I was on a game forum and I saw this question come up, I was interested on the ways that It can be accomplished so I posed it here

Apparently, NVIDIA uses this as a test question when they hire new programmers...

Quote:

---

Q: The function rand5() returns an integer between 1 and 5, inclusive (that's 1,2,3,4,5), with equal probability. Use rand5() to create a function called rand7() that returns an integer between 1 and 7, inclusive, with equal probability.

You must try to find the most efficient, elegant way to solve this problem.

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It will be interesting to see how it can be done

Anyways, I'm off
Adoado

inline int rand7()
{
return rand5() * 1.4;
}

Maybe some asm code is missing with which it would look more advanced

your function will never return 3 and 6.

hm... this is tricky. i think i could do it with some if() clauses but i didn't find an elegant solution yet.

I have been trying to think of a method myself, but yeah - this is hard

rand7()
{
return rand5()+rand2(); // ;P
}

Code:

---

int rand7()
{
  int answer = 8;
  while(answer>7)
    answer = rand5()+rand5()-1; // number between 1 and 9
  return answer;
}

---

best i could do in a minute. sort of elegant, but could potentially be an infinite loop (theoretically, but i think a computer's "random" table would cause a number 7 or lower to pop up fairly quickly).

julz

EDIT: oooh sorry! just realised while eating that each number does not have equal probability of turning up -- it's similar to the probability of '7' turning up when throwing two dice being higher than any other number.

Code:

---

int rand7()
{
  float r7=0;
  
  for(int i=0; i<7; i++)
  {
   r7+=rand5();
  }

  return int((r7/7)*2);
}  

---

This ought to work unless I'm deluding myself.. however its probably not very elegant.. like all my code

@ello: If only it was that simple

Lol if NVIDIA started making hardware that could potentially have infinite loops I would be using an ATI card

*2 makes the "1" impossible, but i think i see where your mind was at; i was thinking something similar a few minutes ago.

i really want to get this before i go to bed!

julz

thinking out-loud time.

Code:

---

int rand7()
{
  int answer = 0;
  for(int i=7; i<7; i++)
    answer+=rand5(); // thanks matt!
  // answer should now be between 7 and 35, right?
  answer %= 7; // now it's between 0-6
  return answer + 1; // and returns in the range 1-7
}

---

but there's a problem! there's a 1/28 higher likelihood of a 1 turning up, because in the range 7-35 there are five 7's when we %7, and four of everything else. 35 minus 7 is 28 --> possible results from the for-loop. how can we bring this down to 27?

it's ugly, but:
Code:

---

int rand7()
{
  int answer;
  do {
    answer = 0;
    for(int i=7; i<7; i++)
      answer+=rand5();
  } while(answer>34);
  answer %= 7;
  return answer + 1;
}

---

right?

of course, if you don't like "do"s, it makes a lot more sense to do this:
Code:

---

int rand7()
{
  int answer = 35;
  while(answer>34)
  {
    answer = 0;
    for(int i=7; i<7; i++)
      answer+=rand5();
  }
  answer %= 7;
  return answer + 1;
}

---

julz