randomvar.html

一本完整的描述Unix Shell 编程的工具书的所有范例

 147 
 148 displacement=$((0-minimum))
 149 for ((i=${minimum}; i<=${maximum}; i+=divisibleBy)); do
 150    answer[i+displacement]=0
 151 done
 152 
 153 
 154 # Now loop a large number of times to see what we get.
 155 loopIt=1000   #  The script author suggests 100000,
 156               #+ but that takes a good long while.
 157 
 158 for ((i=0; i<${loopIt}; ++i)); do
 159 
 160    #  Note that we are specifying min and max in reversed order here to
 161    #+ make the function correct for this case.
 162 
 163    randomBetween ${max} ${min} ${divisibleBy}
 164 
 165    # Report an error if an answer is unexpected.
 166    [ ${randomBetweenAnswer} -lt ${min} -o ${randomBetweenAnswer} -gt ${max} ] && echo MIN or MAX error - ${randomBetweenAnswer}!
 167    [ $((randomBetweenAnswer%${divisibleBy})) -ne 0 ] && echo DIVISIBLE BY error - ${randomBetweenAnswer}!
 168 
 169    # Store the answer away statistically.
 170    answer[randomBetweenAnswer+displacement]=$((answer[randomBetweenAnswer+displacement]+1))
 171 done
 172 
 173 
 174 
 175 # Let's check the results
 176 
 177 for ((i=${minimum}; i<=${maximum}; i+=divisibleBy)); do
 178    [ ${answer[i+displacement]} -eq 0 ] && echo "We never got an answer of $i." || echo "${i} occurred ${answer[i+displacement]} times."
 179 done
 180 
 181 
 182&nbsp;exit 0</PRE
></TD
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><HR></DIV
><P
>Just how random is <TT
CLASS="VARNAME"
>$RANDOM</TT
>? The best
	  way to test this is to write a script that tracks
	  the distribution of <SPAN
CLASS="QUOTE"
>"random"</SPAN
> numbers
	  generated by <TT
CLASS="VARNAME"
>$RANDOM</TT
>. Let's roll a
	  <TT
CLASS="VARNAME"
>$RANDOM</TT
> die a few times . . .</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="RANDOMTEST"
></A
><P
><B
>Example 9-27. Rolling a single die with RANDOM</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# How random is RANDOM?
   3&nbsp;
   4&nbsp;RANDOM=$$       # Reseed the random number generator using script process ID.
   5&nbsp;
   6&nbsp;PIPS=6          # A die has 6 pips.
   7&nbsp;MAXTHROWS=600   # Increase this if you have nothing better to do with your time.
   8&nbsp;throw=0         # Throw count.
   9&nbsp;
  10&nbsp;ones=0          #  Must initialize counts to zero,
  11&nbsp;twos=0          #+ since an uninitialized variable is null, not zero.
  12&nbsp;threes=0
  13&nbsp;fours=0
  14&nbsp;fives=0
  15&nbsp;sixes=0
  16&nbsp;
  17&nbsp;print_result ()
  18&nbsp;{
  19&nbsp;echo
  20&nbsp;echo "ones =   $ones"
  21&nbsp;echo "twos =   $twos"
  22&nbsp;echo "threes = $threes"
  23&nbsp;echo "fours =  $fours"
  24&nbsp;echo "fives =  $fives"
  25&nbsp;echo "sixes =  $sixes"
  26&nbsp;echo
  27&nbsp;}
  28&nbsp;
  29&nbsp;update_count()
  30&nbsp;{
  31&nbsp;case "$1" in
  32&nbsp;  0) let "ones += 1";;   # Since die has no "zero", this corresponds to 1.
  33&nbsp;  1) let "twos += 1";;   # And this to 2, etc.
  34&nbsp;  2) let "threes += 1";;
  35&nbsp;  3) let "fours += 1";;
  36&nbsp;  4) let "fives += 1";;
  37&nbsp;  5) let "sixes += 1";;
  38&nbsp;esac
  39&nbsp;}
  40&nbsp;
  41&nbsp;echo
  42&nbsp;
  43&nbsp;
  44&nbsp;while [ "$throw" -lt "$MAXTHROWS" ]
  45&nbsp;do
  46&nbsp;  let "die1 = RANDOM % $PIPS"
  47&nbsp;  update_count $die1
  48&nbsp;  let "throw += 1"
  49&nbsp;done  
  50&nbsp;
  51&nbsp;print_result
  52&nbsp;
  53&nbsp;exit 0
  54&nbsp;
  55&nbsp;#  The scores should distribute fairly evenly, assuming RANDOM is fairly random.
