Language: C++

#include <iostream>
using namespace std;

int main() {
	double i,j,k,n,m,l;
	cin >> n >> m >> l;
	double a[n][m],b[m][l],c[n][l];
	for(i=0;i<n;i++){
		for(k=0;k<l;k++){
			c[i][k] =0;
		}
	}
	for(i=0;i<n;i++){
		for(j=0;j<m;j++){
			cin >> a[i][j];
		}
	}
	for(j=0;j<m;j++){
		for(k=0;k<l;k++){
			cin >> b[j][k];
		}
	}
	for(i=0;i<n;i++){
		for(k=0;k<l;k++){
			for(j=0;j<m;j++){
				c[i][k] += a[i][j]*b[j][k];
			}
			cout << c[i][k];
			if(k != l-1)
		            cout << " ";
		}
	cout << endl;
	}
	return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:7:21: error: conversion from 'double' to 'long unsigned int' in a converted constant expression
    7 |         double a[n][m],b[m][l],c[n][l];
      |                     ^
a.cc:7:21: error: could not convert 'm' from 'double' to 'long unsigned int'
    7 |         double a[n][m],b[m][l],c[n][l];
      |                     ^
      |                     |
      |                     double
a.cc:7:21: error: size of array 'a' has non-integral type 'double'
a.cc:7:18: error: conversion from 'double' to 'long unsigned int' in a converted constant expression
    7 |         double a[n][m],b[m][l],c[n][l];
      |                  ^
a.cc:7:18: error: could not convert 'n' from 'double' to 'long unsigned int'
    7 |         double a[n][m],b[m][l],c[n][l];
      |                  ^
      |                  |
      |                  double
a.cc:7:18: error: size of array 'a' has non-integral type 'double'
a.cc:7:29: error: conversion from 'double' to 'long unsigned int' in a converted constant expression
    7 |         double a[n][m],b[m][l],c[n][l];
      |                             ^
a.cc:7:29: error: could not convert 'l' from 'double' to 'long unsigned int'
    7 |         double a[n][m],b[m][l],c[n][l];
      |                             ^
      |                             |
      |                             double
a.cc:7:29: error: size of array 'b' has non-integral type 'double'
a.cc:7:26: error: conversion from 'double' to 'long unsigned int' in a converted constant expression
    7 |         double a[n][m],b[m][l],c[n][l];
      |                          ^
a.cc:7:26: error: could not convert 'm' from 'double' to 'long unsigned int'
    7 |         double a[n][m],b[m][l],c[n][l];
      |                          ^
      |                          |
      |                          double
a.cc:7:26: error: size of array 'b' has non-integral type 'double'
a.cc:7:37: error: conversion from 'double' to 'long unsigned int' in a converted constant expression
    7 |         double a[n][m],b[m][l],c[n][l];
      |                                     ^
a.cc:7:37: error: could not convert 'l' from 'double' to 'long unsigned int'
    7 |         double a[n][m],b[m][l],c[n][l];
      |                                     ^
      |                                     |
      |                                     double
a.cc:7:37: error: size of array 'c' has non-integral type 'double'
a.cc:7:34: error: conversion from 'double' to 'long unsigned int' in a converted constant expression
    7 |         double a[n][m],b[m][l],c[n][l];
      |                                  ^
a.cc:7:34: error: could not convert 'n' from 'double' to 'long unsigned int'
    7 |         double a[n][m],b[m][l],c[n][l];
      |                                  ^
      |                                  |
      |                                  double
a.cc:7:34: error: size of array 'c' has non-integral type 'double'
a.cc:10:26: error: invalid types 'double [1][1][double]' for array subscript
   10 |                         c[i][k] =0;
      |                          ^
a.cc:15:33: error: invalid types 'double [1][1][double]' for array subscript
   15 |                         cin >> a[i][j];
      |                                 ^
a.cc:20:33: error: invalid types 'double [1][1][double]' for array subscript
   20 |                         cin >> b[j][k];
      |                                 ^
a.cc:26:34: error: invalid types 'double [1][1][double]' for array subscript
   26 |                                 c[i][k] += a[i][j]*b[j][k];
      |                                  ^
a.cc:26:45: error: invalid types 'double [1][1][double]' for array subscript
   26 |                                 c[i][k] += a[i][j]*b[j][k];
      |                                             ^
a.cc:26:53: error: invalid types 'double [1][1][double]' for array subscript
   26 |                                 c[i][k] += a[i][j]*b[j][k];
      |                                                     ^
a.cc:28:34: error: invalid types 'double [1][1][double]' for array subscript
   28 |                         cout << c[i][k];
      |                                  ^