Re: efficient way to use matched string in variable substitution

[Greg Wooledge]

On Tue, Aug 24, 2021 at 01:31:35PM +0200, Léa Gris wrote:
> Le 23/08/2021 à 21:41, L A Walsh écrivait :
> > 
> > 
> > On 2021/08/23 12:10, Greg Wooledge wrote:
> > > On Mon, Aug 23, 2021 at 11:36:52AM -0700, L A Walsh wrote:
> > > > Starting with a number N, is there
> > > > an easy way to print its digits into an array?
> 
> n=988421
> 
> # Need extglob for the replacement pattern
> shopt -s extglob
> 
> # Split string characters into array
> IFS=' ' read -r -a array <<<"${n//?()/ }"
> 
> # Debug print array
> declare -p array

unicorn:~$ f7() { local arr; IFS=' ' read -ra arr <<< "${1//?()/ }"; }
unicorn:~$ time for ((i=1; i<=10000; i++)); do f7 682390; done
real 0.393  user 0.358  sys 0.022

Interesting.  I expected that one to be slower, with the overhead for
the here string.  You may note that this one has some "sys" time,
unlike the others.  E.g.

unicorn:~$ f6() { local i n=${#1} arr; for ((i=0; i<n; i++)); do 
arr[i]="${1:i:1}"; done; }
unicorn:~$ time for ((i=1; i<=10000; i++)); do f6 682390; done
real 0.381  user 0.381  sys 0.000

Looks like the efficiency of "read -ra" vs. a shell loop just about makes
up for the system calls used for the here string (f6 and f7 are almost
tied in overall speed, with f6 just a *tiny* bit faster).  Good to know.