strange behavior of function leading to complex numbers

i have defined a function named f_gamma which is as follows:

function F = f_gamma(x,xdata)

      if xdata <= x(1)
          F = 0;
          return;
      end

      F = x(2).*((xdata-x(1)).^x(3)).*exp(-(xdata-x(1))/x(4));
  end

in my understanding, for every xdata<=x(1) the function should return zero. but it doesnt. let me give you an example:

lets set x (the constants) = [8, 5, 3, 0.5]; therefore, for all xdata <= 8 the function should return F = 0.

if i call the function now like this

f_gamma(x,2)

ans = 0

everything is correct.

however, if i call it like this, e.g. for plotting

f_gamma(x,1:50)

ans(2) = 229898430,869035 - 250127664,068870i

where does this complex number come from? am i missing something? your help is much appreciated. :)

Answer 1

[dpb]

if xdata <= x(1)
  ...

...in my understanding, for every xdata<=x(1) the function should return zero.

Your have a major mis understanding --

help if
  ...
  The statements are executed if the real part of the expression 
  has all non-zero elements.

So, as written, if there are any elements in xdata>=x(1) the expression is false.

For element-by-element evaluation of your function you need something like...

F=zeros(size(xdata));  % allocate; set zero locations
ix = xdata>x(1);       % logical address of "good" values
F(ix)=x(2).*((xdata(ix)-x(1)).^x(3)).*exp(-(xdata(ix)-x(1))/x(4));

Or, you could just go ahead and compute the complex values, then zero them out...

    F=x(2).*((xdata-x(1)).^x(3)).*exp(-(xdata-x(1))/x(4));
    F(xdata<=x(1))=0;   % zero those needed to be

[Oliver]

duh. i knew i was missing something obvious, thanks, dpb! i really appreciate your code example, too.

Answer 2

[Adam]

If you call

xData <= x(1)

when xData is a vector you will get a vector of results of the same length.

You need to use

all( xData <= x(1) )

if you want a single value representing that all the data in your array meet the condition.

any( xData <= x(1) )

would be the equivalent if you just want to check that at least one value meets the condition.

[Oliver]

Thanks, Adam!