Nested for loop not outputing values for inner loop

Question

I have a problem with this nested for loop:

eta = [1e-3:1e-2:9e-1];
HN =5;
for ii = 1:numel(eta)
    for v = 1:HN
        DeltaEta(v) = eta(ii)*6;
    end
end

This code gives the output of DeltaEta as a 1x5 vector.

However, I want the result to be 90x5 vector where DeltaEta is computed 5 times for each value of eta.

I believe the problem is with the way I am nesting the loops.

It seems trivial but I can't get the desired output, any leads would be appreciated.

You can often linearize a for loop with matlab, for example you can reach your goal with : eta.'.*ones(1,HN)*6 — obchardon
– obchardon, Commented Feb 19, 2019 at 11:18

Wolfie · Accepted Answer · 2019-02-19 11:11:46Z

1

You're assigning outputs to DeltaEta(v), where v = 1,2,..,HN. So you're only ever assigning to

DeltaEta(1), DeltaEta(2), ..., DeltaEta(5)

You can solve this with a 2D matrix output, indexing on ii too...

eta = [1e-3:1e-2:9e-1];
HN = 5;
DeltaEta = NaN( numel(eta), HN );
for ii = 1:numel(eta)
    for v = 1:HN
        DeltaEta(ii,v) = eta(ii)*6;
    end
end
% optional reshape at end to get column vector
DeltaEta = DeltaEta(:);

Note, there is no change within your inner loop - DeltaEta is the same for all values of v. That means you can get rid of the inner loop

eta = [1e-3:1e-2:9e-1];
HN = 5;
DeltaEta = NaN( numel(eta), HN );
for ii = 1:numel(eta)
    DeltaEta( ii, : ) = eta(ii) * 6;
end

And now we can see a way to actually remove the outer loop too

eta = [1e-3:1e-2:9e-1];
HN = 5;
DeltaEta = repmat( eta*6, HN, 1 ).';

answered Feb 19, 2019 at 11:11

Wolfie

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Comments

MrAzzaman · Accepted Answer · 2019-02-19 11:06:57Z

0

To answer your question as asked, you need to index on ii as well as v:

eta = [1e-3:1e-2:9e-1];
HN =5;
for ii = 1:numel(eta)
    for v = 1:HN
        DeltaEta(ii,v) = eta(ii)*6;
    end
end

However this is in general a bad idea -- if you catch yourself using for-loops in MATLAB (particularly doubly-nested for-loops) you should consider if there might be a better way that uses MATLAB's strong vectorisation abilities.

answered Feb 19, 2019 at 11:06

MrAzzaman

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