Random number problem

[gumbogogo]

Hi, I'm needing to generate a list of 10,000 different numbers between -1 and 1. I then have to arrange them in ascending order. Basically I have one array(randomnum[]) filled with the random numbers and I am filling another one(position[]) with the numbers in ascending order. I'm not entirely sure if how I'm generating the numbers is correct, but I feel like everything else is correct. Any help would be appreciated.

#include <iostream>
#include <stdlib.h> 
#include <time.h>
#include <stdio.h> 
using namespace std;

void random(int startpoint, int endpoint);
...
...
...//main function, calls function random(-1, 2)
...
...
void random(int startpoint, int range) 
{
	double randomnum[10000], position[10000];
	int i, j;
	
	srand((unsigned)time(0));

	for (i=0;i<10000;i++){
		position[i] = 1;
		for (j=0;j<10000;j++){
			randomnum[j] = startpoint + (range * rand()) / (RAND_MAX + 1);   //here's where I believe my problem to be
			if ((i == 0) && (randomnum[j] < position[i])) 
				position[i] = random[j];
			if ((randomnum[j] <= position[i])&&(randomnum[j] > position[i-1])&&(i > 0))
				position[i] = randomnum[j];
		}
		cout << position[i];
	}
}

[pogrady]

Have you been able to compile this?

[gumbogogo]

Yes, right now it's giving me a -1, 0, and then a whole bunch of 1s(probably 9998).

[cire]

Those are awful big arrays to allocate on the stack. I don't think you really need the position array unless you need to preserve the original order for some reason.

This would be a bit simpler with C++11.

#include <random>
#include <iostream>
#include <iomanip>
#include <vector>
#include <algorithm>

std::vector<double> generateRandom(double min, double max, unsigned num)
{
    std::mt19937 engine((std::random_device()())) ;
    std::uniform_real_distribution<double> rng(min, max) ;

    std::vector<double> result(num) ;
    std::generate(result.begin(), result.end(), [&](){return rng(engine);}) ;
    return std::move(result) ;
}

void display(const std::vector<double>& v )
{
    std::cout << "{\n"; 
    for ( auto element : v )
        std::cout << '\t' << std::setw(11) << element << '\n' ;
    std::cout << "}\n" ;
}
    
int main()
{
    unsigned nRandoms = 10 ; // or 10000.

    // randoms is an unsorted vector of random values.
    auto randoms = generateRandom(-1, 1, nRandoms) ;
    display(randoms) ;

    auto sortedRandoms = randoms ;
    std::sort(sortedRandoms.begin(), sortedRandoms.end()) ;
    // sorted Randoms is a sorted vector of random values.
    display(sortedRandoms) ;
}

Sample output:

{
          -0.129061
          -0.326383
          -0.275018
            0.43802
           0.861415
          -0.602108
          -0.945427
           0.450466
          -0.261912
          -0.321775
}
{
          -0.945427
          -0.602108
          -0.326383
          -0.321775
          -0.275018
          -0.261912
          -0.129061
            0.43802
           0.450466
           0.861415
}

[gumbogogo]

I do need to keep the originally generated list (generatednum[]). I've checked that list and it is only producing -1s and 0s (0000-100-1-10-1 etc.). Definitely something wrong with my rand()

[cire]

You might try:

randomnum[j] = startpoint + rand()/(RAND_MAX/static_cast<double>(range));
for line 23. It's integer division as it stands now.

[gumbogogo]

Thank you that worked. Now however, my sorted list is wrong, haha. I believe what's happening is that I'm generating a new list for every iteration of i, so it's using different numbers each time. Could I just define

randomnum[10000]=startpoint + rand()/(RAND_MAX/static_cast<double>(range));

outside of my for loops?

EDIT: Nope

[gumbogogo]

I have it figured out, thank you everyone