ch = fgetc(fp)) - 48;

    //Initialize Grid, set all cells to 0
    C = malloc(N * sizeof(int *));
    for (i = 0; i < N; i++)
        C[i] = malloc(N * sizeof(int));

    for (i = 0; i < N; i++)
        for (j = 0; j < N; j++)
            C[i][j] = 0;

    i = 0;
    j = 0;
    do {
        ch = fgetc(fp);
        if (ch != ' ' && ch != '\n') {
            C[i][j] = ch - 48;
            j++;
            i += j / N;
            j %= N;
        }
    } while(i < N && j < N);

    //Print 2D Array:
    for (i = 0; i < N; i++) {
        for (j = 0; j < N; j++)
            printf("%d ", C[i][j]);
        printf("\n");
    }
    fclose(fp);
}

Reading a grid from a text file and storing it within a two-dimensional array?

1 Answers1