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1 perry 1 RAND
2 !!!RAND
3 NAME
4 SYNOPSIS
5 DESCRIPTION
6 RETURN VALUE
7 NOTES
8 CONFORMING TO
9 SEE ALSO
10 ----
11 !!NAME
12
13
14 rand, srand - random number generator.
15 !!SYNOPSIS
16
17
18 __#include
19 __ ''seed''__);
20 __
21 !!DESCRIPTION
22
23
24 The __rand()__ function returns a pseudo-random integer
25 between 0 and __RAND_MAX__.
26
27
28 The __srand()__ function sets its argument as the seed
29 for a new sequence of pseudo-random integers to be returned
30 by __rand()__. These sequences are repeatable by calling
31 __srand()__ with the same seed value.
32
33
34 If no seed value is provided, the __rand()__ function is
35 automatically seeded with a value of 1.
36 !!RETURN VALUE
37
38
39 The __rand()__ function returns a value between 0 and
40 RAND_MAX. The __srand()__ returns no value.
41 !!NOTES
42
43
44 The versions of __rand()__ and __srand()__ in the
45 Linux C Library use the same random number generator as
46 __random()__ and __srandom()__, so the lower-order
47 bits should be as random as the higher-order bits. However,
48 on older __rand()__ implementations, the lower-order bits
49 are much less random than the higher-order
50 bits.
51
52
53 In ''Numerical Recipes in C: The Art of Scientific
54 Computing'' (William H. Press, Brian P. Flannery, Saul A.
55 Teukolsky, William T. Vetterling; New York: Cambridge
56 University Press, 1992 (2nd ed., p. 277)), the following
57 comments are made:
58
59
60
61
62 j=1+(int) (10.0*rand()/(RAND_MAX+1.0));
63
64
65 and never by anything resembling
66
67
68 j=1+(rand() % 10);
69
70
71 (which uses lower-order bits).
72
73
74 Random-number generation is a complex topic. The
75 ''Numerical Recipes in C'' book (see reference above)
76 provides an excellent discussion of practical random-number
77 generation issues in Chapter 7 (Random
78 Numbers).
79
80
81 For a more theoretical discussion which also covers many
82 practical issues in depth, please see Chapter 3 (Random
83 Numbers) in Donald E. Knuth's ''The Art of Computer
84 Programming'', volume 2 (Seminumerical Algorithms), 2nd
85 ed.; Reading, Massachusetts: Addison-Wesley Publishing
86 Company, 1981.
87 !!CONFORMING TO
88
89
90 SVID 3, BSD 4.3, ISO 9899
91 !!SEE ALSO
92
93
94 random(3), srandom(3), initstate(3),
95 setstate(3)
96 ----
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