将文本文件读入 C 中的二维数组

Question

I have a text file that looks like the following.

5
2   7
3   4   2

Basically, row 1 has 1 element, row 2 has 2 elements, row 3 has 3 elements, ... up to n rows with n elements.

I need to read them, and put into a 2-d array in this fashion:

a[0][0] = 5
a[1][0] = 2
a[1][1] = 7
a[2][0] = 3
a[2][1] = 4
a[2][2] = 2
...

You get the drift.

However I know n (the number of rows) only at run time. How would I go about reading the file into a 2x2 array?

I know I can do the following if I know the number of elements in each line of text, and if that number is the same for each line.

fa = fopen("file.txt", "r");
for(i = 0; i < n; i++) {
    fgets(str, 1000, fa);
    sscanf(str, "%d %d %d", &a[i][0], &a[i][1], &a[i][2]);
}
fclose(fa);

But it wouldn't help me here as I have varying number of elements in each row.
How do I go about this?

Answer 1

Allocate memory for the array like this:

int** array;

array = malloc(n * sizeof(*array)); /* Assuming `n` is the number of rows */
if(!array) /* If `malloc` failed */
{
    fputs(stderr, "Error allocating memory; Bailing out!");
    exit(-1);
}

int count = 1;
for(int i = 0; i < n; i++)
{
    array[i] = malloc(count * sizeof(**array));
    if(!array[i]) /* If `malloc` failed */
    {
        for(int j = 0; j < i; j++) /* free previously allocated memory */
        {
            free(array[j]); 
        }
        free(array);

        fputs(stderr, "Error allocating memory; Bailing out!");
        exit(-1);
    }
    count++;
}

Then, read data from the file into the array by using:

FILE* fp = fopen("file.txt", "r");
if(!fp)
{
   for(int i = 0; i < n; i++) /* free previously allocated memory */
   {
      free(array[i]); 
   }
   free(array);

   fputs(stderr, "Error opening 'file.txt'; Bailing out!");
   exit(-1);
}

int max = 1;

for(int i = 0; i < n; i++)
{
    for(count = 0; count < max; count++)
    {
        fscanf(fp, "%d", &array[i][count]);
    }
    max++;
}

Then print the results:

max = 1;

for(int i = 0; i < n; i++)
{
    for(count = 0; count < max; count++)
    {
        printf("array[%d][%d] = %d", i, count, array[i][count]);
    }
    max++;
}

And finally, free the allocated memory:

for(int i = 0; i < n; i++)
{
   free(array[i]); 
}
free(array);