⎕RL is a 2-element vector. Its second element is an integer that identifies the random number generator that is currently in use, and its first element contains the base or seed. Together these items define how the system generates random numbers using Roll and Deal.

In a clear ws, the second element of ⎕RL is 1, and the first element is initialised to the value defined by the default_rl parameter which itself defaults to 16807 if it is not defined.

The facility to select the random number generator by assignment to ⎕RL is a new feature in Version 15.0 and replaces the use of (16807⌶). This function is deprecated and will be removed in the next release.

The 3 random number generators are listed in the table below. The 4th column of the table contains the values of seeds that may be assigned to them.

Id | Name | Algorithm | Valid Seed Values |
---|---|---|---|

0 | RNG0 | Lehmer linear congruential generator. | 0, ⍬, or an integer in the range 1 to ¯2+2*31 |

1 | RNG1 | Mersenne Twister. | 0, ⍬, an integer in the range 1 to ¯2+2*31 or a 625-element integer vector |

2 | RNG2 | Operating System random number generator. | ⍬ |

The default random number generator in a CLEAR WS is 1 (Mersenne Twister). This algortithm RNG1 produces 64-bit values with good distribution.

The Lehmer linear congruential generator RNG0 was the only random number generator provided in versions of Dyalog APL prior to Version 13.1. The implementation of this algorithm has several limitations including limited value range (2*31), short period and non-uniform distribution (some values may appear more frequently than others). It is retained for backwards compatibility.

Under Windows, the Operating System random number generator algorithm RNG2 uses the CryptGenRandom() function. Under UNIX/Linux it uses /dev/urandom.

Random number sequences may be predictable or not and repeatable or not. A predictable and repeatable sequence is obtained by starting with the same specific value for the seed. A non-predictable sequence is obtained by starting with a seed which is itself chosen at random, but such a sequence is repeatable if the value of the seed (chosen at random) is visible. A non-predictable and non-repeatable sequence of random numbers is obtained where the initial seed is chosen completely at random and is unknown.

Using RNG0 or RNG1:

- To obtain an entirely predictable random sequence, set the seed to a non-zero value
- To obtain a non-predictable, but repeatable sequence, set the seed to 0
- To obtain a non-predictable, non-repeatable series of random numbers, set the seed to ⍬

RNG2 does not support a user modifiable random number seed, so when using this scheme, it is not possible to obtain a repeatable random number series and the seed must always be ⍬.

If the seed is ⍬, Dyalog is able to take advantage of certain optimisations which deliver maximum performance. This is the best choice unless you require a repeatable sequence.

⎕RL does not behave quite like a regular 2-element variable; it has its own rules relating to assignment and reference.

⎕RLreturns a 2-element vector whose second element identifies the scheme in use (0, 1 or 2).

Using RNG0, ⎕RL[1] is an integer which represents the seed for the next random number in the sequence.

Using RNG1, the system internally retains a block of 312 64-bit numbers which are used one by one to generate the results of roll and deal. When the first block of 312 have been used up, the system generates a second block. In this case, ⎕RL[1] is an integer vector of 32-bit numbers of length 625 (the first is an index into the block of 312) which represents the internal state of the random number generator. This means that, as with RNG0, you may save the value of ⎕RL in a variable and reassign it later.

Using RNG2, the seed is purely internal and ⎕RL[1] is always zilde.

⎕RL may only be assigned in its entirety. Indexed and selective assignment may not be used to assign values to individual elements.

To preserve compatibility with Versions of Dyalog prior to Version 15.0 (in which ⎕RL specifies just the seed) if the value assigned to ⎕RL represents a valid seed for the random number generator in use, it is taken to be the new seed. Otherwise, the value assigned to ⎕RL must be a 2-element vector, whose first item is the seed and whose second item is 0, 1 or 2 and specifies the random number generator to be used subsequently.

)CLEAR clear ws ⎕RL←16807 10?10 4 1 6 5 2 9 7 10 3 8 5↑⊃⎕RL 10 0 16807 1819658750 ¯355441828 X←?1000⍴1000 5↑⊃⎕RL 100 ¯465541037 ¯1790786136 ¯205462449 996695303

⎕RL←16807 10?10 4 1 6 5 2 9 7 10 3 8 Y←?1000⍴1000 X≡Y 1 5↑⊃⎕RL 100 ¯465541037 ¯1790786136 ¯205462449 996695303

⎕RL←16807 0 ⍝ Select RNG0 ⎕RL 16807 0 ?9 9 9 2 7 5 ?9 7 ⎕RL 984943658 0 ⎕RL←16807 ?9 9 9 2 7 5 ?9 7 ⎕RL 984943658 0

⎕RL←16807 1 ⍝ Select RNG1 5↑⊃⎕RL 100 ¯465541037 ¯1790786136 ¯205462449 996695303

When you set the seed to 0, a random seed is created for you:

⎕RL←0 0 ⎕RL 865618822 0 ⎕RL←0 ⎕RL 1100783275 0

This gives you a new, unpredictable random sequence yet it is repeatable because you can see the actual seed after you set it:

?10⍴100 14 22 18 30 42 22 71 32 32 12 ⎕RL←1100783275 ?10⍴100 14 22 18 30 42 22 71 32 32 12

When you set the seed to zilde, you get the same random initialisation but you can’t see the actual seed either:

⎕RL←⍬ ⎕RL 0