How to create idendical variables in MATLAB from an array of variable names?

Question

I have the following code in Matlab:

a = zeros(23,1)
b = zeros(23,1)
c = zeros(23,1)

How can I write it more compactly? I was looking for a solution that is something like this:

str = {'a','b','c'}
for i = str{i}
    i = zeros(23,1)
end

But I can't find a way to do it properly without an error message. Can someone help please?

The first one is good, no need to change it. You can also always do d=zeros(23,3) and index each of the second dimension. — Ander Biguri
– Ander Biguri, Commented Aug 15, 2019 at 13:39

rahnema1 · Accepted Answer · 2019-08-15 13:51:19Z

5

Here is a compact way using deal :

[a, b, c] = deal(zeros(23,1));

answered Aug 15, 2019 at 13:51

rahnema1

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Comments

Cris Luengo · Accepted Answer · 2019-08-15 15:37:52Z

3

You can also use a struct if the variable name is important:

str = {'a','b','c'};
data = struct
for ii = 1:numel(str)
    data.(str{ii}) = zeros(23,1);
end

The struct is more efficient than the table. You can now address data.a, data.b, etc.

But if the name is not useful, it's best to put your data into a cell array:

N = 3;
data = cell(N,1);
for ii = 1:N
    data{ii} = zeros(23,1);
end

or simply:

data = cell(3,1);
[data{:}] = deal(zeros(23,1));

Now you address your arrays as data{1}, data{2}, etc., and they're always easy to address in loops.

answered Aug 15, 2019 at 15:37

Cris Luengo

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2 Comments

Wolfie Over a year ago

Structs aren't necessarily more efficient than tables... It depends what you're doing

Cris Luengo Over a year ago

@Wolfie: sure. But A.field is significantly faster if A is a struct than a table. A table is a user class, so indexing into it runs some M-file code. A struct is a built-in type. There's a Q&A somewhere on here where these two are timed.

Wolfie · Accepted Answer · 2019-08-15 13:53:50Z

What you're tempted to do is very bad practise, but can be done like this

str = {'a','b','c'};
for ii = 1:numel(str)
    eval( [str{ii} ' = zeros(23,1)'] );
end

Why is this bad practise?

Your code legibility has just gone way down, you can't clearly see where variables are declared.
eval should be avoided

You could use deal to make things a bit nicer, but this doesn't use the array of variable names

[a, b, c] = deal( zeros(23, 1) );

Even better, it's likely you can optimise your code by using a matrix or table instead of separate 1D arrays. The table option means you can still use your variable name array, but you're not using eval for anything!

% Matrix
M = zeros( 23, 3 ); % Index each column as a/b/c using M(:,1) etc

% Table, index using T.a, T.b, T.c
T = array2table( zeros(23,3), 'VariableNames', {'a','b','c'} );

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