MATLAB: == operator says two identical arrays aren’t the same

==arrayMATLAB

I have an array which I call graph and it is made like this.
graph = sin(1:pi/10:10*pi);
There's a variable called period_two.
period_two = 22;
Then period_two is used to make two arrays out of the first one.
graph(2:period_two-1)
graph(period_two:2*period_two-3)
And the output MatLab gives me is:
graph(2:period_two-1)
ans =
    Columns 1 through 8
      0.9672    0.9983    0.9317    0.7739    0.5403    0.2538   -0.0575   -0.3632
    Columns 9 through 16
     -0.6333   -0.8415   -0.9672   -0.9983   -0.9317   -0.7739   -0.5403   -0.2538
    Columns 17 through 20
      0.0575    0.3632    0.6333    0.8415
and
graph(period_two:2*period_two-3)
ans =
    Columns 1 through 8
      0.9672    0.9983    0.9317    0.7739    0.5403    0.2538   -0.0575   -0.3632
    Columns 9 through 16
     -0.6333   -0.8415   -0.9672   -0.9983   -0.9317   -0.7739   -0.5403   -0.2538
    Columns 17 through 20
      0.0575    0.3632    0.6333    0.8415
But if I use the == operator to check if the two arrays are the same, I get.
graph(2:period_two-1) == graph(period_two:2*period_two-3)
ans =
       1     1     0     0     0     0     0     0     0     0     1     1     0     0     0     0     0     0     0     0
Can you tell me why it says that the most of the values are not the same, when I can see that the two arrays are identical?

Best Answer

Floating point numbers are not infinitely precise, and you probably have differences too small to be noticed when displaying only 4 digits after the decimal.
Read this topic on determining equality, specifically the discussion about eps.
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