可以生成多位随机字母和数字，常用作验证码的生成或注册序列号的生成。程序里测试生成并打印了60位的随机字母和数字的字符串。(A number of random letters and numbers can be generated, which are often used to generate authentication codes or register sequence numbers. In the program, the test generates and prints a string of 60 bits of random letters and numbers.)