randomvar.html

Shall高级编程

  62 #  Sufficiently long "random" sequences may exhibit
  63 #+ chaotic and other "non-random" behavior.
  64 
  65 # Exercise (easy):
  66 # ---------------
  67 # Rewrite this script to flip a coin 1000 times.
  68&nbsp;# Choices are "HEADS" and "TAILS".</PRE
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><P
>As we have seen in the last example, it is best to
	  <I
CLASS="FIRSTTERM"
>reseed</I
> the <TT
CLASS="PARAMETER"
><I
>RANDOM</I
></TT
>
	  generator each time it is invoked. Using the same seed
	  for <TT
CLASS="PARAMETER"
><I
>RANDOM</I
></TT
> repeats the same series
	  of numbers.
	    <A
NAME="AEN5744"
HREF="#FTN.AEN5744"
>[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 9-31. Reseeding RANDOM</B
></P
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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
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><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
> pseudo-device file
	  provides a method of generating much more <SPAN
CLASS="QUOTE"
>"random"</SPAN
>
	  pseudorandom numbers than the <TT
CLASS="VARNAME"
>$RANDOM</TT
>
	  variable.  <TT
CLASS="USERINPUT"
><B
>dd if=/dev/urandom of=targetfile
	  bs=1 count=XX</B
></TT
> creates a file of well-scattered
	  pseudorandom numbers.  However, assigning these numbers
	  to a variable in a script requires a workaround, such
	  as filtering through <A
HREF="extmisc.html#ODREF"
>od</A
>
	  (as in above example, <A
HREF="textproc.html#RND"
>Example 15-14</A
>, and
	  <A
HREF="contributed-scripts.html#INSERTIONSORT"
>Example A-38</A
>), or using <A
HREF="extmisc.html#DDREF"
>dd</A
> (see <A
HREF="extmisc.html#BLOTOUT"
>Example 15-58</A
>),
	  or even piping to <A
HREF="filearchiv.html#MD5SUMREF"
>md5sum</A
>
	  (see <A
HREF="colorizing.html#HORSERACE"
>Example 33-14</A
>).</P
><P
><A
NAME="AWKRANDOMREF"
></A
></P
><P
>There are also other ways to generate pseudorandom
          numbers in a script. <B
CLASS="COMMAND"
>Awk</B
> provides a
	  convenient means of doing this.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="RANDOM2"
></A
><P
><B
>Example 9-32. Pseudorandom numbers, using <A
HREF="awk.html#AWKREF"
>awk</A
></B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# random2.sh: Returns a pseudorandom number in the range 0 - 1.
   3&nbsp;# Uses the awk rand() function.
   4&nbsp;
   5&nbsp;AWKSCRIPT=' { srand(); print rand() } '
   6&nbsp;#            Command(s) / parameters passed to awk
   7&nbsp;# Note that srand() reseeds awk's random number generator.
   8&nbsp;
   9&nbsp;
  10&nbsp;echo -n "Random number between 0 and 1 = "
  11&nbsp;
  12&nbsp;echo | awk "$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
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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
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></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
BORDER="0"
CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN5704"
HREF="randomvar.html#AEN5704"
>[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 only <I
CLASS="FIRSTTERM"
>simulate</I
>
	    randomness, and computer-generated sequences of
	    <SPAN
CLASS="QUOTE"
>"random"</SPAN
> numbers are therefore referred to as
	    <I
CLASS="FIRSTTERM"
>pseudorandom</I
>.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN5744"
HREF="randomvar.html#AEN5744"
>[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 <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>23</I
></SPAN
> as <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>series
	      #23</I
></SPAN
>.</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
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