How do can I check whether 2 columns in an array have equal values in a row?

I have an array of 5 columns and I would like to check whether columns 1 and 5 have the same value (1) and report the result to a vector. If the values are not the same, I want the vector to have values of 1 (e.g. [1, 1, 1, 1, 1, 1]). If the values in columns 1 and 5 are the same for a given row, I want to increase the value by 1 (e.g. 1, 1, 1, 1, 2, 2).

I receive the following error: Operands to the and && operators must be convertible to logical scalar values. When I change the && to &, the function returns ans = []. In the code below, the array is named positions and the vector into which I would like the values reported is named SimIDs.

Please let me know any additional information I can provide.

i=1;
SimIDs=[];

if positions(:,1)==1 && positions(:,5)==1
    i=i+1;

    SimIDs=[SimIDs,i];
end

Answer 1

[Robert]

Your if statement is examining the entire column because you use : in your indexing expression instead of a loop index, but you increment i as though you were in a loop.

Instead, try using cumsum on your logical result as follows

SimIDs = cumsum(positions(:,1)==1 & positions(:,5)==1) + 1;
% add one to start at one rather than zero

[jrb13]

Thank you very much! This is working for me. My data actually begin with a row with columns 1 and 5 having a value of 1, so I eliminated the +1. I don't quite understand how it works. The cumulative sum of the elements in A would at the least be 2, no?

[Robert]

Yes, if the first value will always be 1, not zero, then your SimIDs will never start at zero anyhow. You don't need the +1.

Answer 2

[Walter Roberson]

mask = positions(:,1) == positions(:,5);
SimIDs = ones(size(mask));
SimIds(mask) = positions(mask,1) + 1;