How to replicate C++ rand in another language's random function?

Question

I am using an internal language in my company and they only have Random() that returns a float between 0 and 1.

I need to port a piece of C++ code that use rand():

int b = rand() % (i+1);

I looked at the docs, but not sure how I can use my Random() to generate a number between 0 and i+1 which is what the above code does, right?

I tried multiplying i+1 with Random() but didn't get the same results, that's why I am not sure if what I am doing is correct.

I expect difference between the results due to different random functions, but still I want to be sure I am translating it correctly.

codaddict · Accepted Answer · 2012-10-08 03:07:38Z

4

You need to multiply Random() with i and not i+1.

C++ rand() returns an integer between 0 and RAND_MAX but your Random() returns a float between 0 and 1, so multiplying the output of Random() with i and taking the integer portion of the result will give you an integer in [0,i] which is what rand() %(i+1) gives.

answered Oct 8, 2012 at 3:07

codaddict

457k83 gold badges501 silver badges537 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Joan Venge Over a year ago

Thanks I don't understand why the original code uses i+1 though? It loops backwards starting with the last index, so wouldn't i+1 give an index larger than the max index which is (num_items - 1)?

codaddict Over a year ago

(a +ve int) % N returns a number between 0 and N-1 (both inclusive).

rici Over a year ago

If the original said rand() % (i + 1) then you definitely want to replace it with Random()*(i + 1). I'm willing to be that Random() produces a number in the range [0, 1) , so multiplying by i produces the range [0, i) and truncating to int will result in [0, i-1].

Mark Ransom Over a year ago

@rici, If you're unsure whether Random() could return 1.0 or not you can multiply by i+0.999999999999999 instead of i+1.

PRB · Accepted Answer · 2012-10-08 03:14:26Z

Rand() give you a number between 0 and RAND_MAX, which after applying the mod operator you end up with a number between 0 and i (including i).

To do the same with Random() you'll need to multiply by ( i+1 ), then take the floor of that (round down):

b = floor( Random() * (i+1) )

This will give you a number from 0 to i (including the fence posts) as required.

Collectives™ on Stack Overflow

How to replicate C++ rand in another language's random function?

2 Answers 2

4 Comments

Comments

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

4 Comments

Comments

Related