  56&nbsp;#  With $MAXTHROWS at 600, all should cluster around 100, plus-or-minus 20 or so.
  57&nbsp;#
  58&nbsp;#  Keep in mind that RANDOM is a pseudorandom generator,
  59&nbsp;#+ and not a spectacularly good one at that.
  60&nbsp;
  61&nbsp;#  Randomness is a deep and complex subject.
  62&nbsp;#  Sufficiently long "random" sequences may exhibit
  63&nbsp;#+ chaotic and other "non-random" behavior.
  64&nbsp;
  65&nbsp;# Exercise (easy):
  66&nbsp;# ---------------
  67&nbsp;# Rewrite this script to flip a coin 1000 times.
  68&nbsp;# Choices are "HEADS" and "TAILS".</PRE
></TD
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><HR></DIV
><P
>As we have seen in the last example, it is best to
	  <SPAN
CLASS="QUOTE"
>"reseed"</SPAN
> the <TT
CLASS="VARNAME"
>RANDOM</TT
>
	  generator each time it is invoked. Using the same seed
	  for <TT
CLASS="VARNAME"
>RANDOM</TT
> repeats the same series
	  of numbers.
	    <A
NAME="AEN5208"
HREF="#FTN.AEN5208"
>[2]</A
>
	  (This mirrors the behavior of the
	  <TT
CLASS="REPLACEABLE"
><I
>random()</I
></TT
> function in
	  <I
CLASS="FIRSTTERM"
>C</I
>.)</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="SEEDINGRANDOM"
></A
><P
><B
>Example 9-28. Reseeding RANDOM</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# seeding-random.sh: Seeding the RANDOM variable.
   3&nbsp;
   4&nbsp;MAXCOUNT=25       # How many numbers to generate.
   5&nbsp;
   6&nbsp;random_numbers ()
   7&nbsp;{
   8&nbsp;count=0
   9&nbsp;while [ "$count" -lt "$MAXCOUNT" ]
  10&nbsp;do
  11&nbsp;  number=$RANDOM
  12&nbsp;  echo -n "$number "
  13&nbsp;  let "count += 1"
  14&nbsp;done  
  15&nbsp;}
  16&nbsp;
  17&nbsp;echo; echo
  18&nbsp;
  19&nbsp;RANDOM=1          # Setting RANDOM seeds the random number generator.
  20&nbsp;random_numbers
  21&nbsp;
  22&nbsp;echo; echo
  23&nbsp;
  24&nbsp;RANDOM=1          # Same seed for RANDOM...
  25&nbsp;random_numbers    # ...reproduces the exact same number series.
  26&nbsp;                  #
  27&nbsp;                  # When is it useful to duplicate a "random" number series?
  28&nbsp;
  29&nbsp;echo; echo
  30&nbsp;
  31&nbsp;RANDOM=2          # Trying again, but with a different seed...
  32&nbsp;random_numbers    # gives a different number series.
  33&nbsp;
  34&nbsp;echo; echo
  35&nbsp;
  36&nbsp;# RANDOM=$$  seeds RANDOM from process id of script.
  37&nbsp;# It is also possible to seed RANDOM from 'time' or 'date' commands.
  38&nbsp;
  39&nbsp;# Getting fancy...
  40&nbsp;SEED=$(head -1 /dev/urandom | od -N 1 | awk '{ print $2 }')
  41&nbsp;#  Pseudo-random output fetched
  42&nbsp;#+ from /dev/urandom (system pseudo-random device-file),
  43&nbsp;#+ then converted to line of printable (octal) numbers by "od",
  44&nbsp;#+ finally "awk" retrieves just one number for SEED.
