Randomize

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Randomize

Seeds the random number generator that Rnd uses, and can choose its algorithm.

Syntax

Declare Sub Randomize ( ByVal seed As Double = -1.0, ByVal algorithm As Long = 0 )

Parameters

• seed
• algorithm

Description

Sets the random seed that helps Rnd generate random numbers, and selects the algorithm to use.
The constants for algorithm are defined in fbprng.bi. 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: for all algorithms except algorithm #4, because the fractional part of the seed is clipped, the 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, or even possibly worse by inducing sequence overlapping. 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.

Note:

Randomize is thread-safe (by using an internal mutex), but not thread specific.
Unfinished structures for other random number generators are also available in the standard header "fbprng.bi".

Remarks

Usage

Randomize [ seed ][, algorithm ]

Dialect Differences

The default algorithm used depends on the current dialect in use:

Differences from QB

• The algorithm parameter is new to FreeBASIC. QBASIC only had one algorithm (replicated in FB in algorithm number 4, and set as the default in the -lang qb dialect).

See also

• Rnd

Examples

'' 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

Next

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Category: Mathematical Functions
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Reproduced from the FreeBASIC Wiki Manual under the GNU Free Documentation License 1.3 — with no Invariant Sections, no Front-Cover Texts and no Back-Cover Texts. Copyright © the FreeBASIC project and wiki contributors. See License.