Random and negative numbers

Question

I have to generate numbers in range [-100; +2000] in c++. How can I do this with rand if there is only positive numbers available? Are there any fast ways?

score 42 · Accepted Answer · edited Sep 30 '10 at 13:11

42

generate a random number between 0 and 2100 then subtract 100.

A quick google search turned up a decent looking article on using Rand(). It includes code examples for working with a specific range at the end of the article.

edited Sep 30 '10 at 13:11

Fred Larson

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answered Sep 30 '10 at 12:58

Kendrick

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score 9 · Answer 2 · edited Nov 14 '19 at 17:36

9

You can use the C++ TR1 random functions to generate numbers in the desired distribution.

std::random_device rseed;
std::mt19937 rng(rseed());
std::uniform_int_distribution<int> dist(-100,2100);

std::cout << dist(rng) << '\n';

edited Nov 14 '19 at 17:36

Uglylab

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answered Sep 30 '10 at 15:24

Blastfurnace

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For those without TR1, boost::random is quite close. – MSalters Oct 01 '10 at 08:28

score 8 · Answer 3 · answered Sep 30 '10 at 12:59

8

Generate a random number between 0 and 2100, and subtract 100.

answered Sep 30 '10 at 12:59

Jez

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score 4 · Answer 4 · edited Mar 04 '14 at 15:21

4

Here is the code.

#include <cstdlib>
#include <ctime>

int main()
{
    srand((unsigned)time(0));
    int min = 999, max = -1;
    for( size_t i = 0; i < 100000; ++i )
    {
        int val = (rand()%2101)-100;
        if( val < min ) min = val;
        if( val > max ) max = val;
    }

}

edited Mar 04 '14 at 15:21

SimplePi

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answered Sep 30 '10 at 13:04

John Dibling

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This will not give an even distribution! Also, finding the minimum and maximum of 100000 such numbers is not relevant to the question. – darklon Sep 30 '10 at 13:49
Also if by chance `val` was `-100` every time, the resulting `max` would be wrong. Ditto for minimum. – Ben Voigt Sep 30 '10 at 15:36
@Cornelius, @Ben: Who cares? It's just a demo. If I had posted just `(rand()%2101)-100` that would have been better than a compilable, runnable sample? Please. – John Dibling Sep 30 '10 at 16:08

score 3 · Answer 5 · answered Sep 30 '10 at 12:58

3

Can you generate a number from 0-2100 and subtract 100?

answered Sep 30 '10 at 12:58

n8wrl

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score 1 · Answer 6 · answered Sep 30 '10 at 13:55

1

In C++0x they will enhance this to provide better support for it with a standard library.

http://www2.research.att.com/~bs/C++0xFAQ.html#std-random

answered Sep 30 '10 at 13:55

David

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score 0 · Answer 7 · answered Sep 30 '10 at 13:07

Currently my C++ syntax is a little rusty, but you should write a function that takes two parameters: size and offset.

So you generate numbers with the given size as maximum value and afterwards add the (negative) offset to it.

The function would look like:

int GetRandom(int size, int offset = 0);

and would be called in your case with:

int myValue = GetRandom(2100, -100);

Random and negative numbers

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