Strange behaviour with elseif

Hi, I noticed something totally strange with a repeated elseif statement. Here is the code:

A = [0.0001:0.0001:0.0009];

for i=1:length(A)

      x=A(i);
      if x == 0.0001
          x
      elseif x == 0.0002
          x
      elseif x == 0.0003
          x
      elseif x == 0.0004
          x
      elseif x == 0.0005
          x
      elseif x == 0.0006
          x
      elseif x == 0.0007
          x
      elseif x == 0.0008
          x
      elseif x == 0.0009
          x
      else
          display('Error')
      end
end

When I run it, I get twice the "Error" displayed, for x=0.0003 and x=0.0008. I could reproduce it on two different Matlab version (R2009a & R2010a). Any idea? Thanks a lot.

Answer 1

[Andreas Goser]

The behaviour you observe is not a bug, but a result of normal numerical effects on different installations (surely processors, OS; maybe releases). A bit background reading:

http://www.mathworks.com/support/solutions/en/data/1-16FOQ/?solution=1-16FOQ

It is not a best practice to do comparisions like that. My guess is that there is a fundamentally different way for you to write the code, but in case you like it that way, I suggest introducing the comparison with a tolarance like that:

A=0.0001;
A_test=0.0001+eps;

if A==A_test
    disp('equal')
else
    disp('not equal')
end

if abs(A-A_test)<(2*eps)
    disp('numerically equal')
else
    disp('numerically not equal')
end