matlab. vectorization within if/else if/else state

Question

I need some help with the following code:

if x(:,3)>x(:,4)
output=[x(:,1)-x(:,2)];
elseif x(:,3)<x(:,4)
output=[x(:,2)-x(:,1)];
else
output=NaN
end

Here is a sample data:

matrix x              output
10   5   1   2        -5
10   5   2   1         5     
NaN  1   1   3         NaN

I'm not sure how to make the code work. It just takes the first argument and ignores the else if and else arguments. Please help. Thank you.

Answer 1

Using sign to avoid indexing for different conditions.

B=diff(x,1,2);
B(B(:,3)==0,3) = NaN;
output = B(:,1) .* sign(B(:,3));

Or in a shorter and less readable form:

B=diff(x,1,2);
output = B(:,1) .* (sign(B(:,3))+0./sign(B(:,3)));

Answer 2

if x(:,3)>x(:,4) doesn't really work, if expects either true or false not a vector. So it only evaluates the first element of the vector x(:,3)>x(:,4) which is why it appears to ignore your elseif.

So you must either use a loop or even better you can use logical indexing like this:

x= [10   5   1   2        
10   5   2   1        
NaN  1   1   3]

output = NaN(size(x,1),1)
I = x(:,3)>x(:,4);
output(I) = x(I,1)-x(I,2);
I = x(:,3)<x(:,4);
output(I) = x(I,2)-x(I,1)

Answer 3

Here is how you can do it:

output = NaN(size(x,1),1);

idx1 = x(:,3)>x(:,4);
idx2 = x(:,3)<x(:,4);

output(idx1) = x(idx1,1)-x(idx1,2);
output(idx2) = x(idx2,2)-x(idx2,1);