rand() was not declared in this scope, please help‏

[eric lin]

dear cygwin/gcc users:
 
  I compile a simple code which i copied from a book, "The Complete Reference of C++"  Fourth Edition by Herbert Schildt in page 575, rand()
which is clearly not be included in
#include <iostream>
 
I tried 
#inlcude <cmath>
 
it still not compile, i.e. same compile error
 
please help(see which lib I should include)
/* I tried in ubuntu linux on my laptop, it also not compile by same reason */
 
Eric

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