How to pass cell indexing as an input argument to a function, such that cell inside the function can use the indexing directly?

Question

I am wondering if we can pass cell indexing as an input argument to a function, such that a cell inside the function can parse it.

For example, given a cell like x = num2cell(reshape(1:24, [2,3,4])), we can do indexing

>> x(:,1,2)

ans =

  2×1 cell array

    {[7]}
    {[8]}

However, if I want to pass (:,1,2) as an input argument to a custom function (see the attempt below, which is not working for sure), what can we do?

f({:,1,2}) % not working

function y = f(varargin)
x = num2cell(reshape(1:24, [2,3,4]));
y = x(varargin{:});
end

There are two issues here. 1) varargin here itself will be a cell. So if you go inside your function f at runtime, you will see that varargin is a cell that contains 1 element - another cell (this is what contains the inputs). When you then call x(varargin{:}), you are trying to use the inner cell as in index - hence the error. If you change this to f(2,1,2), it will now work. 2) the use of :. The colon operator cannot be directly passed if the context is not known. Hence, f(:,1,2) will not work. You can use the colon character ':' instead - f(':',1,2) will work. — magnesium
– magnesium, Commented Jul 23 at 8:48

Wolfie · Accepted Answer · 2025-07-23 13:37:04Z

5

Because you are using a cell array to input the indices, varargin is a cell array with your cell nested as the first element. You don't need varargin and can simply let f expect a single input which is your indexing cell
You can't pass : around like this, but : and the char ':' will be treated the same when indexing so you can do

f({':',1,2}) % Note the colon operator is a character

function y = f(idx)
% Take an indexing array 'idx' and use it to index a subset
% of the generated array 'x'
x = num2cell(reshape(1:24, [2,3,4]));
y = x(idx{:});
end

Alternatively, if you wanted to use varargin to achieve the same functionality, you could do

f(':',1,2) % Note the colon operator is a character

function y = f(varargin)
% Take an indexing array 'idx' and use it to index a subset
% of the generated array 'x'
x = num2cell(reshape(1:24, [2,3,4]));
y = x(varargin{:});
end

The 2nd option means you don't need to put the indices into a cell array, but would probably be less clear if you had other inputs to f.

Here is a MathWorks blog post about passing around indices in cell arrays, including a reference to using the colon operator in quotes:

https://blogs.mathworks.com/loren/2006/11/10/all-about-the-colon-operator/

edited Jul 23 at 13:37

answered Jul 23 at 10:04

Wolfie

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1 Comment

ThomasIsCoding Jul 23 at 12:01

This is excellent! Thanks for the elegant solution!

ThomasIsCoding · Accepted Answer · 2025-07-23 08:48:33Z

0

I found one possible workaround, but will keep the question open for more options

function y = f(indexingStruct)
x = num2cell(reshape(1:24, [2,3,4]));
y = subsref(x, indexingStruct);
end

indexingPattern = substruct('()', {':', 1, 2});
f(indexingPattern)

which gives

ans =

  2×1 cell array

    {[7]}
    {[8]}

answered Jul 23 at 8:48

ThomasIsCoding

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