Deleting certain elements from an array without looping

Question

Teshan Rezel 2021-11-8

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https://ww2.mathworks.cn/matlabcentral/answers/1581624-deleting-certain-elements-from-an-array-without-looping

回答： Rik 2021-11-8

Hi folks,

I have a cell array containing all the file names in a subfolder. I want to remove the filenames that are not 21 characters in length.

I have tried:

fileNames(length(fileNames(:)) ~= 21) = [];

but this only deletes one entry each time I run it, and not the correct ones eather!

May I please ask where I've gone wrong and how to fix it?

full code:

subFolders = dir(sourcePath);
subFolders = subFolders([subFolders(:).isdir]);
subFolders = subFolders(~ismember({subFolders(:).name},{'.','..'}));
fileNum    = numel(subFolders);
folderDir = fullfile(subFolders(i).folder, subFolders(i).name);
folderDir  = dir(folderDir);
folderDir  = folderDir(~ismember({folderDir(:).name},{'.','..'}));
fileNames  = {folderDir.name};

Thanks!

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Chunru 2021-11-8

Can you provide a small part of fileNames so we know what it looks like?

Teshan Rezel 2021-11-8

@Rik apologies, I'll ammend my question to include this!

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Answer 1

Rik 2021-11-8

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The code Chunru wrote will work, but I have a few suggestions.

Instead of using length, you should consider using numel as a habit. In this case dir will alway provide you with vectors, but there is no situation I know of where length is a better choice than either numel or size. I have never seen a non-contrived example where max(size(A)) was actually the intended behavior.

I would also like to point you to the legacy syntax of cellfun, which is the main (or only) situation where cellfun is faster than a loop:

fileNames = {'ThisIsADummyFile', '012345678901234567891'}
fileNames = 1×2 cell array
    {'ThisIsADummyFile'}    {'012345678901234567891'}
idx = cellfun('prodofsize', fileNames)~=21
idx = 1×2 logical array
   1   0
fileNames(idx) =[]
fileNames = 1×1 cell array
    {'012345678901234567891'}

You can also use idx to remove elements of the struct that dir returned.

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Answer 2

Chunru 2021-11-8

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https://ww2.mathworks.cn/matlabcentral/answers/1581624-deleting-certain-elements-from-an-array-without-looping#answer_826489

在 MATLAB Online 中打开

fileNames = {'ThisIsADummyFile', '012345678901234567891'}
fileNames = 1×2 cell array
    {'ThisIsADummyFile'}    {'012345678901234567891'}
idx = cellfun(@(x) length(x)~=21, fileNames)
idx = 1×2 logical array
   1   0
fileNames(idx) =[]
fileNames = 1×1 cell array
    {'012345678901234567891'}