Matlab vector to comma-separated list conversion in one line

Question

In Matlab I often want to assign multiple values from a numeric vector to a given field of a structure array.

b = 1:3;
x(1).a = b(1);
x(2).a = b(2);
x(3).a = b(3);

It seems like there should be a way to make this assignment in a single line but two lines is the best I can come up.

c = num2cell(b);
[x.a] = c{:};

Is there a way to convert a numeric vector into a comma-separated list? I'm looking for something like:

[x.a] = num2csl(b);

Note that I am assuming that length(x) == length(b) here.

@rahnema1 I've always wanted that to work in Matlab. I had no idea that it actually worked in Octave. Thanks. — Reed Espinosa
– Reed Espinosa, Commented Dec 13, 2016 at 20:06

Community · Accepted Answer · 2017-05-23 10:30:38Z

6

Yes, you can just use struct. If you provide a cell array as the value for a given fieldname, MATLAB will create a struct the same size as that field and use each element within the cell array to populate the corresponding struct in the resulting array.

x = struct('a', num2cell(b))

In general, there is no way to easily return a comma-separated list from a function

edited May 23, 2017 at 10:30

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answered Dec 13, 2016 at 19:07

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2 Comments

Reed Espinosa Over a year ago

My structure x frequently has other fields that I don't want to overwrite so I really need a comma-separated list but it sounds like the answer is that there is currently no reasonable way to do that. Thanks!

Suever Over a year ago

Often it's also important to remember that there is really no harm in having two lines, especially if they improve readability.

jez · Accepted Answer · 2019-01-26 03:00:35Z

3

This can only be done with a function if x already exists and has the correct number of elements:

b = 1:3;

x = repmat(struct, size(b));
[x.a] = num2csl(b);

This works if num2csl.m makes use of the special output convention varargout as follows:

function varargout = num2csl(a)
varargout = num2cell(a);

If on the other hand, x does not already exist, then this one-liner will initialize it, as previously pointed out by Suever:

x = struct('a', num2cell(b));

edited Jan 26, 2019 at 3:00

answered Dec 13, 2016 at 19:38

jez

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