  45&nbsp;RANDOM=$SEED
  46&nbsp;random_numbers
  47&nbsp;
  48&nbsp;echo; echo
  49&nbsp;
  50&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
><A
NAME="URANDOMREF"
></A
></P
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="100%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The <TT
CLASS="FILENAME"
>/dev/urandom</TT
> device-file provides
	  a method of generating much more <SPAN
CLASS="QUOTE"
>"random"</SPAN
>
	  pseudorandom numbers than the <TT
CLASS="VARNAME"
>$RANDOM</TT
>
	  variable.  <TT
CLASS="USERINPUT"
><B
>dd if=/dev/urandom of=targetfile
	  bs=1 count=XX</B
></TT
> creates a file of well-scattered
	  pseudorandom numbers.  However, assigning these numbers
	  to a variable in a script requires a workaround, such as
	  filtering through <A
HREF="extmisc.html#ODREF"
>od</A
> (as in
	  above example and <A
HREF="textproc.html#RND"
>Example 12-13</A
>), or using <A
HREF="extmisc.html#DDREF"
>dd</A
> (see <A
HREF="extmisc.html#BLOTOUT"
>Example 12-55</A
>),
	  or even piping to <A
HREF="filearchiv.html#MD5SUMREF"
>md5sum</A
>
	  (see <A
HREF="colorizing.html#HORSERACE"
>Example 33-14</A
>).</P
><P
><A
NAME="AWKRANDOMREF"
></A
></P
><P
>There are also other ways to generate pseudorandom
          numbers in a script. <B
CLASS="COMMAND"
>Awk</B
> provides a
	  convenient means of doing this.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="RANDOM2"
></A
><P
><B
>Example 9-29. Pseudorandom numbers, using <A
HREF="awk.html#AWKREF"
>awk</A
></B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# random2.sh: Returns a pseudorandom number in the range 0 - 1.
   3&nbsp;# Uses the awk rand() function.
   4&nbsp;
   5&nbsp;AWKSCRIPT=' { srand(); print rand() } '
   6&nbsp;#            Command(s) / parameters passed to awk
   7&nbsp;# Note that srand() reseeds awk's random number generator.
   8&nbsp;
   9&nbsp;
  10&nbsp;echo -n "Random number between 0 and 1 = "
  11&nbsp;
  12&nbsp;echo | awk "$AWKSCRIPT"
  13&nbsp;# What happens if you leave out the 'echo'?
  14&nbsp;
  15&nbsp;exit 0
  16&nbsp;
  17&nbsp;
  18&nbsp;# Exercises:
  19&nbsp;# ---------
  20&nbsp;
  21&nbsp;# 1) Using a loop construct, print out 10 different random numbers.
  22&nbsp;#      (Hint: you must reseed the "srand()" function with a different seed
  23&nbsp;#+     in each pass through the loop. What happens if you fail to do this?)
  24&nbsp;
  25&nbsp;# 2) Using an integer multiplier as a scaling factor, generate random numbers 
  26&nbsp;#+   in the range between 10 and 100.
  27&nbsp;
  28&nbsp;# 3) Same as exercise #2, above, but generate random integers this time.</PRE
></TD
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><HR></DIV
><P
>The <A
HREF="timedate.html#DATEREF"
>date</A
> command also lends
	  itself to <A
HREF="timedate.html#DATERANDREF"
>generating pseudorandom
	  integer sequences</A
>.</P
></TD
></TR
></TABLE
></DIV
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
BORDER="0"
CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN5174"
HREF="randomvar.html#AEN5174"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>True <SPAN
CLASS="QUOTE"
>"randomness,"</SPAN
> insofar as
	    it exists at all, can only be found in certain
	    incompletely understood natural phenomena such as
	    radioactive decay. Computers can only simulate
	    randomness, and computer-generated sequences of
	    <SPAN
CLASS="QUOTE"
>"random"</SPAN
> numbers are therefore referred to as
	    <I
CLASS="EMPHASIS"
>pseudorandom</I
>.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN5208"
HREF="randomvar.html#AEN5208"
>[2]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>The <I
CLASS="FIRSTTERM"
>seed</I
> of a
	      computer-generated pseudorandom number series
	      can be considered an identification label. For
	      example, think of the pseudorandom series with a
	      seed of <I
CLASS="EMPHASIS"
>23</I
> as <I
CLASS="EMPHASIS"
>series
	      #23</I
>.</P
><P
>A property of a pseurandom number series is the length of
	      the cycle before it starts repeating itself. A good pseurandom
	      generator will produce series with very long cycles.</P
></TD
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