Filling a matrix without for-loops and ifs

So i have this piece of code:

for i = 1:L

n1 = M(i,1);

n2 = M(i,2);

for j = 1:N

if j == n1

A(i,j) = 1;

elseif j == n2

A(i,j) = -1;

else

A(i,j) = 0;

end

end

end

M is a (L,2) matrix which tells me where does a line start and where it ends.

What i am looking to do is fill a matrix "A" , that is already filled with zeros, with a 1 where the line starts [M(:,1)], and a -1 where the line ends [M(:,2)], and all other elements with zeros, line per line.

For example if my M matrix is:

M =

1 2

2 3

1 3

The matrix A would be:

A =

1 -1 0

0 1 -1

1 0 -1

What i would like to know, is if i can fill this matrix "A" without using for's and if's, just make my code cleaner and maybe faster to run.

I was talking with a professor and told me to do it like this:

A(ind(:,1) = 1

A(ind(:,2) = -1

But i feel something is missing and cant find how to do it properly.

Sorry for the long post, thank you for your time.

Cheers

Answer 1

[James Tursa]

Check out the sub2ind( ) function:

https://www.mathworks.com/help/matlab/ref/sub2ind.html?searchHighlight=sub2ind&s_tid=doc_srchtitle

In particular, your code would look something like:

A = zeros(_____); % <-- You need to figure out what the size of A should be
A(sub2ind(_____)) =  1;  % <-- You need to figure out the proper inputs for sub2ind( )
A(sub2ind(_____)) = -1;  % <-- You need to figure out the proper inputs for sub2ind( )