Issue with find() function

If I create an array x = 0.1:0.05:0.6; and then ask find(x==0.15), I get an empty matrix. It also fails for x==0.40? I don't understand this at all.

[Cedric]

If you understand the answers below, read about EPS. In the end, you will probably end up doing something like:

 pos = find( abs(x - 0.15) < eps(1) ) ;

Answer 1

[Star Strider]

It’s called ‘floating point approximation error’. The concept is similar to the decimal approximation of 1/3 = 0.33333.... To get around it, you need to state the conditions in your find calls to allow for some imprecision.

Run these to get an idea of how the concept applies to your problem:

x = 0.1:0.05:0.6;

ix11 = find(x  <= 0.15, 1, 'last')
ix12 = find(x  >= 0.15, 1, 'first')
x1 = x([ix11  ix12])

ix21 = find(x <= 0.40, 1, 'last')
ix22 = find(x >= 0.40, 1, 'first')
x2 = x([ix21 ix22])

Answer 2

[Azzi Abdelmalek]

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

Answer 3

[Image Analyst]

Here's some pretty general code based on the FAQ Azzi referred you to:

x = 0.1 : 0.05 : 0.6  % Sample data
targetValue = 0.15  % Whatever value you want to find
tolerance = .005  % We want to find x within this value of the target.

% Find values in range.
indexesInRange = abs(x - targetValue) <= tolerance
xThatAreInRange = x(indexesInRange)

% Or, in an if statement, if you want to use it in an if statement instead...
if abs(x(2) - targetValue) <= tolerance
  % x(2) is close enough to the targetValue.
else
  % x(2) is far away from the targetValue.
end