array element not equal to equivalent double

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]

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]

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]

thanks :)