Thread: Saving matrices with data structures-"get" and "set" confusion

Hi everyone,
I´m taking a programming class and currently we´re doing data structures. Today, we discussed how to save square matrices column-wise with data structures. The teacher said that after the first few steps where you declare the structure, allocate the matrix, free it and get the dimension (which mostly make sense to me I think), you have to "get" and "set" the matrix by putting in something like

Code:

void setMatrixEntry(Matrix* A, int i, int j, double Aij) 
{

A->entries[i+j*A->m] = Aij;

}


double getMatrixEntry(Matrix* A, int i, int j) 
{

return A->entries[i+j*A->m]
}

I´m really confused here because I can´t figure out what "A->entry[i+j*A->m] = Aij;" and "return A->entry[i+j*A->m]" do exactly or how you get to them.

I´d be really grateful for any help!

The data structure is

Code:

typedef struct _Matrix_ {

int m;

int n;

double* entry;

} Matrix;

and the allocation is:

Code:

Matrix* newMatrix(int m, int n) {

int i = 0;

Matrix* A = malloc(sizeof(Matrix));

A->m = m;

A->n = n;

A->entry = malloc(m*n*sizeof(double));

for (i=0; i<m*n; ++i) {

A->entry[i] = 0;

}

return A;

}

Is that what you meant?

Thank you, I think I understand how to get to it now!

I´ve still got one question, though...
What exactly is
return A->entries[i + j * A->m]; supposed to do? The idea is that a user puts in entries and they get saved in the matrix (I think). Why do we need A->entries[i + j * A->m]; ?

Ok, thank you!

And just to make sure I´ve understood this...does setMatrixEntry give every entry the value/name Aij?

Originally Posted by Suuper Susi

Ok, thank you!

And just to make sure I´ve understood this...does setMatrixEntry give every entry the value/name Aij?

Every entry? Can you explain why you think that? It sounds like you are just guessing and you don't understanding laserlight's explanations at all. I'm being genuine, you have a misunderstanding that needs to be sorted out.