Minute error in array causes == to be ineffective

Hi, I am trying to use an if statement using the == comparator to search an array of numbers. Something like this:

for i = 1:100
     if a == array(i,1)
          % do stuff here
     end
end

Doing this, however, leads to the if statement never being true. When I would expect a and the array value to be equal, the statement is still false, due to a very minute difference of something around the order of 10^-8.

What is causing this very small error and what are some of the workarounds you guys suggest? Alternatively, is there a smarter way to search an array of numbers?

Answer 1

[Wayne King]

Hi Elliot, this is a reality of dealing with floating point numbers. You could use a tolerance

a = randn(10,1);
for ii = 1:length(a)
  if (abs(a(ii)-0)<1e-6)
        disp('true');
  else
        disp('false');
end
end

For example

    x = 1+1e-7;
    y = 1.000000;

They look the same, but

    abs(x-y)<1e-6

returns a 1 (true)

Answer 2

[Walter Roberson]

http://matlab.wikia.com/wiki/FAQ#Why_is_0.3_-_0.2_-_0.1_.28or_similar.29_not_equal_to_zero.3F