MATLAB: Problem with ismember with certain numbers in a loop

Question

Here is the code I have writen, as far as I can see B should alternating 1 and 0 (B=1, 0, 1, 0, 1 etc) but at (as far as I can tell) random points it does not do that. I can't see what I have done wrong and I don't know if I have misunderstood something with "ismember".

I have noticed this with number=0.3250 for example, ismember(number, setcom)=0 but ismember(0.3250, setcom)=1. If however I pause the code type in compand line "number" I get 0.3250 and I get ismember(number, setcom)=0, but if in the comand line I also type "number =0.3250" and then ismember(number, setcom)=1.

I don't understand why it works for most numbers in the loop but not all. Any help would be very much appreciated.

set=[0.125:0.0125:0.5];

setcom=[0.125:0.025:1];

B=zeros(length(set), 1);

for i=1:length(set)

number=set(i);

compare=ismember(number,setcom);

B(i,1)=compare;

end

Answer 1

Use ismembertol instead. You are likely dealing with ‘floating-point approximation error’. See Floating-Point Numbers for a full discussion.