Creating a function to generate a random number

Question

Note: I cannot use any of the built in functions to solve this.

This is the question:

Given a function that returns a random integer number between 1 and 5, create a function that creates a random integer between 1 and 7.

what did you try and where did you get stuck? (sounds like a homework thread if you cant use builtin stuff?) — usethedeathstar, Nov 21 '13 at 07:49
pfft. this is homework and you clearly haven't made the slightest effort. — SpliFF, Nov 21 '13 at 07:49
@SpliFF This is a somewhat famous google interview question. That already has an answer on this site — daniel gratzer, Nov 21 '13 at 07:50
I think the question itself is a joke / trick question. A function that returns a random integer number between 1 and 5 ALREADY meets the second criteria because every number in the range 1 - 5 is also between 1 and 7 — SpliFF, Nov 21 '13 at 07:53
This is an interesting question (although it should show an attempt - I can think of a number of invalid approaches), *but the title is absolutely horrid*. — user2864740, Nov 21 '13 at 07:53
@Spliff it most likely have to have a quite even distribution between the numbers when it has been run a lot of times, else it really makes no sense. — spydon, Nov 21 '13 at 07:55
http://stackoverflow.com/questions/137783/expand-a-random-range-from-15-to-17 eh...answered a long time ago. — Zegar, Nov 21 '13 at 08:02

score 1 · Answer 1 · edited May 23 '17 at 12:12

Apparently the following doesn't work. See the linked answer and the other responses as well.

Well, here is an approach I am thinking of. It could be wrong. I am going with the assumption that the result should be randomly distributed within the larger range.

Generate 7 random numbers using the given random functions (that generates numbers [1,5]) and calculate their sum.

This will result in a value between 7 (1 * 7) and 35 (5 * 7).

Because this value is evenly divisible by the target range and randomly distributed, then it seems like it would be valid to collapse the intermediate range [7,35] back to [1,7] without losing uniformity.

Or maybe that's not quite it - but it seems like exploiting a common multiple between the numbers is key.

score 1 · Accepted Answer · edited May 23 '17 at 10:31

1

As it has been answered in Adam Rosenfield's solution a while ago you may try to use an algoritm similar to this one below:

int i;
do
{
  i = 5 * (rand5() - 1) + rand5();  // i is now uniformly random between 1 and 25
} while(i > 21);
// i is now uniformly random between 1 and 21
return i % 7 + 1;  // result is now uniformly random between 1 and 7

EDIT: Translation to python:

def rand7():
i = 0;

while True:
    i = 5 * (rand5() -1) + rand5()
    if i > 21:
        break
return i % 7 + 1

edited May 23 '17 at 10:31

Community

1
1

answered Nov 21 '13 at 08:09

Zegar

1,005
10
18

But all right I've added a python version. Please check it as I'm not experienced in this language. – Zegar Nov 21 '13 at 08:42

score -1 · Answer 3 · answered Nov 21 '13 at 08:04

-1

import random
def winningFunc():
   return originalFuncReturning1to5() + random.randint(0,2)

answered Nov 21 '13 at 08:04

SpliFF

38,186
16
91
120

reason for negative vote? – SpliFF Nov 21 '13 at 08:05
Because he can't use built-in functions. – spydon Nov 21 '13 at 08:06
`Note: I cannot use any of the built in functions to solve this.` – Zegar Nov 21 '13 at 08:07
Even if he would be able to use built-in functions this is not very good as a random functions as it's not uniform. – spydon Nov 21 '13 at 08:15

Creating a function to generate a random number

3 Answers3