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9 random digits (Posted on 2003-05-02) Difficulty: 3 of 5
Suppose you want to make a random 9 digit number, using every number from 1 to 9 exactly once. You have a process called random(top) that gives a random number up to top (if top was 5, it would give random numbers from 1 to 5)

a) How could you do this?

b) If top couldn't be more than 9, how could you do this using random(top) only 9 times (or less)?

See The Solution Submitted by Gamer    
Rating: 3.4000 (5 votes)

Comments: ( Back to comment list | You must be logged in to post comments.)
re: The goal of this problem.... | Comment 4 of 20 |
(In reply to The goal of this problem.... by Brian Smith)

No, Gamer specifically says he wants a random 9-digit number, so 1 through 99,999,999 are not acceptable outputs, and he wants every digit from 1 to 9 to appear exactly once in the output number, also far from random as zeros and repeated digits are disallowed. Only 9! = 362,880 numbers out of a billion satisfy this is about 1 in 2756, so it's highly selective rather than just any random number in the range.
  Posted by Charlie on 2003-05-02 08:27:21

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