Question

I have a cell array in Matlab:

strings = {'one', 'two', 'three'};

How can I efficiently calculate the length of all three strings? Right now I use a for loop:

lengths = zeros(3,1);
for i = 1:3
    lengths(i) = length(strings{i});
end

This is however unusable slow when you have a large amount of strings (I’ve got 480,863 of them). Any suggestions?

Answer 1

You can also use:

cellfun(@length, strings)

It will not be faster, but makes the code clearer.
Regarding the slowness, you should first run the profiler to check where the bottleneck is. Only then should you optimize.

Edit: I just recalled that ‘length’ used to be a built-in function in cellfun in older Matlab versions. So it might actually be faster! Try

 cellfun('length',strings)

Edit(2) : I have to admit that my first answer was a wild guess. Following @Rodin s comment, I decided to check out the speedup.

Here is the code of the benchmark:

First, the code that generates a lot of strings and saves to disk:

function GenerateCellStrings()
    strs = cell(1,10000);
    for i=1:10000
        strs{i} = GenerateRandomString();
    end
    save strs;
end

function st = GenerateRandomString()
    MAX_STR_LENGTH = 1000;
    n = randi(MAX_STR_LENGTH);
    st = char(randi([97 122], 1,n ));

end

Then, the benchmark itself:

 function CheckRunTime()
    load strs;
    tic;
    disp('Loop:');
    for i=1:numel(strs)
        n = length(strs{i});
    end
    toc;

    disp('cellfun (String):');
    tic;
    cellfun('length',strs);
    toc;

    disp('cellfun (function handle):');
    tic;
    cellfun(@length,strs);
    toc;

end

And the results are:

Loop:
Elapsed time is 0.010663 seconds.
cellfun (String):
Elapsed time is 0.000313 seconds.
cellfun (function handle):
Elapsed time is 0.006280 seconds.

Wow!! The ‘length’ syntax is about 30 times faster than a loop! I can only guess why it becomes so fast. Maybe the fact that it recognizes length specifically. Might be JIT optimization.

Edit(3) – I found out the reason for the speedup. It is indeed recognition of length specifically. Thanks to @reve_etrange for the info.