MATLAB: How to change a value in an array when a condition is met

arraylogical indexingreplace

Hi, I have the following array:
Velocity = [x, y, velocityx, velocityy]
Velocity =
     2     9    -1    -1
     2     3    -1     1
    10     8     1    -1
     6     3     0    -1
     7     4    -1     0
     7     4     0    -1
     9     3     0     0  %This is the problem part
     7     3     0    -1
     7     8     1     1
    10     7     0    -1
I want to change the 0 in the last column by a 1 or a -1 when there is a 0 in the third column. I have already tried this with the code below, but the problem with this code is that than all values will be the same instead of changing between 1 and -1.
idx = Velocity(:,3) == 0
p = [1 -1]
Velocity(idx,4) = p(randperm(length(p),1))
How can I solve this problem?

Best Answer

idx = Velocity(:,3)==0;
vec = [1;-1];
Velocity(idx,4) = vec(randi(numel(vec),nnz(idx),1))
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