# list comprehension in MATLAB using Python interface

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dleal on 25 Jul 2022
Answered: Al Danial on 5 Aug 2022
Hi all,
using MATLAB's Python interface, is there a way to do a list comprehension in MATLAB?
Sample problem python list comprehension:
list_1 = [foo(x) for x in L] % L = py.list({1,2,3,4}); list_1 = [x+1 for x in L];
(In my actual use case, L is a list of classes, and foo() is getattr(L{ii}, '__dict'__') ).
I can only think of two ways:
1) using a MATLAB for loop:
list_1 = py.list();
for ii = 1:int64(py.len(L))
list1.append(foo(L{ii}));
end
which ends up being slow because of the overhead of calling Python functions in a loop.
2) defining a Python function that simulates a list comprehension and importing it to MATLAB:
% sampleScript.py
def list_compr(L):
return [foo(x) for x in L]
%--- in matlab ---
list_1 = py.sampleScript.list_compr(L);
which, I think is not very flexible, because I would need a different function for a slightly different list comprehension.
Do you have any suggestions? thank you,

Al Danial on 5 Aug 2022
MATLAB's cellfun() and arrayfun() act like Python list comprehensions. You can give them Python functions to act on each item in the cell or array. For example, if foo.py has
def foo(x):
return 2*x
then in MATLAB you can do
>> F = py.importlib.import_module('foo');
>> a = 20:29
20 21 22 23 24 25 26 27 28 29
>> list_l = arrayfun( @(x)(F.foo(x)), a)
40 42 44 46 48 50 52 54 56 58
list_l is actually a MATLAB matrix so if you wanted a Python list you'd need to convert it, ie
>> py.list(list_l)
Python list with values:
[40.0, 42.0, 44.0, 46.0, 48.0, 50.0, 52.0, 54.0, 56.0, 58.0]
Use string, double or cell function to convert to a MATLAB array.

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