Hello, 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?
+21
A:
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.
Kendrick
2010-09-30 12:58:26
+2
A:
Here's teh codez.
#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;
}
}
John Dibling
2010-09-30 13:04:37
This will not give an even distribution! Also, finding the minimum and maximum of 100000 such numbers is not relevant to the question.
Cornelius Scarabeus
2010-09-30 13:49:12
Also if by chance `val` was `-100` every time, the resulting `max` would be wrong. Ditto for minimum.
Ben Voigt
2010-09-30 15:36:44
@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
2010-09-30 16:08:18
+1
A:
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);
Oliver
2010-09-30 13:07:09
+1
A:
In C++0x they will enhance this to provide better support for it with a standard library.
David
2010-09-30 13:55:26
+2
A:
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<int> dist(-100,2100);
std::cout << dist(rng) << '\n';
Blastfurnace
2010-09-30 15:24:11