Not able to print this 2D array (weird output) in C

Question

I am trying to read a text file with 100 numbers like 1 2 45 55 100 text file here (all on a single line) and then put them in a 10x10 array (2D array).

736.2 731.6 829.8 875.8 568.3 292.2 231.1 868.9 66.7 811.9 292.0 967.6 419.3 578.1 322.5 471.7 980.0 378.8 784.1 116.8 900.4 355.3 645.7 603.6 409.1 652.1 144.1 590.6 953.1 954.0 502.0 689.3 685.6 331.9 565.1 253.9 624.1 796.2 122.8 690.7 608.0 414.8 658.3 27.3 992.9 980.8 499.0 972.8 359.7 283.1 89.7 260.1 638.4 735.4 863.6 47.5 387.5 7.7 638.1 340.6 961.7 140.1 29.8 647.3 471.9 594.9 901.2 96.0 391.1 24.0 786.7 999.1 438.7 445.0 26.4 431.6 425.9 525.4 404.4 785.6 808.5 494.1 45.7 447.0 229.5 909.3 494.4 617.0 917.0 132.5 957.5 878.8 272.6 987.4 526.1 744.5 582.3 427.3 840.5 973.3

Here is my code:

#include <stdio.h>

#define NR 10
#define NC 10

int main(void) {
    int numbers[9][9];
    int i = 0;
    int count;
    int j = 0;
    FILE *file;

    file = fopen("numbers.txt", "r");
    for (count = 1; count < 101; count++) {   
        fscanf(file, "%d", &numbers[i][j]);
        j++;
        if ((count != 1) && (count % 10 == 0)) {
            i++;
            j = 0;
        }
    }
      
    fclose(file);
    
    int p = 0;
    int q = 0;
    for (p = 0; p < NR; p++) {
        for (q = 0; q < NC; q++) {
            printf("%d", numbers[p][q]);
        }
        printf("\n");
    }
    return 0;
}

Answer 1

There are multiple issues in your code:

• the matrix is too small, make it numbers[NR][NC].
• you do not check for fopen failure: you will have undefined behavior if the file numbers.txt is not in the current directory or cannot be open for reading.
• you read the file contents as integers, but the file contains floating point numbers with a . decimal separator: the second and subsequent fscanf() will get stuck on the . and keep returning 0 without modifying the destination number, leaving the matrix mostly uninitialized. Make the matrix double numbers[NR][NC], read the numbers with %lf and test for conversion failure.
• the counting method in the reading loop is weird. Just use 2 nested for loops with proper counter and tests.
• printing the matrix contents, you should output at least a space between numbers so the output is readable.

Here is a modified version:

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

#define NR 10
#define NC 10

int main() {
    double numbers[NR][NC];
    FILE *file;

    file = fopen("numbers.txt", "r");
    if (file == NULL) {
        fprintf(stderr, "cannot open numbers.txt: %s\n", strerror(errno));
        return 1;
    }
    for (int i = 0; i < NR; i++) {   
        for (int j = 0; j < NC; j++) {   
            if (fscanf(file, "%lf", &numbers[i][j]) != 1) {
                fprintf(stderr, "error reading number at row %d, col %d\n",
                        i + 1, j + 1);
                fclose(file);
                return 1;
            }
        }
    }
    fclose(file);
    
    for (int p = 0; p < NR; p++) {
        for (int q = 0; q < NC; q++) {
            printf(" %5g", numbers[p][q]);
        }
        printf("\n");
    }
    return 0;
}

Answer 2

As SparKot noted in a comment, to read a 10x10 matrix, you need to define the matrix with 10x10 elements:

int numbers[10][10];

That has to be one of the weirder ways of reading a 10x10 matrix that I've ever seen. Why not go for a simple approach of nested loops. Since the data contains floating-point numbers, you need to read them as double (or perhaps float) values.

for (int i = 0; i < 10; i++)
{
    for (int j = 0; j < 10; j++)
    {
        double double_val;
        if (fscanf(file, "%lf", &double_val) != 1)
        {
            fprintf(stderr, "failed to read matrix[i][j]\n", i, j);
            exit(EXIT_FAILURE);
        }
        numbers[i][j] = double_val;
    }
}

The mess with double_val works around the data containing floating point numbers and your original code trying to read integers. You'll get one valid value; thereafter, fscanf() will return 0 because the . is not a part of a valid integer. This highlights the importance of checking the return value from fscanf() and its relatives.

Frankly, you should be using double numbers[10][10]; for the data from the file. Then you could read directly into the array:

if (fscanf("%lf", &numbers[i][j]) != 1)

But you'd need to check (and probably change) all the rest of the code too.

Answer 3

Clear all a common condition that causes programs to crash; they are often associated with a file named core. code is showing segmentation fault.