Post by @aesthr@wandering.shop

Anyone who doesn't know about this really shouldn't be speaking on what computers are supposedly good at

(This is not a bug. This is documented and expected behavior in pretty much every widely used programming language today. See: IEEE 754)

R console screenshot:

> 0.1 + 0.2 == 0.3
[1] FALSE — R console screenshot: > 0.1 + 0.2 == 0.3 [1] FALSE

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Ferret

@ferret@hub.workersofthe.world 1 hr. ago

@Æ. bash seems cool with it

if [[ 0.1+0.2==0.3 ]]; then echo "true"; fi
true

Æ.

@aesthr@wandering.shop 42 min. ago

@ferret yeah, but that's not great either, right? because it diverges from the behavior of standard floating point numbers that's established nearly everywhere else

Æ.

@aesthr@wandering.shop 40 min. ago

@ferret actually, it's much worse

bash doesn't do floating point at all. I didn't know until today

the bad thing here is that it fails silently without a warning or error

esther@Jupiter:~$ if [[ 0.1+0.2==42 ]]; then echo "true"; fi
true — esther@Jupiter:~$ if [[ 0.1+0.2==42 ]]; then echo "true"; fi true

Zack Weinberg

@zwol@masto.hackers.town 13 min. ago

@aesthr @ferret it's even worse than that; this isn't doing arithmetic at all! there are no spaces in "0.1+0.2==42" and so `[[ 0.1+0.2==42 ]]` is interpreted as equivalent to `[[ -n "0.1+0.2==42" ]]`, i.e. test whether the *string* "0.1+0.2==42" is *not empty*

Zack Weinberg

@zwol@masto.hackers.town 12 min. ago

@aesthr @ferret it's quite difficult to get bash to do arithmetic inside [[ ]]; i had to resort to $(( ))

$ (set -x; [[ 0.1 + 0.2 == 42 ]]; echo $?)
bash: unexpected token `+', conditional binary operator expected
bash: syntax error near `+'

$ (set -x; [[ (0.1 + 0.2) == 42 ]]; echo $?)
bash: unexpected token `+', conditional binary operator expected
bash: expected `)'
bash: syntax error near `+'

(cont'd...)

Zack Weinberg

@zwol@masto.hackers.town 11 min. ago

@aesthr @ferret

$ (set -x; [[ ( 0.1 + 0.2 ) == 42 ]]; echo $?)
bash: unexpected token `+', conditional binary operator expected
bash: expected `)'
bash: syntax error near `+'

but finally

$ (set -x; [[ $(( 0.1 + 0.2 )) == 42 ]]; echo $?)
bash: 0.1 + 0.2 : arithmetic syntax error: invalid arithmetic operator (error token is ".1 + 0.2 ")

and

$ (set -x; [[ $(( 1 + 2 )) == 42 ]]; echo $?)
+ [[ 3 == 42 ]]
+ echo 1
1

(but that last [[ 3 == 42 ]] is still a string comparison!)

Zack Weinberg

@zwol@masto.hackers.town 7 min. ago

@aesthr @ferret this is one of the many reasons why I tend to tell beginners "the moment something starts to look complicated or difficult to do in shell, the moment you are tempted to reach for _any bash extension at all_, that is the moment you should stop and rewrite the entire script in _any other language_, I don't care which one, as long as it isn't Tcl or C shell, you'll be having a better time than with Bourne shell".