Problem finding find command

Hello, I am facing this very irrating probelm using matlab find() command. I have matrix with angle values from 0 till 180 and I wanted to find a index of particular angle. I prompt user to enter angle to find and after getting input I store in a variable "x" and then using find command i.e. find(a.angle.value==x) I try to find index but its giving me empty matrix except for angles 0 and 180 degrees any other angle in between its giving me empty matrix. My angles goes from 0 till 180 with a step size of 2 degrees so there are no decimal point issue. I even tried to put with four decimal points like 20.0000 but still giving me empty matrix. Any ideas what went wrong ?

Please find below the Sample data

a.angle.value=[0 1.99999999771467 3.99999999542933 5.99999999314400   ........ 180].

[Stephen23]

@antennist: please upload some sample data that shows this behavior. Do this by editing your question and clicking on the paperclip button. Please also add a complete minimum working example that shows this behavior (i.e. just the code that will let us replicate the effect that you are observing).

[Stephen23]

@antennist: I am guessing that you come from a culture that uses the comma for the decimal radix, and you actually meant to use the period (as is used in English):

 a.angle.value = [0, 1.99999999771467, 3.99999999542933, 5.99999999314400   ........ 180]

[antennist]

yes in here we use comma as a point. My apologies I have edited my post

Answer 1

[Stephen23]

Floating point numbers are how decimal values are stored in your computer: they do not have infinite precision, and they can only approximate many values. Even if it looks like the "actual" value is being shown to you this does not mean that the internal representation inside your computer is the same value.

Of course this means that every operation you perform on floating point values will accumulate these errors. It is up to you (and every other programmer) to design programs that take this "floating point error" into account.

If you actually gave us some data we could show this to you.

Read all about it:

http://www.mathworks.com/matlabcentral/answers/69-why-does-1-2-3-1-3-not-equal-zero

http://www.mathworks.com/matlabcentral/answers/57444-faq-why-is-0-3-0-2-0-1-not-equal-to-zero

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

And if you want to see what the decimal equivalent of a floating point value is, then try this FEX submission:

http://www.mathworks.com/matlabcentral/fileexchange/22239-num2strexact--exact-version-of-num2str-

As its author points out: "Don't confuse the exact conversion with significance!"

EDIT after antennist added actual values to their question:

What value do you expect 1.99999999771467 to be equal to? Is 1.99999999771467 equal to 2 ? Do you expect 2==1.99999999771467 to be true ?

"Any ideas what went wrong ?"

There is nothing wrong with find. However comparing floating point numbers for equality is a very bad idea, because after calculating or even defining floating point numbers they can have small differences that are not visible when you look at them in the command window (or variable viewer, etc), but those differences mean that the values are not equal.

To compare floating point values in a robust way, then you should include a tolerance in the comparison:

>> tol = 0.1;
>> num = 4;
>> vec = [0, 1.99999999771467, 3.99999999542933, 5.99999999314400];
>> find(abs(vec-num)<tol)
ans =  3

[antennist]

@Stephen Cobeldick I think I got your point. When I copy paste the original data here now I can see floating point issue. Does that mean I have to add tolerance limit ?

[Stephen23]

@antennist: yes, see my edited post.

And in future: never rely on what you believe your code is doing, always look at what the code is really doing.

[antennist]

@Stephen Cobeldick Thanx alot for your help and I will keep your advice in mind.

[Stephen23]

@antennist: I hope that you got is working. If my answer helped you to resolve your question, then please accept it, which is an easy way for you to say "thank you" to us volunteers.