Best way to retrieve items from a cell array using a cell array of indices?

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NV 2025-1-16

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https://ww2.mathworks.cn/matlabcentral/answers/2172959-best-way-to-retrieve-items-from-a-cell-array-using-a-cell-array-of-indices

评论： Star Strider 2025-1-17

Hi I am porting a script I had in Python to Matlab. Also returning to MATLAB after a decade, so pardon me if it's too basic.

The key issue I am having is, retrieving premapped items from a cell array. Example, I have:

label_names={"assds", "Sasas", "Asasa", "assds"}
indices={[1 3 2]} %note that this can have varying lengths from 1 to full length of the label array.
label_names(indices) %of course returns an error

Error is "Unable to use a value of type cell as an index."

What is the best way to achieve this - retreive items based on a cell array of indices - as cleanly as possible?

PS: The reason they are cell arrays has to do with varying lengths. To avoid X-Y problem, lets assume rest of the code is done optimally.

Thanks!

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Walter Roberson 2025-1-16

If the cell array always has exactly one element, then why use a cell array?

If the cell array potentially has more than one element, then what is the intended meaning? Should the different elements be used as different subscript positions? If the cell array was {[1 3], [2 4]} then should the call be equivalent to X([1 3], [2 4]) producing a 2 x 2 output, or should it be [X(1,2), X(3,4)] producing a 1 x 2 output ?

NV 2025-1-17

Hi Walter,

Thanks. The reason here is, what's shown above happens within a loop. The above two arrays are grabbed from a larger cell array which is why they are of varying lengths. The indices is always a 1D array with list of indices I want to retrieve.

I resolved this after posting this by doing a vertcat as follows:

indices=vertcat(indices{:})

I was hoping there is a direct way to do this (or may be this is the direct way).

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Answer 1

Star Strider 2025-1-16

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2172959-best-way-to-retrieve-items-from-a-cell-array-using-a-cell-array-of-indices#answer_1557560

编辑：Star Strider 2025-1-16

在 MATLAB Online 中打开

Creating ‘indices’ as a cell array is the problem.

Refer to it as a double array instead —

label_names={"assds", "Sasas", "Asasa", "assds"}
label_names = 1x4 cell array
    {["assds"]}    {["Sasas"]}    {["Asasa"]}    {["assds"]}
indices=[1 3 2] %note that this can have varying lengths from 1 to full length of the label array.
indices = 1×3
     1     3     2
<mw-icon class=""></mw-icon>
<mw-icon class=""></mw-icon>
label_names(indices)
ans = 1x3 cell array
    {["assds"]}    {["Asasa"]}    {["Sasas"]}

Equivalently (although redundantly) —

label_names={"assds", "Sasas", "Asasa", "assds"}
label_names = 1x4 cell array
    {["assds"]}    {["Sasas"]}    {["Asasa"]}    {["assds"]}
indices={[1 3 2]} %note that this can have varying lengths from 1 to full length of the label array.
indices = 1x1 cell array
    {[1 3 2]}
label_names(indices{:})
ans = 1x3 cell array
    {["assds"]}    {["Asasa"]}    {["Sasas"]}

.

EDIT — Corrected typographical errors.

.

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NV 2025-1-17

Thanks! This works but it comes in as a cell array due to other reasons, but doing the conversion as follows worked. But I get the point regarding not using cell arrays for this purpose:

indices=vertcat(indices{:})

Star Strider 2025-1-17

As always, my pleasure!

There is cerrtainly nothing wrong with using a cell array if that works for you.

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