Matrix multiplication - C++ [closed] - c++

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void power(int a[][21], int n, int d) {
int e[21][21], k, i, j, l;
if (d == 1) {
for (i = 1; i <= n; i++) {
for (j = 1; j <= n; j++)
cout << a[i][j] << " ";
cout << endl;
}
} else {
for (i = 1; i <= n; i++)
for (j = 1; j <= n; j++)
e[i][j] = 0;
for (l = 1; l < d; l++)
for (i = 1; i <= n; i++)
for (j = 1; j <= n; j++)
for (k = 1; k <= n; k++)
e[i][j] = e[i][j] + (a[i][k] * a[k][j]);
for (i = 1; i <= n; i++) {
for (j = 1; j <= n; j++)
cout << e[i][j] << " ";
cout << endl;
}
}
}
I am trying to build a function that calculates the power of a matrix. The program works for the 1st and 2nd power of the matrix but if I want to calculate the 3rd power the function will return incorrect values. I think the problem is at retaining the previous results but I can't figure out on how to fix it.

Answer 1

This block of code
for (l = 1; l < d; l++)
for (i = 1; i <= n; i++)
for (j = 1; j <= n; j++)
for (k = 1; k <= n; k++)
e[i][j] = e[i][j] + (a[i][k] * a[k][j]);
sets e to be a*a no matter what d is.
You need to have a temporary matrix to make things work.
Bootstrap:
e = a;
In the loop:
temp = e;
e = temp * a;
temp = e;
e = temp * a;
... etc.
Something along the lines of:
for (i = 0; i < n; i++)
{
for (j = 0; j < n; j++)
{
e[i][j] = a[i][j];
}
}
for (l = 1; l < d; l++)
{
for (i = 0; i < n; i++)
{
for (j = 0; j < n; j++)
{
temp[i][j] = e[i][j];
}
}
for (i = 0; i < n; i++)
{
for (j = 0; j < n; j++)
{
e[i][j] = 0;
for (k = 0; k < n; k++)
{
e[i][j] += (temp[i][k] * a[k][j]);
}
}
}
}
PS I have used 0-based indexing to access the matrices. I am not sure why you have used 1-based indexing.