Hi,
I'm doing a little write-up on Java basics and comparing some of them
to Clojure (things like mutable shared state, side effects and so
on). When I came to "numbers" I was surprised by some of the things I
found in Clojure.
(== (double 0.5) (float 0.5)) ;; -> true
(== (double 0.2) (float 0.2)) ;; -> false
The docs
(https://clojure.github.io/clojure/clojure.core-api.html#clojure.core/==)
say, that `==` compares the numbers type-independently. But why are
the two __representations__ (types?) of the numerical value `0.2`
different then?
I understand that `(float 0.2)` gets _converted_ to `double` and this
conversion is done like this (just the way Java does it -- should be
`f2d` in byte code).
(double (float 0.2)) ;; -> 0.20000000298023224
So that's not equal to `(double 0.2)`. But why not convert `float` to
`double` like this:
(defn to-double [f]
(Double. (str f)))
Here we're not converting the (inexact) `float` approximation of `0.2`
to `double` but we use what human readers perceive as the (exact)
_numerical value_.
This would parallel the way `BigDecimal` literals work in Clojure:
0.3M ;; -> 0.3M
(BigDecimal. 0.3) ;; ->
0.299999999999999988897769753748434595763683319091796875M
(BigDecimal. (str 0.3)) ;; -> 0.3M
When we use numbers in sets and maps, more questions come up:
(into #{} [0.5 (float 0.5)]) ;; -> #{0.5}
(into #{} [0.2 (float 0.2)]) ;; -> #{0.2 0.2}
First it seems that `==` is used to check for equality, but I think
it's not the case.
(= 0.5 (float 0.5)) ;; -> true
Ahh -- seems that `(float 0.5)` gets converted to `double` before
comparing via `=`.
Getting `#{0.2 0.2}` is bad: we won't be able to read this set literal
back in.
#{0.2 0.2} ;; -> java.lang.IllegalArgumentException: Duplicate key: 0.2
So my question is: does anyone know about tutorials, docs etc. on
clojure.org or elsewhere where I can find advices/best practice/a
"clojure specifications" (like the JLS does it for Java) on this
topic.
Henrik
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