What's wrong with this array declaration?

Hi, I got an assignment of doing a program to make the sum, substraction, etc. of a matrix A and a matrix B of given dimensions (Via keyboard, via detection, any method was possible).

I used dynamic memory allocation with three double ** A, **B, **C and used calloc to create the 2 dim arrays but, then I found this code on the net:

Code:

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#include<stdio.h>

int main(){

  int N,M,L;

  printf("Number of rows in Matrix A: ");
  scanf("%d",&N);
  printf("Number of columns in Matrix A: ");
  scanf("%d",&M);
  printf("Number of columns in Matrix B: ");
  scanf("%d",&L);

  double A[N][M],B[M][L],C[N][L];

  // REST OF CODE
  return 0;
}

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No warning, compiles, and is shorter than memory allocation, as far as I know, array dimensions must be const int but here they are read from the stdin with a plain scanf and it surprisingly works. Is there anything wrong with doing this?

It is alright, nowadays. ( Variable Length - Using the GNU Compiler Collection (GCC) )
But if your matrices are going to be passed around a lot, you're better off using dynamic allocation.

I see, this is pretty new to me.
My lecturer is always angry at us when we forget to make dimensions constant, I might use this to trim simple programs then, thanks for the reply :)

Quote:

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Originally Posted by dxgalaxy

I see, this is pretty new to me.
My lecturer is always angry at us when we forget to make dimensions constant, I might use this to trim simple programs then, thanks for the reply :)

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Beware that this code may not be accepted by your teacher.
Education standards are generally *much* back dated that than the language standards.

Use dynamic allocation as you'd be expected to. (And will need to, in any serious C program.)