rand between -x and x

[NoNameoN]

I everybody, I'm searching for a method to use rand()and srand() to have a randomic number between -x and x.
x is a normal double variable... how can I solv it?
I sow that in C you can use seed...

[keskiverto]

between -x and x

Do you mean [-x, x] or [-x, x) or (-x, x] or (-x, x)?

http://www.cplusplus.com/reference/random/uniform_real_distribution/

[Ganado]

Does it need to be using rand(), or are you only saying that because that's what you're familiar with?

I second using uniform_real_distribution, it's much easier to work with, despite the slightly more complicated setup.
See the example in keskiverto's link.

[salem c]

Something simple like
rand() % (2*x) - x;

Adjust with some +/-1 depending on whether you want open or closed intervals.

[Ganado]

OP's question was for reals, so you'd need to incorporate RAND_MAX in there to normalize it, if you need to go that route.

But note that this comes with trade-offs:
http://www.azillionmonkeys.com/qed/random.html

[salem c]

Indeed. Standard C rand() is only good enough for student coin toss homework.

"Randomness is too important to be left to chance".
http://www.quotationspage.com/quote/461.html

[deleted account xyzzy]

The C standard recommends not using rand if there are alternatives available. (C11 standard, note 295)

C++ certainly does have that.

A bit out-dated and long-winded, but the issues raised are still valid about avoiding the use of rand:
https://web.archive.org/web/20180123103235/http://cpp.indi.frih.net/blog/2014/12/the-bell-has-tolled-for-rand/

std::random device is more robust than rand for most trivial cases and is as easy to use when instantiated. No need to seed the bugger.

The C++ random engines and distributions look "scary" because they offer a lot more options vs. rand. Most programming courses don't cover <random> (or <chrono>).

Random number generation, from C to shining C++:
https://www.learncpp.com/cpp-tutorial/59-random-number-generation/

[NoNameoN]

Hi everybody, thanks all for the answers.
I should do it only for university homework so it isn't "important"...
I'm a beginner and I want to make a program where a variable takes a value from a random number between -x and x.
So if I have
double x = 5.0;
What I want is
double y = rand();
And y should have a value between -5.0 and 5.0 .... so -4.2 || 0.0 || 3.8 ecc...

[lastchance]

#include <iostream>
#include <string>
#include <cstdlib>
#include <ctime>
using namespace std;

double u01()
{
   string str = "0.";
   for ( int i = 0; i < 14; i++ ) str += "0123456789"[rand() % 10];
   return stod( str );
}

int main()
{
   int N = 10;
   srand( time(0) );

   int x;
   cout << "Input x: ";   cin >> x;
   cout << N << " random numbers in [" << -x << ", " << x << ") are:\n";
   while( N-- ) cout << -x + 2 * x * u01() << ' ';
}

Input x: 5
10 random numbers in [-5, 5) are:
-1.83806 -2.27038 3.10626 3.85746 -1.06539 -3.0556 -4.70587 1.08927 4.3542 1.82575 

[keskiverto]

#include <iostream>
#include <random>

int main()
{
   using std::cout;
   int N = 10;
   int x;
   cout << "Input x: ";   std::cin >> x;
   cout << N << " random numbers in [" << -x << ", " << x << ") are:\n";
   std::default_random_engine generator;
   std::uniform_real_distribution<double> distribution( -x, x );
   while( N-- ) cout << distribution(generator) << ' ';
}

I should do it only for university homework so it isn't "important"...

Isn't it important what one learns in university?

[Ganado]

double u01()
{
   string str = "0.";
   for ( int i = 0; i < 14; i++ ) str += "0123456789"[rand() % 10];
   return stod( str );
}

This is creative.

[lastchance]

But it definitely shouldn't be used in anger!

[thmm]

@keskiverto,

Isn't it important what one learns in university?

Not if it is some very old stuff <= C++98

[deleted account xyzzy]

A university/school teaching C++11 or later? Especially to beginners?!? WHOA! That is a radical idea!

[TheToaster]

My university is currently teaching C++14, but they allow me to submit code written in C++17 or some C++20 features since they are running the latest version of g++ provided I give the exact compilation instructions or makefile

[deleted account xyzzy]

That is not the norm for most schools or universities. The curriculum is very often years out of date, usually taught by people who were trained by a curriculum just as old. If not older.

[keskiverto]

Learner2 wrote:

[quote=keskiverto]Isn't it important what one learns in university?

Not if it is some very old stuff <= C++98[/quote]
Exactly. One does probably pay (time and money) for studying in university.

Is it ok for a shop to sell food that has "Best before" date long gone?
Is it wise for a customer to waste resources (money) on worthless products?

I deem both sides guilty.