C Storing Matrix in Array of Chars and Printing - c

Question

Hey all I am trying to store a matrix in an array of chars and then print it out.
My code that I have written:
#include<stdio.h>
#include<stdlib.h>
int main() {
int i;
int j;
int row=0;
int col=0;
int temp=0;
char c;
int array[3][2] = {{}};
while((c=getchar()) !=EOF && c!=10){
if((c==getchar()) == '\n'){
array[col++][row];
break;
}
array[col][row++]=c;
}
for(i=0; i<=2; i++){
for(j=0; j<=3; j++){
printf("%c ", array[i][j]);
}
printf("\n");
}
}
Using a text file such as:
1 2 3 4
5 6 7 8
9 1 2 3
I would like to be able to print that back out to the user, however what my code outputs is:
1 2 3 4
3 4 5 6
5 6 7 8
I cannot figure out what is wrong with my code, some how I am off an iteration in one of my loops, or it has something to do with not handling new lines properly. Thanks!

Answer 1

A few problems that I can see are:
As user3386109 mentioned in the comments, your array should be array[3][4] to match the input file.
The line array[col++][row]; does nothing but increment col, and then uselessly indexes the array and throws away the value. You can do the same thing with just col++;. However, you're not even using col at any later point in the code, so really you don't even need that. The break; all by itself does what you need. Which leads me to...
You're not populating the array like you think you are. You're incrementing col and then immediately breaking out of the loop. So how does the entire array ever get populated? Just by pure luck. As it turns out with your array declared as array[3][4], the array access array[0][4] (which isn't even technically supposed to exist) is equivalent to array[1][0]. This is because all multidimensional arrays (in C and just about any other language) are laid out in memory as flat arrays, because memory itself uses linear addressing. In C, this flattening of multidimensional arrays is done in so-called Row-major order, meaning that as you traverse the raw memory from first address to last, the corresponding multidimensional indices (i,j,k,...z, or in your case just i,j) increment in such a way that the last index will change the fastest. So, not only does col never get incremented except for right before you break out of the loop, but row never gets reset to 0, which means you're storing values in array[0][0], array[0][1], ... array[0][11], not array[0][0] .. array[0][3], array[1][0] .. array[1][3], array[2][0] .. array[2][3] as you were expecting. It was just luck that, thanks to row-major ordering, these two sets of indices were actually equivalent (and C doesn't do array bounds checking for you because it assumes you're doing it yourself).
This is just personal preference, but you will usually see arrays referenced as array[row][col], not array[col][row]. But like I said, that's just preference. If it's easier for you to visualize it as [col][row], then by all means do it that way. Just make sure you do it consistently and don't accidentally switch gears midway through your code to doing [row][col].
Your code will break and only print out part of the matrix if you accidentally put a trailing space at the end of one of your rows of numbers, because of the weird way you're checking for the end of input (doing a second getchar after each initial getchar and checking to see if the second character is \n). This method isn't wrong per se, in the sense that it will work, but it's not very robust and relies on your input data being precisely formatted and containing no trailing spaces. Anyone who has ever spent hours trying to figure out why their Makefile didn't work, only to find out that it was because they had leading spaces instead of tabs can attest to the fact that those kinds of errors can be extremely time-consuming and frustrating to track down. Precisely formatted input data is always a good thing, but your code shouldn't break in unexpected an non-obvious ways (such as only printing out half of a matrix) when it doesn't get perfect input. Edit: It only occurred to me later on that you were actually intending to do two mutually exclusive things here: increment col for the next line of input, and break out of the loop after having (presumably) detected the end of input. You need to figure out which thing you're doing here, although thanks to item #3, your code actually (and oddly) works just by taking user3386109's advice and changing array[3][2] to array[3][4].
I can only assume you used <= 2 and <= 3 in your for loops instead of < 3 and < 4, respectively, because you prefer doing it that way. That's fine, but it generally makes for easier-to-read code if your for loop conditions match up with your array dimensions. Just speculating here, but perhaps that's why you had array[3][2] when you really meant array[3][4].