list comprehension in MATLAB using Python interface

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,

Answer 1

[Al Danial]

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.