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This primer demonstrates usage of the standard module Random.chpl.

use Random;

This primer shows two ways to generate a sequence of random numbers: The first is by creating an array of random numbers using the top-level Random.fillRandom function. The second way is to use a randomStream instance.

Using fillRandom

Call Random.fillRandom with an array argument. The array will be filled with random numbers.

var rands: [1..10] real;

Now writeln(rands) would output these random values, but then this program’s output would not be deterministic.

To produce deterministic output, specify the starting seed to use when filling the array.

var randsSeeded: [0..9] real;
var seed = 17;
fillRandom(randsSeeded, seed);
writeln("randsSeeded = ", randsSeeded); // Here the output is deterministic

Other numeric types are supported:

var rand16s: [1..10] uint(16);
fillRandom(rand16s, seed);
writeln("rand16s = ", rand16s); // Here the output is deterministic

Using randomStream

The second way to generate a sequence of random numbers is by creating a randomStream instance. The first argument in its initializer is the type of the elements that the instance should generate. The second argument is a seed value of type int.

var randStream = new randomStream(real);
var randStreamSeeded = new randomStream(real, seed);

Then the instance can be used to obtain the numbers. This can be done in a large chunk by calling fill:

var randsFromStream: [1..10] real;

Or random numbers can be requested one at a a time:

var firstRand = randStreamSeeded.next();
writeln(firstRand == randsSeeded[0]);

Note that since since we are using the same seed, the numbers generated will match those generated earlier by fillRandom(randsSeeded, seed).

The next random number generated will follow the most recent…

var nextRand = randStreamSeeded.next();
writeln(nextRand == randsSeeded[1]);

…unless the position to look at has been changed. In this case, the 6th random number in the stream will be generated. Note that the position argument must be greater than or equal to 0.

var rand6 = randStreamSeeded.next();
writeln(rand6 == randsSeeded[6]);

The position can be earlier or later than the most recent.

var rand3 = randStreamSeeded.next();
writeln(rand3 == randsSeeded[3]);

The stream can also be used to iterate over a domain. Note that this doesn’t yield the 5th through 10th elements of the stream, but rather the next 5 random numbers.

for i in randStreamSeeded.next({5..10}) {