Filter MATLAB non-numerical array data based on criteria

Question

Two questions, one fairly simple question (at least it seems it should be simple) and one that may take a bit more work. Feel free to contribute to either or both.

First, I'd like to create a string array based off of an existing string array based on a criteria. Take for example a similar operation with a double array:

>> nums = [ 1 2 1 2]

nums =

     1     2     1     2

>> big_nums = (nums == 2) .* nums

big_nums =

     0     2     0     2

I'd like to do something similar with a string array, however I don't know what function to use:

>> sizes = ["XL" "L" "XL" "L"]

sizes = 

  1×4 string array

    "XL"    "L"    "XL"    "L"

>> large_sizes = (sizes == "L") .* sizes
Undefined operator '.*' for input arguments of type 'string'.

I'd like the output to be

large_sizes = 

  1×4 string array

    ""    "L"    ""    "L"

Second question. Suppose I have a 2 dimensional cell array. I'd like to filter data based on criteria:

>> data = {"winter", 1; "spring", 2; "summer", 3; "fall", 4}

data =

  4×2 cell array

    ["winter"]    [1]
    ["spring"]    [2]
    ["summer"]    [3]
    ["fall"  ]    [4]

>> nice_weather = ( (data(1,:) == "fall") + (data(1,:) == "spring") ) .* data
Error using  == 
Cell must be a cell array of character vectors.

I'd like a code that results in one of two arrays: nice_weather =

  4×2 cell array

    [""]          [1]
    ["spring"]    [2]
    [""]          [3]
    ["fall"]      [4]

----- OR -----

nice_weather =

  2×2 cell array

    ["spring"]    [2]
    ["fall"]      [4]

For this question, I am also open to separating data into multiple arrays (for example, one array for strings and one array for numbers).

Thanks!

Community · Accepted Answer · 2020-06-20 09:12:55Z

This solution uses the strcmpi function from MATLAB (no toolbox required) to compare two strings (insensitive to the case).

1D Cell Array:

sizes = {'XL' 'L' 'XL' 'L'};    % Changed " to ' & used cell array
idx = strcmpi(sizes,'L');       % Logical index
sizelist = {sizes{idx}}

Or you could try something like
sizes(~idx) = {"" ""} % manual just for example

For this to automatically adjust the number of blanks "", you could use repmat like this sizes(~idx) = repmat({""},1,sum(~idx))

Output:

sizes = 1×4 cell array
{[""]} {'L'} {[""]} {'L'}

2D Cell Array:

data = {'winter', 1; 'spring', 2; 'summer', 3; 'fall', 4};   % Changed " to '
nicemo1 = 'spring';   
nicemo2 = 'fall';

idx = strcmpi(data(:,1),nicemo1) | strcmp(data(:,1),nicemo2);  % Obtain logical index
data(idx,:)

Output:

ans = 2×2 cell array
{'spring'} {[2]}
{'fall' } {[4]}

Tested with MATLAB R2018b.
Also beware variables like sizes as dropping a letter masks a useful function, size.

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