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TL;DR: Struggling with 2 dimensional arrays.

I'm trying to create two two dimensional array from a list of integers from a text file. This is programmed in C.

tester.txt contains:

2 1 2 3 4 5 6 7 8

The first number means that both arrays have 2 rows and 2 columns, if it were any other number the columns/rows would be represented as such.

tester.txt should ouput the following:

1 2    5 6
3 4    7 8

Here is my code:

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

int main()
{
    int i,j,k;
    FILE *filepointer;
    int nrows;
    int size;

    fputs("Enter a filename: ", stdout);
    fflush(stdout);

    if ( fgets(filename, sizeof filename, stdin) != NULL )
    {
        char *newline = strchr(filename, '\n'); /* search for newline character */
        if ( newline != NULL )
        {
            *newline = '\0'; /* overwrite trailing newline */
        }
        printf("filename = \"%s\"\n", filename);
    }

    filepointer=fopen(filename,"r");
    fseek(filepointer, 0, SEEK_END); // seek to end of file
    size = ftell(filepointer);
    printf("Size=%d\n",size);
    fseek(filepointer, 0, SEEK_SET);

    int holderarray[size];

    for(i=0; i<size; i++)
        fscanf(filepointer, "%d", &holderarray[i]);

    nrows=holderarray[0];
    printf("Number of rows/columns=%d\n",nrows);

    if (filepointer == NULL)
    {
        fprintf(stderr, "Can't open input file in.list!\n");
        exit(1);
    }
}

Everything works as expected up until this point. I can't visualize how to add the first half of the values to the new 2 dimensional arrays, hopefully you guys can help. Here's my brainstorming in codeblocks.

int matrix1[nrows][nrows];
int matrix2[nrows][nrows];


for (i=1; i<sizeof(holderarray);i++)
{
    for (j=0;j<nrows;j++)
    {
        matrix[i][j]=holderarray[j];
    }

for (i=0;i<sizeof(nrows);i++)
{
    for (j=0;j<sizeof(nrows);j++)
    {
        printf("%d",matrix[i][j]);
    }

}
return 0;
share|improve this question
Your holder-array is being sized to the byte-count of the file; not the number of integers within. If this is intentional as a mechanism for ensuring you don't overrun a short array it seems a bit overkill. – WhozCraig Nov 19 '12 at 8:56
oh I had no idea, how do I size by number of integers within? – Unknown Nov 19 '12 at 9:07
You don't. This is a txt file. you need to process the character data within, translating to the native data types you need (in your case, ints).Creative use of scanf() after reading the number chars (and translating to your int matrix size, of course), would make for a fairly straight-forward solution. – WhozCraig Nov 19 '12 at 9:13

2 Answers

up vote 0 down vote

you can get them by looping using getc 

1. you read the first char in line and define the array structure , cast to integer 
2. initialize the arrays  eg you read 2 so 2*2 is the size of the array, 3*3 is the size of the array and number of the elements to read in every array 
3. continue reading in to reach the first array bound based 2*2 = 4 3*3= 9 based on the first line. 

4. fill the other array since the first array is full,
share|improve this answer
Yeah ok, I understand how that would work. It's not three nested loops. I'm trying to visualize it, but I'm having a hard time/ – Unknown Nov 19 '12 at 8:41
up vote 0 down vote

You can't dynamically declare arrays in standard C like this based on variables that aren't known at compile time:

int matrix1[nrows][nrows];
int matrix2[nrows][nrows];

What you need to do, if you aren't using C99 or a later version, is use the malloc function that dynamically allocates memory for you:

int **matrix1, **matrix2;
int i;

matrix1 = malloc(nrows * sizeof(int*));
matrix2 = malloc(nrows * sizeof(int*));

for(i = 0; i < nrows; i++) {
  matrix1[i] = malloc(nrows * sizeof(int));
  matrix2[i] = malloc(nrows * sizeof(int));
}

Two-dimensional arrays in C are treated as pointers to pointers. Each pointer is a reference to a contiguous chunk of memory that contains integers (i.e. the rows of your matrix), and then we use a pointer to a pointer as a reference to another contiguous chunk of memory that contains references/pointers to the first element in all of these rows.

This image (not mine) may help: DYNAMIC 2D ARRAY

Note that this dynamically allocated memory should be freed when you are finished using it:

for(int i = 0; i < nrows; i++) {
  free(matrix1[i]);
  free(matrix2[i]);
}

free(matrix1);
free(matrix2);
share|improve this answer
Yes, I understand, but by the time we reach int matrix1[nrows][nrows], nrows is instantiated. – Unknown Nov 19 '12 at 8:39
That works only if you are using C99, so make sure you have the -std=c99 flag when compiling. Sorry for being unclear; I'll edit my answer. – dtidmarsh Nov 19 '12 at 8:49

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