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?<

Answer 1

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;
    }

}

Answer 2

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.

Answer 3

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);

Answer 4

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

Answer 5

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';

Answer 6

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