Re: Kludge for handling REPL arithmetic expressions

[Chris Elvidge]

On 04/08/2021 12:39 pm, Greg Wooledge wrote:
> On Wed, Aug 04, 2021 at 11:12:01AM +0100, Chris Elvidge wrote:
> > I was intrigued by this. So tried:
> > =() { echo "scale=5; $@" | bc; }
>
> I'd suggest $* here.  You don't really want to preserve separate
> arguments in this case.
>
> Also, you really want bc -l, which also means you really don't want
> to reduce the scale to merely 5.  (If you want to reduce the
> percision of the answer, capture it with a command substitution, and
> then feed it to printf for rounding.  Reducing the scale within bc
> leads to extremely wrong answers in some cases.)
>
> > Seems to work for simple calculations.
> >
> > A couple of questions, though.
> > 1) How do you 'export -f ='? Or is it impossible?
>
> Probably impossible, as it is an invalid shell "identifier".
>
> > 2) How do you type in '= log(2048,2)' without getting a syntax error?
>
> With quotes.  Also, "log()" is spelled "l()" in bc.
>
> unicorn:~$ =() { echo "$*" | bc -l; }
> unicorn:~$ = 'l(2048.2)'
> 7.62471663764133723968
>
> Or with rounding:
>
> unicorn:~$ =() { local n; n=$(echo "$*" | bc -l) || return; printf '%.5f\n' 
> "$n"; }
> unicorn:~$ = 'l(2048.2)'
> 7.62472
>
> This is a cute little experiment, but it's not a path I'd choose for
> myself.  What I do instead is keep a dedicated terminal window open
> in a specific spot on my screen, with bc -l running interactively.
> If I need to calculate something, I just focus on that window and type
> my expressions there.
>
> Also, I guess checking bc's exit status is pointless...
>
> unicorn:~$ = 1/0
> Runtime error (func=(main), adr=3): Divide by zero
> 0.00000
> unicorn:~$ echo '1/0' | bc -l
> Runtime error (func=(main), adr=3): Divide by zero
> unicorn:~$ echo $?
> 0
>
> *sigh*

Thanks Greg (and Eric for the idea and leading me to rediscover Euclid's 
Algorithm).

I've reduced it to:
=() { bc -l <<<"$*"; }

and (using d for /)

command_not_found_handle ()
{
    if [[ "$1" != =* ]]; then
        printf "%s: command not found\n" "$1" 1>&2;
        return 127;
    fi;
    set -- ${*/d/\/};
    set -- ${*#=};
    printf '%.3f\n' $(bc -l <<<"$*")
}


--
Chris Elvidge
England