Random matrix multiplication [closed] - c++

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I need to create at least 2 matrix 4x4, multiplicate them and display the result, but I'm getting this thing as as result img
I'm creating matrix a[i][j] and matrix b[k][l], and trying to pass the result to a new matrix called c[i][j]
Besides that, I need to make 2 kinds of matrix multiplication, one like this, and another one like this
Can you please please please help? Code below
#include<iostream>
#include<stdlib.h>
#include<time.h>
using namespace std;
int matriz1() {
int a[4][4], i, j;
for (i = 0; i < 4; ++i)
{
for (j = 0; j < 4; ++j)
{
a[i][j] = rand() % 100 + 1;
}
}
for (i = 0; i < 4; ++i)
{
for (j = 0; j < 4; ++j)
std::cout << a[i][j] << '\t';
std::cout << '\n';
}
std::cout << '\n';
std::cout << "x" << std::endl;
std::cout << '\n';
std::cout << "Matriz 2:" << std::endl;
int b[4][4], k, l;
for (k = 0; k < 4; ++k)
{
for (l = 0; l < 4; ++l)
{
b[k][l] = rand() % 100 + 1;
}
}
for (k = 0; k < 4; ++k)
{
for (l = 0; l < 4; ++l)
std::cout << b[k][l] << '\t';
std::cout << '\n';
}
std::cout << '\n';
int c[4][4], m, n, x;
for (i = 0; i < 4; i++) {
for (j = 0; j < 4; j++) {
for (k = 0; k < 4; k++) {
c[i][j] += a[i][k] * b[k][j];
}
}
}
cout << " RESULTADO!!!!!!!!!!!!!!!!!!!!!!" << endl;
for (i = 0; i < 4; i++) {
for (j = 0; j < 4; j++) {
cout << c[i][j] << "\t";
}
cout << "\n";
}
return 0;
}
int main()
{
srand(time(0));
std::cout << "Matriz 1:" << std::endl;
std::cout << matriz1() << std::endl;
}
SOLVED IN THE COMMENTS! Stop disliking my post its my first post

Answer 1

You do this:
c[i][j] += a[i][k] * b[k][j];
but you never initialized c array, it contains random values, likely something like
0xCDCDCDCD (-842150451). Initialize it like this:
int c[4][4] = {}
You have repeated code, so consider to break it up in functions, e.g. you can initialize matrices as functions and output one as another. THat would make code more readable and easier to find errors.

Answer 2

A common "Beginner's Problem" is writing too much code. One consequence is that there are too many places where bugs and other flaws can hide.
This is in 'C', but the only 'C++' aspect of your code is using cout for output. printf() can also be used with C++.
#include <stdio.h>
#include <stdlib.h>
void fill4x4( int a[][4] ) {
for( int r = 0; r < 4; r++ )
for( int c = 0; c < 4; c++ )
a[r][c] = rand() % 100 + 1;
}
void show4x4( int a[][4] ) {
for( int r = 0; r < 4; r++ )
for( int c = 0; c < 4; c++ )
printf( "%5d%c", a[r][c], " \n"[c] );
puts( "" );
}
int showPair( int a, int b, int pos ) {
printf( "%2dx%-2d%c", a, b, " \n"[pos] );
return a * b;
}
void mult4x4( int a[][4], int b[][4], int d[][4], bool x ) {
for( int r = 0; r < 4; r++ )
for( int c = 0; c < 4; c++ )
// assign (=), not accumulate (+=)
// notice the 'exchange' of row/col access of 'b[][]'
if( x )
d[r][c] = showPair( a[r][c], b[r][c], c );
else
d[r][c] = showPair( a[r][c], b[c][r], c );
puts( "" ); show4x4( d );
}
int main()
{
srand(time(0));
int a[4][4]; fill4x4( a ); show4x4( a );
int b[4][4]; fill4x4( b ); show4x4( b );
int d[4][4];
mult4x4( a, b, d, true );
mult4x4( a, b, d, false );
return 0;
}
Copy, paste, compile and run this to see the (random) output.
EDIT: Pointed out by a question from the OP, here's a further compaction of one function that may-or-may-not be self-evident:
void mult4x4( int a[][4], int b[][4], int d[][4], bool x ) {
for( int r = 0; r < 4; r++ )
for( int c = 0; c < 4; c++ )
d[r][c] = showPair( a[r][c], x?b[r][c]:b[c][r], c );
puts( "" ); show4x4( d );
}
Further EDIT: A common problem for those with time to fill is mucking around, shrinking code that already works. Here's the result of some of that..
#include <stdio.h>
#include <stdlib.h>
char *fmt1 = "%5d%.2s";
char *fmt2 = "%2dx%-2d%.2s";
char *tail1 = " ::";
char *tail2 = " \n";
void show2x4x4( int a[][4], int b[][4] ) {
for( int r = 0, c; r < 4; r++ ) {
for( c = 0; c < 4; c++ ) printf( fmt1, a[r][c], &tail1[c+c] );
for( c = 0; c < 4; c++ ) printf( fmt1, b[r][c], &tail2[c+c] );
}
puts( "" );
}
int main() {
srand(time(0));
int r, c, v, a[4][4], b[4][4], d[4][4], e[4][4], *px, *py;
// fill 2 4x4 arrays of random ints as if both were 1x16
for( px = &a[0][0], py = &b[0][0], v = 0; v < 4 * 4; v++ ) {
int num = rand();
*px++ = num % 100; // two digits only
*py++ = num / 100 % 100;
}
show2x4x4( a, b ); // display
// show and perform the calc of the product matrices
for( r = 0; r < 4; r++ ) {
for( c = 0; c < 4; c++ ) {
printf( fmt2, a[r][c], b[r][c], &tail1[c+c] );
d[r][c] = a[r][c] * b[r][c];
}
for( c = 0; c < 4; c++ ) { // note b[] swaps col & row
printf( fmt2, a[r][c], b[c][r], &tail2[c+c] );
e[r][c] = a[r][c] * b[c][r];
}
}
puts( "" );
show2x4x4( d, e ); // show both products
return 0;
}
Output
29 66 71 20 : 35 88 22 22
35 80 36 85 : 53 1 28 54
12 14 12 71 : 95 98 92 19
62 61 89 17 : 94 63 32 43
29x35 66x88 71x22 20x22 :29x35 66x53 71x95 20x94
35x53 80x1 36x28 85x54 :35x88 80x1 36x98 85x63
12x95 14x98 12x92 71x19 :12x22 14x28 12x92 71x32
62x94 61x63 89x32 17x43 :62x22 61x54 89x19 17x43
1015 5808 1562 440 : 1015 3498 6745 1880
1855 80 1008 4590 : 3080 80 3528 5355
1140 1372 1104 1349 : 264 392 1104 2272
5828 3843 2848 731 : 1364 3294 1691 731