Looping with indices that are not equally spaced

I'm trying to run a loop on a group of indices I obtained using "find". The indices will not always be consecutive. So, running a for loop like:

for i = indices

won't work. I'm trying to avoid doing something like:

for i = 1:length(VECTOR)

Any ideas?

[Oleg Komarov]

not clear why it won't work. Depends how you structure the operations inside the loop. Post more code.

[Daniel Shub]

What do you mean it doesn't work? What would you expect to get with:

indices = [1,2,3,5,7,13,11];

for i = indices, i, end

Answer 1

[Laura Proctor]

Actually, it will work.

for idx = [ 1 -2 10 12.5 0 ]
    disp(idx)
end

Isn't MATLAB cool?

Answer 2

[John F]

So, it works with a row vector of indices, but not a column?

I tried that bit of code but transposed idx, and the loop didn't work. Strange?

[Laura Proctor]

You are correct - check out Loren's Blog, it explains this behavior much better than I can:

http://blogs.mathworks.com/loren/2010/01/27/matlab-behavior-for/

[Daniel Shub]

Yeah, but check out what it does do with a column. You should have a read of:

http://blogs.mathworks.com/loren/2006/07/19/how-for-works/

Answer 3

[Frederick Abangba Akendola]

Please, how do I write a “For” loop with irregular interval? For example; 2,4,8,16,32

[Walter Roberson]

for K = 2.^(1:5)
    result = whatever involving K
end

However, most of the time you want to create one output per input. The general way to do that is

K_vals = 2.^(1:5);
numK = numel(K_vals);
results = zeros(size(K_vals));
for K_idx = 1 : numK
    K = K_vals(K_idx);
    results(K_id) = whatever involving K
end

plot(K_vals, results)