Are BigFloats unable to exactly represent 0.1?

0.1 is a number that can be exactly represented by a Float16, or Float32, or a Float64.
(at least according to how we print them)
But not it would seem (at least according to printing) by a BigFloat.

julia> parse(Float64, "0.1")
0.1

julia> parse(Float32, "0.1")
0.1f0

julia> parse(Float16, "0.1")
Float16(0.1)

julia> parse(BigFloat, "0.1")
0.1000000000000000000000000000000000000000000000000000000000000000000000000000002

Is this true? Or is it printing wrong?

(I suspect the answer is going to be insightful about floating point representations and printing being round-trip-able with parsing)

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julia> 1//10 < 0.1
true
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As Jeffrey pointed out, this assumption is wrong. Another way to see it:

julia> @printf "%.20f" 0.1f0
0.10000000149011611938
julia> @printf "%.50f" 0.1
0.10000000000000000555111512312578270211815834045410
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You can also prove by hand that 1/10 is an (infinitely) repeating decimal in base-2, which is why it can’t be represented exactly by binary floating-point with any finite precision: https://softwareengineering.stackexchange.com/a/237018

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How does the show method know that we want to see 0.1 and not the real value that’s stored?

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It’s for the same reason that 0.1 + 0.2 != 0.3 (https://0.30000000000000004.com)

Julia prints the shortest value that uniquely identifies the float in question when parsed back. The algorithm in question is called “Ryu”, you can find the julia implementation here.

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Is it that we don’t use Ryu for BigFloat but instead use some inferior algorithm?

BigFloat is really a wrapper around the MPFR library, which has its own printing algorithm.

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And for fun

julia> bitstring(3)
"0000000000000000000000000000000000000000000000000000000000000011"

julia> bitstring(1.0)
"0011111111110000000000000000000000000000000000000000000000000000"

julia> bitstring(0.1)
"0011111110111001100110011001100110011001100110011001100110011010"

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BigFloats are binary floating point and it CANNOT exactly represent 0.1 because it is base-2 instead of base-10

I think part of the confusion is that the Julia expression 0.1 is a Float64 close, but not equal to 1//10. So Float64(0.1) is really the same thing as 0.1. I try always to be careful to distinguish 0.1 and 1/10 = 0.1 which are different (Julia left and common number right.)

This also implies that a == 0.30000000000000004 is the correctly rounded answer to the question 0.1 + 0.2 == ? with rounding error 0.00000000000000002 (and not 0.00000000000000004 as one might think.)

With some custom pretty-printing on top.

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