array element not equal to equivalent double

Question

Pierre-Vincent 2022-11-15

评论： Pierre-Vincent 2022-11-15

I have the following code:

array=0.0001:0.0001:0.6;
x=0.0402
x = 0.0402
y=array(402)
y = 0.0402
test=x==y
test = logical
   0

the result of the test is 0 and I really don't understand why... Both are supposed to be double. Can somebody help?

(the final idea is to find the index of X in the array with the find function)

Answer 1

Matt J 2022-11-15

编辑：Matt J 2022-11-15

Because computers cannot do exact math. The difference between x and y beyond the 4th decimal place make this clear:

array=0.0001:0.0001:0.6;
x=0.0402
x = 0.0402
y=array(402)
y = 0.0402
x-y
ans = -6.9389e-18

Matt J 2022-11-15

编辑：Matt J 2022-11-15

Instead, you can do,

array=0.0001:0.0001:0.6;
x=0.0402;
idx=interp1(array,1:numel(array),x,'nearest')
idx = 402

Pierre-Vincent 2022-11-15

thanks :)