Iterating through an array/vector Matlab

Question

First off, I do have to apologize, because I'm very sure that I'm simply making a simple mistake. I am making my first program in MatLab, and have been reading up on the relevant documentation, but simply still can't seem to solve my problem.

I am trying to implement the equation for information entropy in MatLab (I'm sure it probably already exists, but that's beside the point), but I am having issues with arrayfun as it seems to be calling entropySingle with no arguments.

I have the following functions in appropriately named files

function y = entropySingle(x)
y = x * log2(x);
end

and

function y = entropy(x)
if ~isvector(x)
    error('Input must be a vector');
end 
x = arrayfun(entropySingle, x);
y = sum(x);
end

and I'm calling entropy([1/3 1/4 1/6 1/8 1/12 1/24]). The error occurs on line 2 of entropySingle, but why is it being called with a null pointer? Thanks in advance,

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

1

You need to use element wise multiplication:

y = x .* log2(x);

Not using that small . before the multiplication tell matlab this is about matrix multiplication where it is not.

Also, don't use the name entropy. You are overwriting a built-in matlab function, and this just invites more trouble to your code and life in general.

edited Jun 20, 2020 at 9:12

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answered Jan 31, 2016 at 7:52

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2 Comments

CBlumey Over a year ago

Thanks, I changed the code to name it something different and use .* instead, but I am still experiencing the same errors.

CBlumey Over a year ago

However, if I use an anonymous function, @(x) x .* log2(x), then everything works fine.

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