if statement returns false to checking if 0.05 +0.01 = 0.06

Hi Everyone,

I'm trying to write a .m script that would determine if points in an array are within 0.01 of each other (i.e. 0,0.01,0.02,0.03) which would indicate the points are 'adjacent', as opposed to (0,86,0.98, etc). To do that I ask over the size of the array, is this element + 0.01 = the next element (as seen below in the code). If that's true, do some stuff. However if you check the print out of the data (under Sample date) it checks 0.05 and finds it is not within 0.01 of 0.06 (returns 0 also known as false)!

This is such a basic logic problem I don't know where to start. Anyone have any idea what's going on?

for j = 1 : 1 : ij
  if j<ij
      lin_SOC(j)
      lin_SOC(j)+0.01==lin_SOC(j+1)
   if lin_SOC(j)+0.01==lin_SOC(j+1) %if this is true then points are ajacent (i.e. in same region)
       if region_size==0 % if region size is 0 we're at the beginning of a new linear region
           lin_start_point =[lin_start_point;lin_SOC(j)]
           region_size
       end
       region_size=region_size+1;
   else
        if region_size~=0 % if region size is 0 we're at the beginiong of a new linear region
           lin_end_point =[lin_end_point;lin_SOC(j)]
           region_size
       end

         num_regions=num_regions+1; %number of non-linear regio
         region_size=0; %reset nonlinear region size counter
     end
    end

   end

~~Sample Data~~

lin_SOC=
    0.0500
ans =
     0
lin_SOC =
    0.0600
ans =
     0
lin_SOC =
   0.0700
ans =
     1
lin_start_point =
    0.0700
region_size =
     0
lin_SOC =
    0.0800
ans =
     1
lin_SOC =
    0.0900
ans =
     0

Answer 1

[Stephen23]

This is the comparison you are trying:

>> (0.05+0.01) == 0.06
ans =
     0

But when you compare floating point numbers you should use a tolerance value like this:

>> tol = 1e-3;
>> abs((0.05+0.01) - 0.06) <= tol
ans =
     1

And you should read about floating point numbers to know why this is required:

http://www.mathworks.com/help/matlab/matlab_prog/floating-point-numbers.html

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

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

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

http://www.mathworks.com/company/newsletters/news_notes/pdf/Fall96Cleve.pdf

http://www.mathworks.com/matlabcentral/answers/41536-why-is-2-24-10-22-4-not-equal-to-0

https://www.mathworks.com/matlabcentral/newsreader/view_thread/66553

http://www.mathworks.com/company/newsletters/news_notes/pdf/Fall96Cleve.pdf

http://blogs.mathworks.com/loren/2006/08/23/a-glimpse-into-floating-point-accuracy/

http://www.mathworks.com/help/fixedpoint/ug/floating-point-numbers.html

And some external links on this topic:

http://www.lahey.com/float.htm

http://matlabgeeks.com/tips-tutorials/floating-point-comparisons-in-matlab/

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