Q36736: RANDOMIZE Statements Reseed but Don’t Restart RND Sequence

Article: Q36736
Product(s): See article
Version(s): 1.00 1.01 1.02 2.00 2.01 3.00 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom B_BasicInt B_GWBasicI B_MQuickB | mspl13_basic
Last Modified: 11-JAN-1990

The first invocation of the RANDOMIZE statement determines a given set
of random numbers returned from successive calls to the RND function.
Not invoking the RANDOMIZE statement in a program is equivalent to
invoking RANDOMIZE 0 before invoking RND. Note that a second (or
subsequent) RANDOMIZE x statement does not restart the number sequence
from the beginning of the set for a given x, but it randomly changes
(reseeds) the sequence from what it would have been from that point
on. This behavior is by design. Example 2 below illustrates this in
detail.

If you want to return the same sequence of random numbers several
times within a given program run, you can invoke the RND function with
the exact same negative number argument followed by a sequence of RND
invocations with a positive argument or no argument. Invoking RND with
a negative argument eliminates the effect of a previous RANDOMIZE
statement. Please see Example 1 below for further illustration.

This behavior of the RANDOMIZE statement and the RND function occurs
in most versions of Microsoft BASIC, including the following:

1. Microsoft QuickBASIC Versions 1.00, 1.01, 1.02, 2.00, 2.01, 3.00,
   4.00, 4.00b, and 4.50 for the IBM PC

2. Microsoft BASIC Compiler Versions 5.35 and 5.36 for MS-DOS

3. Microsoft BASIC Compiler Versions 6.00 and 6.00b for MS-DOS and MS
   OS/2

4. Microsoft BASIC PDS Version 7.00 for MS-DOS and MS OS/2

5. Microsoft GW-BASIC Interpreter Versions 3.20, 3.22, and 3.23 for
   MS-DOS

6. Microsoft QuickBASIC Version 1.00 for the Apple Macintosh

7. Microsoft BASIC Compiler Version 1.00 for the Apple Macintosh

8. Microsoft BASIC Interpreter Versions 1.00, 1.01, 2.00, 2.10, and
   3.00 for the Apple Macintosh

Other BASICs may behave differently.

If you would like an alternative to this behavior of RND and
RANDOMIZE, you may use your own formula to generate random numbers as
shown in a separate article. (Query in this database for the keywords
RANDOM and EQUATION.)

The following is Example 1:

(This example of invoking RND once with a negative argument always
returns the same sequence of random numbers for subsequent invocations
of RND.)

CLS
RANDOMIZE TIMER   ' Ignored unless you remove RND(-1) below.
FOR j = 1 TO 2
 ' Passing a negative value to the RND function supersedes the effect
 ' of the previous RANDOMIZE TIMER statement:
 PRINT RND(-1)   ' Remove this line for a different sequence on every
                 ' loop iteration. Otherwise, each j loop iteration
                 ' (and separate program run) returns the same
                 ' three-number sequence for the inner i loop.
 FOR i = 1 TO 3
    PRINT RND
 NEXT I
 PRINT
NEXT J

The following is Example 2:

FOR k = 1 TO 5
  PRINT RND
NEXT k

Below is the default random number set output from the above program,
when run with QB.EXE Version 4.00 on an IBM PC. The following sequence
of random numbers varies with different versions of Microsoft
QuickBASIC and BASIC Compilers:

   .7107346
   .99058
   .8523988
   .3503776
   4.363585E-02

The following code shows the effect of RANDOMIZE 0 at start-up:

PRINT  "Set the seed to zero at startup"
RANDOMIZE 0
FOR k = 1 TO 5
  PRINT RND
NEXT k
PRINT "Again, reset the seed to zero"
RANDOMIZE 0
FOR k = 1 TO 5
  PRINT RND
NEXT k

The above program has the following output:

   Set the seed to zero at startup
    .7107346
    .99058
    .8523988
    .3503776
    4.363585E-02
   Again, reset the seed to zero
    .7987763
    .6497337
    .5426014
    .9642789
    8.590406E-02

Note: The second invocation of RANDOMIZE 0 does not restart the
sequence from the beginning. This is by design. If you remove the
second RANDOMIZE 0 statement and run the program again, the 6th
through 10th numbers are different than above. This shows that
multiple RANDOMIZE statements reseed the sequence (and change the
random number set displayed), but they do not restart the sequence
from the beginning.

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