[R] strange issue with "which" on "seq"

[james.foadi at diamond.ac.uk]

Dear R community,
I am puzzled by the following lines:

> v <- seq(-0.5,0.5,by=0.1)
> v
  [1] -0.5 -0.4 -0.3 -0.2 -0.1  0.0  0.1  0.2  0.3  0.4  0.5
> which(v == -0.4)
  [1] 2
> which(v == 0)
  [1] 6
> which(v == 0.1)
integer(0)
> which(v == 0.2)
integer(0)
> which(v == 0.3)
integer(0)
> which(v == 0.4)
  [1] 10
> which(v == 0.5)
  [1] 11

Why "which" can only match some of the values in "v"? Are the numbers generated by "seq" not exact fractional numbers?
Please, help me to understand this.

J

Dr James Foadi PhD
Membrane Protein Laboratory (MPL)
Diamond Light Source Ltd
Diamond House
Harewell Science and Innovation Campus
Chilton, Didcot
Oxfordshire OX11 0DE

Email    :  james.foadi at diamond.ac.uk
Alt Email:  j.foadi at imperial.ac.uk

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