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7.8.7 Random Number generation
The function rand() computes a sequence of pseudo-random integers in the range zero to RAND_MAX, which is defined in <stdlib.h>. One way to produce random floating-point numbers greater than or equal to zero but less than one is
#define frand() ((double) rand() / (RAND_MAX+1.0))
(If your library already provides a function for floating-point random numbers, it is likely to have better statistical properties than this one.)
The function srand(unsigned) sets the seed for rand. The portable implementation of rand and srand suggested by the standard appears in Section 2.7.
Exercise 7-9. Functions like isupper can be implemented to save space or to save time. Explore both possibilities.