Random provides an interface to Ruby's pseudo-random number generator, or PRNG. The PRNG produces a deterministic sequence of bits which approximate true randomness. The sequence may be represented by integers, floats, or binary strings.

The generator may be initialized with either a system-generated or user-supplied seed value by using ::srand.

The class method #rand provides the base functionality of Kernel#rand along with better handling of floating point values. These are both interfaces to Random::DEFAULT, the Ruby system PRNG.

::new will create a new PRNG with a state independent of Random::DEFAULT, allowing multiple generators with different seed values or sequence positions to exist simultaneously. Random objects can be marshaled, allowing sequences to be saved and resumed.

PRNGs are currently implemented as a modified Mersenne Twister with a period of 2**19937-1.

Included Modules
Class Public methods
Random.new(seed = Random.new_seed) → prng

Creates a new PRNG using seed to set the initial state. If seed is omitted, the generator is initialized with ::new_seed.

See ::srand for more information on the use of seed values.

static VALUE
random_init(int argc, VALUE *argv, VALUE obj)
    VALUE vseed;
    rb_random_t *rnd = get_rnd(obj);

    if (rb_check_arity(argc, 0, 1) == 0) {
        vseed = random_seed();
    else {
        vseed = argv[0];
        rb_check_copyable(obj, vseed);
        vseed = rb_to_int(vseed);
    rnd->seed = rand_init(&rnd->mt, vseed);
    return obj;
Random.new_seed → integer

Returns an arbitrary seed value. This is used by ::new when no seed value is specified as an argument.

Random.new_seed  #=> 115032730400174366788466674494640623225
static VALUE
    VALUE v;
    uint32_t buf[DEFAULT_SEED_CNT+1];
    fill_random_seed(buf, DEFAULT_SEED_CNT);
    v = make_seed_value(buf, DEFAULT_SEED_CNT);
    explicit_bzero(buf, DEFAULT_SEED_LEN);
    return v;
Random.rand → float
Random.rand(max) → number

Alias of Random::DEFAULT.rand.

static VALUE
random_s_rand(int argc, VALUE *argv, VALUE obj)
    VALUE v = rand_random(argc, argv, Qnil, rand_start(&default_rand));
    check_random_number(v, argv);
    return v;
srand(number = Random.new_seed) → old_seed

Seeds the system pseudo-random number generator, Random::DEFAULT, with number. The previous seed value is returned.

If number is omitted, seeds the generator using a source of entropy provided by the operating system, if available (/dev/urandom on Unix systems or the RSA cryptographic provider on Windows), which is then combined with the time, the process id, and a sequence number.

srand may be used to ensure repeatable sequences of pseudo-random numbers between different runs of the program. By setting the seed to a known value, programs can be made deterministic during testing.

srand 1234               # => 268519324636777531569100071560086917274
[ rand, rand ]           # => [0.1915194503788923, 0.6221087710398319]
[ rand(10), rand(1000) ] # => [4, 664]
srand 1234               # => 1234
[ rand, rand ]           # => [0.1915194503788923, 0.6221087710398319]
static VALUE
rb_f_srand(int argc, VALUE *argv, VALUE obj)
    VALUE seed, old;
    rb_random_t *r = &default_rand;

    if (rb_check_arity(argc, 0, 1) == 0) {
        seed = random_seed();
    else {
        seed = rb_to_int(argv[0]);
    old = r->seed;
    r->seed = rand_init(&r->mt, seed);

    return old;
Random.urandom(size) → string

Returns a string, using platform providing features. Returned value is expected to be a cryptographically secure pseudo-random number in binary form. This method raises a RuntimeError if the feature provided by platform failed to prepare the result.

In 2017, Linux manpage random(7) writes that “no cryptographic primitive available today can hope to promise more than 256 bits of security”. So it might be questionable to pass size > 32 to this method.

Random.urandom(8)  #=> "\x78\x41\xBA\xAF\x7D\xEA\xD8\xEA"
static VALUE
random_raw_seed(VALUE self, VALUE size)
    long n = NUM2ULONG(size);
    VALUE buf = rb_str_new(0, n);
    if (n == 0) return buf;
    if (fill_random_bytes(RSTRING_PTR(buf), n, FALSE))
        rb_raise(rb_eRuntimeError, "failed to get urandom");
    return buf;
Instance Public methods
prng1 == prng2 → true or false

Returns true if the two generators have the same internal state, otherwise false. Equivalent generators will return the same sequence of pseudo-random numbers. Two generators will generally have the same state only if they were initialized with the same seed

Random.new == Random.new             # => false
Random.new(1234) == Random.new(1234) # => true

and have the same invocation history.

prng1 = Random.new(1234)
prng2 = Random.new(1234)
prng1 == prng2 # => true

prng1.rand     # => 0.1915194503788923
prng1 == prng2 # => false

prng2.rand     # => 0.1915194503788923
prng1 == prng2 # => true
static VALUE
random_equal(VALUE self, VALUE other)
    rb_random_t *r1, *r2;
    if (rb_obj_class(self) != rb_obj_class(other)) return Qfalse;
    r1 = get_rnd(self);
    r2 = get_rnd(other);
    if (memcmp(r1->mt.state, r2->mt.state, sizeof(r1->mt.state))) return Qfalse;
    if ((r1->mt.next - r1->mt.state) != (r2->mt.next - r2->mt.state)) return Qfalse;
    if (r1->mt.left != r2->mt.left) return Qfalse;
    return rb_equal(r1->seed, r2->seed);
prng.bytes(size) → a_string

Returns a random binary string containing size bytes.

random_string = Random.new.bytes(10) # => "\xD7:R\xAB?\x83\xCE\xFAkO"
random_string.size                   # => 10
static VALUE
random_bytes(VALUE obj, VALUE len)
    return genrand_bytes(get_rnd(obj), NUM2LONG(rb_to_int(len)));
prng.rand → float
prng.rand(max) → number

When max is an Integer, rand returns a random integer greater than or equal to zero and less than max. Unlike Kernel#rand, when max is a negative integer or zero, rand raises an ArgumentError.

prng = Random.new
prng.rand(100)       # => 42

When max is a Float, rand returns a random floating point number between 0.0 and max, including 0.0 and excluding max.

prng.rand(1.5)       # => 1.4600282860034115

When max is a Range, rand returns a random number where range.member?(number) == true.

prng.rand(5..9)      # => one of [5, 6, 7, 8, 9]
prng.rand(5...9)     # => one of [5, 6, 7, 8]
prng.rand(5.0..9.0)  # => between 5.0 and 9.0, including 9.0
prng.rand(5.0...9.0) # => between 5.0 and 9.0, excluding 9.0

Both the beginning and ending values of the range must respond to subtract (-) and add (+)methods, or rand will raise an ArgumentError.

static VALUE
random_rand(int argc, VALUE *argv, VALUE obj)
    VALUE v = rand_random(argc, argv, obj, get_rnd(obj));
    check_random_number(v, argv);
    return v;
prng.seed → integer

Returns the seed value used to initialize the generator. This may be used to initialize another generator with the same state at a later time, causing it to produce the same sequence of numbers.

prng1 = Random.new(1234)
prng1.seed       #=> 1234
prng1.rand(100)  #=> 47

prng2 = Random.new(prng1.seed)
prng2.rand(100)  #=> 47
static VALUE
random_get_seed(VALUE obj)
    return get_rnd(obj)->seed;