Question

Take this simple example:

a = [1 2i];

x = zeros(1,length(a));
for n=1:length(a)
    x(n) = isreal(a(n));
end

In an attempt to vectorize the code, I tried:

y = arrayfun(@isreal,a);

But the results are not the same:

x =
     1     0
y =
     0     0

What am I doing wrong?

Answer 1

This certainly appears to be a bug, but here’s a workaround:

>> y = arrayfun(@(x) isreal(x(1)),a)

ans =

     1     0

Why does this work? I’m not totally sure, but it appears that when you perform an indexing operation on the variable before calling ISREAL it removes the “complex” attribute from the array element if the imaginary component is zero. Try this in the Command Window:

>> a = [1 2i];         %# A complex array
>> b = a(1);           %# Indexing element 1 removes the complex attribute...
>> c = complex(a(1));  %# ...but we can put that attribute back
>> whos
  Name       Size            Bytes  Class      Attributes

  a          1x2                32  double     complex   
  b          1x1                 8  double                  %# Not complex
  c          1x1                16  double     complex      %# Still complex

Apparently, ARRAYFUN must internally maintain the “complex” attribute of the array elements it passes to ISREAL, thus treating them all as being complex numbers even if the imaginary component is zero.