igraph_rng_t *igraph_rng_default();

Returns: 

See also: 

void igraph_rng_set_default(igraph_rng_t *rng);

Arguments: 

Time complexity: O(1).

int igraph_rng_init(igraph_rng_t *rng, const igraph_rng_type_t *type);

This function allocates memory for a random number generator, with the given type, and sets its seed to the default.

Arguments: 

Returns: 

Time complexity: depends on the type of the generator, but usually it should be O(1).

void igraph_rng_destroy(igraph_rng_t *rng);

Arguments: 

Time complexity: O(1).

int igraph_rng_seed(igraph_rng_t *rng, unsigned long int seed);

Arguments: 

Returns: 

Time complexity: usually O(1), but may depend on the type of the RNG.

unsigned long int igraph_rng_min(igraph_rng_t *rng);

Arguments: 

Returns: 

Time complexity: O(1).

unsigned long int igraph_rng_max(igraph_rng_t *rng);

Arguments: 

Returns: 

Time complexity: O(1).

const char *igraph_rng_name(igraph_rng_t *rng);

Arguments: 

Returns: 

Time complexity: O(1).

long int igraph_rng_get_integer(igraph_rng_t *rng,
                                long int l, long int h);

Arguments: 

Returns: 

Time complexity: depends on the generator, but should be usually O(1).

igraph_real_t igraph_rng_get_unif(igraph_rng_t *rng,
                                  igraph_real_t l, igraph_real_t h);

Arguments: 

Returns: 

Time complexity: depends on the type of the RNG.

igraph_real_t igraph_rng_get_unif01(igraph_rng_t *rng);

Arguments: 

Returns: 

Time complexity: depends on the type of the RNG.

igraph_real_t igraph_rng_get_normal(igraph_rng_t *rng,
                                    igraph_real_t m, igraph_real_t s);

Arguments: 

Returns: 

Time complexity: depends on the type of the RNG.

igraph_real_t igraph_rng_get_geom(igraph_rng_t *rng, igraph_real_t p);

Arguments: 

Returns: 

Time complexity: depends on the type of the RNG.

igraph_real_t igraph_rng_get_binom(igraph_rng_t *rng, long int n,
                                   igraph_real_t p);

Arguments: 

Returns: 

Time complexity: depends on the type of the RNG.

igraph_real_t igraph_rng_get_gamma(igraph_rng_t *rng, igraph_real_t shape,
                                   igraph_real_t scale);

Arguments: 

Returns: 

Time complexity: depends on RNG.

const igraph_rng_type_t igraph_rngtype_mt19937 = {
    /* name= */      "MT19937",
    /* min=  */      0,
    /* max=  */      0xffffffffUL,
    /* init= */      igraph_rng_mt19937_init,
    /* destroy= */   igraph_rng_mt19937_destroy,
    /* seed= */      igraph_rng_mt19937_seed,
    /* get= */       igraph_rng_mt19937_get,
    /* get_real= */  igraph_rng_mt19937_get_real,
    /* get_norm= */  0,
    /* get_geom= */  0,
    /* get_binom= */ 0,
    /* get_exp= */   0,
    /* get_gamma= */ 0
};

The MT19937 generator of Makoto Matsumoto and Takuji Nishimura is a variant of the twisted generalized feedback shift-register algorithm, and is known as the “Mersenne Twister” generator. It has a Mersenne prime period of 2^19937 - 1 (about 10^6000) and is equi-distributed in 623 dimensions. It has passed the diehard statistical tests. It uses 624 words of state per generator and is comparable in speed to the other generators. The original generator used a default seed of 4357 and choosing s equal to zero in gsl_rng_set reproduces this. Later versions switched to 5489 as the default seed, you can choose this explicitly via igraph_rng_seed instead if you require it. For more information see, Makoto Matsumoto and Takuji Nishimura, “Mersenne Twister: A 623-dimensionally equidistributed uniform pseudorandom number generator”. ACM Transactions on Modeling and Computer Simulation, Vol. 8, No. 1 (Jan. 1998), Pages 3–30 The generator igraph_rngtype_mt19937 uses the second revision of the seeding procedure published by the two authors above in 2002. The original seeding procedures could cause spurious artifacts for some seed values. This generator was ported from the GNU Scientific Library.

const igraph_rng_type_t igraph_rngtype_glibc2 = {
    /* name= */      "LIBC",
    /* min=  */      0,
    /* max=  */      RAND_MAX,
    /* init= */      igraph_rng_glibc2_init,
    /* destroy= */   igraph_rng_glibc2_destroy,
    /* seed= */      igraph_rng_glibc2_seed,
    /* get= */       igraph_rng_glibc2_get,
    /* get_real= */  igraph_rng_glibc2_get_real,
    /* get_norm= */  0,
    /* get_geom= */  0,
    /* get_binom= */ 0,
    /* get_exp= */   0,
    /* get_gamma= */ 0
};

It is a linear feedback shift register generator with a 128-byte buffer. This generator was the default prior to igraph version 0.6, at least on systems relying on GNU libc. This generator was ported from the GNU Scientific Library.

const igraph_rng_type_t igraph_rngtype_rand = {
    /* name= */      "RAND",
    /* min=  */      0,
    /* max=  */      0x7fffffffUL,
    /* init= */      igraph_rng_rand_init,
    /* destroy= */   igraph_rng_rand_destroy,
    /* seed= */      igraph_rng_rand_seed,
    /* get= */       igraph_rng_rand_get,
    /* get_real= */  igraph_rng_rand_get_real,
    /* get_norm= */  0,
    /* get_geom= */  0,
    /* get_binom= */ 0,
    /* get_exp= */   0,
    /* get_gamma= */ 0
};

The sequence is x_{n+1} = (a x_n + c) mod m with a = 1103515245, c = 12345 and m = 2^31 = 2147483648. The seed specifies the initial value, x_1. The theoretical value of x_{10001} is 1910041713. The period of this generator is 2^31. This generator is not very good -- the low bits of successive numbers are correlated. This generator was ported from the GNU Scientific Library.

If the user does not use any of the RNG functions explicitly, but calls some of the randomized igraph functions, then a default RNG is set up the first time an igraph function needs random numbers. The seed of this RNG is the output of the time(0) function call, using the time function from the standard C library. This ensures that igraph creates a different random graph, each time the C program is called.

The created default generator is stored internally and can be queried with the igraph_rng_default() function.

If reproducible results are needed, then the user should set the seed of the default random number generator explicitly, using the igraph_rng_seed() function on the default generator, igraph_rng_default(). When setting the seed to the same number, igraph generates exactly the same random graph (or series of random graphs).

By default igraph uses the igraph_rng_default() random number generator. This can be changed any time by calling igraph_rng_set_default(), with an already initialized random number generator. Note that the old (replaced) generator is not destroyed, so no memory is deallocated.

igraph also provides functions to set up multiple random number generators, using the igraph_rng_init() function, and then generating random numbers from them, e.g. with igraph_rng_get_integer() and/or igraph_rng_get_unif() calls.

Note that initializing a new random number generator is independent of the generator that the igraph functions themselves use. If you want to replace that, then please use igraph_rng_set_default().