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Full Discussion: On how to select the right tool for a given task
Top Forums Shell Programming and Scripting On how to select the right tool for a given task Post 302248917 by bakunin on Monday 20th of October 2008 06:10:35 AM
Old 10-20-2008
Quote:
Originally Posted by Perderabo
So, datecalc is not meaningful? I'm hurt... :(
Sorry! ;-)) Of course it is meaningful. I was thinking about the scripts i was writing and couldn't find a single one without any external program. You are correct in pointing out datecalc to me, because things like this (which are quite "abstracted" from concrete problems) i have put into external ksh functions and use these so frequently that i don't think about how they are constructed. I shouldn't have looked into my scripts but my functions. Anyway, i stand corrected and offer my apologies.

Quote:
When I get some time I'll benchmark rot13 in pure ksh vs tr.
I have done that and this was the real surprise for me. Here are the scripts i used (yes, i used ksh93. In AIX the default shell is ksh88 still, but i didn't want to reinvent the substring function of ksh93):

Code:
#!/bin/ksh93

exec 3>&1
exec 1>/dev/null
typeset -i iCnt=1

while [ $iCnt -le 1000 ] ; do
     var="abcde"
     print - $var | tr 'abcdefghijklmnopqrstuvwxyz' 'mnopqrstuvwxyzabcdefghijklm
'
     (( iCnt += 1 ))
done

exec 1>&3
exit 0

Code:
#!/bin/ksh93

exec 3>&1
exec 1>/dev/null
typeset -i iCnt=1

while [ $iCnt -le 1000 ] ; do
     var="abcde"
     typeset -i j=1
     while [ $j -le ${#var} ] ; do
          case ${var:$j:1} in
               a) print -f "%s" "m" ;;
               b) print -f "%s" "n" ;;
               c) print -f "%s" "o" ;;
               d) print -f "%s" "p" ;;
               e) print -f "%s" "q" ;;
               f) print -f "%s" "r" ;;
               g) print -f "%s" "s" ;;
               h) print -f "%s" "t" ;;
               i) print -f "%s" "u" ;;
               j) print -f "%s" "v" ;;
               k) print -f "%s" "w" ;;
               l) print -f "%s" "x" ;;
               m) print -f "%s" "y" ;;
               n) print -f "%s" "z" ;;
               o) print -f "%s" "a" ;;
               p) print -f "%s" "b" ;;
               q) print -f "%s" "c" ;;
               r) print -f "%s" "d" ;;
               s) print -f "%s" "e" ;;
               t) print -f "%s" "f" ;;
               u) print -f "%s" "g" ;;
               v) print -f "%s" "h" ;;
               w) print -f "%s" "i" ;;
               x) print -f "%s" "j" ;;
               y) print -f "%s" "k" ;;
               z) print -f "%s" "l" ;;
          esac
          (( j += 1 ))
     done
     print -f "\n"
     (( iCnt += 1 ))
done

exec 1>&3
exit 0

The the "external" version is "xtime1.sh", the "shell-only" version xtime2.sh:

Code:
bakunin@aix5200-10-03-0744/~# time ./xtime1.sh

real    0m4.70s
user    0m0.33s
sys     0m4.07s

bakunin@aix5200-10-03-0744/~# time ./xtime2.sh

real    0m0.12s
user    0m0.12s
sys     0m0.00s

I reasoned that the tr command has to be executed completely while the long case..esac construct is not completely used because i used characters from the beginning of the alphabet. So changed "$var" above to the complete alphabet and ran again:

Code:
bakunin@aix5200-10-03-0744/~# time ./xtime1.sh

real    0m4.82s
user    0m0.29s
sys     0m4.20s
bakunin@aix5200-10-03-0744/~# time ./xtime2.sh

real    0m0.77s
user    0m0.69s
sys     0m0.06s

You see that the length of the string indeed has some impact but not nearly as much as i would have expected. The tr-version was not impacted by the length of the string, because the most expensive part is the call of tr itself and the work it does is negligeable by comparison.

So also in this case i stand corrected. I heavily understimated the "cost" of system calls and would have supposed that - because the implementation is way longer - there would be a tradeoff. Obviously this is not nearly the case.

Anyway, it was interesting to find out all this and i am glad i still can learn something about shell programming every day. ;-))

bakunin
This User Gave Thanks to bakunin For This Post:
editha1000
 

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