#!/bin/bash

# Copyright (c) Michael Still 2002
# Released under the terms of the GNU GPL

# In this case, Nemo wants to be able to specify a list of items, with
# weights associated with them...

# $1 is the list with weights, in the form:
# "1 frog 2 banana 3 hamster"

# Scary assumption number one, people hand me correctly formatted lists
# Incidentally, this will break with numbers exist in the items I am handed
# e.g. Banana42 will break this
NUMBERS=`echo $1 | sed 's/[^0-9 ]//g'`
WORDS=`echo $1 | sed 's/[0-9]//g'`
WEIGHTED=""

# Build the list of options, including the weights
for NUM in $NUMBERS
do
  WORD=`echo $WORDS | sed 's/ .*$//'`
  WORDS=`echo $WORDS | sed "s/^$WORD *//"`

  COUNT=0
  while [ $COUNT -lt $NUM ]
  do
    WEIGHTED=`echo "$WEIGHTED $WORD"`
    COUNT=$(( $COUNT + 1 ))
  done  
done

# Get the random number
LOBOUND=1
HIBOUND=`echo $WEIGHTED | wc -w`
RANDMAX=32767
BINUMBER=$(( $LOBOUND + ($HIBOUND * $RANDOM) / ($RANDMAX + 1) ))

# Get the item -- I can't use shift, because it is not on the command line
COUNT=1
while [ $COUNT -lt $BINUMBER ]
do
  WEIGHTED=`echo $WEIGHTED | sed 's/^[^ ]*//'`
  COUNT=$(( $COUNT + 1 ))
done

# The first word should be magic selected one
echo $WEIGHTED | sed 's/ .*$//'

• 1: 9964
• 2: 10009
• 3: 9978
• 4: 10015
• 5: 9997
• 6: 10020
• 7: 10011
• 8: 10008
• 9: 10006
• 10: 9992

From mikal@stillhq.com Fri Mar 29 10:26:04 2002
Date: Fri, 29 Mar 2002 10:14:17 +1100 (EST)
From: Michael Still 
To: Linux user group 
Subject: Nemo's bash challenge for the day


Well, I said it could be done...

The brief: Generate a random number, and then return that element from a
list of elements, in bash

The code: (Assuming that the arguements on the command line are the
possible return options, and that the random number generator is running
as a separate script)

LOBOUND=0
HIBOUND=$#
shift $(( $LOBOUND + ($HIBOUND * $RANDOM) / (32767 + 1) ))
echo $1

See the attachments for some exploratory scripts I wrote while coming up
with this truncated sh. There are 54 lines of comments / white spaces, to
the 4 or so lines of actual code.

Have a nice life...

Cheers,
Mikal

PS: How good is the bash random? See the attachment output.count for a
summary of 100,000 numbers between 1 and 10 being generated with the
default seed. It's probably good enough for most people.

PPS: If you want the code, it should also be online at
http://www.stillhq.com/cgi-bin/getpage?area=bashrand&page=index.htm in
about 30 minutes, depending when Andrew goes to sleep.

-- 

Michael Still (mikal@stillhq.com)     UMT+11hrs

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    [ Part 4, ""  Text/PLAIN (Name: "output.count")  2 lines. ]
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#!/bin/bash

# Select a specified item from a list. Copyright (c) Michael Still 2002
# Released under the terms of the GNU GPL

# $1 is the number to get, $* except for $1 is the list of options, delimited
# by a space each

# We can the shift operation to get to the right number
shift $1
echo $1

#!/bin/bash

# Generate a random number. Copyright (c) Michael Still 2002
# Released under the terms of the GNU GPL
#
# (Is it possible to copyright a single line of code?)

# To quote from the rand manpage as to why we bound the random number this way:
#
#       In Numerical Recipes in C: The Art of Scientific Computing
#       (William  H.  Press, Brian P. Flannery, Saul A. Teukolsky,
#       William T.  Vetterling;  New  York:  Cambridge  University
#       Press, 1992 (2nd ed., p. 277)), the following comments are
#       made:
#              "If you want to generate a random integer between 1
#              and 10, you should always do it by using high-order
#              bits, as in
#
#                     j=1+(int) (10.0*rand()/(RAND_MAX+1.0));
#
#              and never by anything resembling
#
#                     j=1+(rand() % 10);
#
#              (which uses lower-order bits)."

# To seed the random number generator, set RANDOM to a value... We can see
# that the bash code (2.05a in this case) already does some seeding for us...
#
# brand ()
# {
#  rseed = rseed * 1103515245 + 12345;
#  return ((unsigned int)((rseed >> 16) & 32767));       /* was % 32768 */
# }
#
# Here I have an example using the current time, which wont work well with
# multiple calls per second
#RANDOM=`date | tr -d ":" | cut -f 4 -d " "`

# These variables just make the equation easier to read, and are not needed
LOBOUND=1
HIBOUND=10
RANDMAX=32767
BINUMBER=$(( $LOBOUND + ($HIBOUND * $RANDOM) / ($RANDMAX + 1) ))

echo $BINUMBER