Seeds the random number generator

declare sub Randomize ( byval seed as double = -1.0, byval algorithm as long = 0 )

Randomize [ seed ][, algorithm ]

A double seed value for the random number generator. If omitted, a value based on Timer will be used instead.
An integer value to select the algorithm (see below, or from fbc version >= 1.08 the standard header "fbmath.bi" for available algorithms). If omitted, the default algorithm for the current language dialect is used.

Sets the random seed that helps Rnd generate random numbers, and selects the algorithm to use.
The constants for algorithm are defined in fbmath.bi (from fbc version 1.08). In the -lang fb dialect, these constants are part of the FB Namespace.
Valid values for algorithm are:
FB_RND_AUTO (0) - Default for current language dialect. This is algorithm FB_RND_MTWIST (3) in the -lang fb dialect, FB_RND_QB (4) in the -lang qb dialect and FB_RND_CRT (1) in the -lang fblite dialect.
FB_RND_CRT (1) - Uses the C runtime library's rand() function. This will give different results depending on the platform.
FB_RND_FAST (2) - Uses a fast implementation. This should be stable across all platforms, and provides 32-bit granularity, reasonable degree of randomness.
FB_RND_MTWIST (3) - Uses the Mersenne Twister. This should be stable across all platforms, provides 32-bit granularity, and gives a high degree of randomness.
FB_RND_QB (4) - Uses a function that is designed to give the same random number sequences as QBASIC. This should be stable across all platforms, and provides 24-bit precision, with a low degree of randomness.
FB_RND_REAL (5) - Available on Win32 and Linux, using system features (Win32 Crypto API, Linux /dev/urandom) to provide cryptographically random numbers. If those system APIs are unavailable, algorithm FB_RND_MTWIST (3) will be used instead.

For any given seed, each algorithm will produce a specific, deterministic sequence of numbers for that seed. If you want each call to Randomize to produce a different sequence of numbers, a seed that is not quite predictable should be used - for example, the value returned from Timer. Omitting the seed parameter will use a value based on this.
Note: using the Timer value directly as a parameter will produce the same seed if used more than once in the same second. However, it is generally not worth calling Randomize twice with unpredictable seeds anyway, because the second sequence will be no more random than the first. In most cases, the Mersenne twister should provide a sufficiently random sequence of numbers, without requiring reseeding between Rnd calls.

When you call Randomize with the QB compatible algorithm, part of the old seed is retained. This means that if you call Randomize several times with the same seed, you will not get the same sequence each time. To get a specific sequence in QB compatible mode, set the seed by calling Rnd with a negative parameter.

- For fbc version >= 1.08, Randomize is became thread-safe (by using an internal mutex), but not thread specific. Structures for other random number generators are also now available in the standard header "fbmath.bi".

'' Seed the RNG to the method using C's rand()
Randomize , 1

'' Print a sequence of random numbers
For i As Integer = 1 To 10
    Print Rnd

Dialect Differences:
The default algorithm used depends on the current dialect in use:
  • With the -lang fb dialect, a 32 bit Mersenne Twister function with a granularity of 32 bits is used.
  • With the -lang qb dialect, a function giving the same output as Rnd in QB is used. The granularity is 24 bits.
  • With the -lang deprecated and -lang fblite dialects, the function in the C runtime available in the system is used. The function has a granularity of 15 bits in Win32, and 32 bits in Linux and DOS.

Differences from QB:
See also:
Back to Mathematical Functions
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