Generating a random double between a range of values

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Im currently having trouble generating random numbers between -32.768 and 32.768. It keeps giving me the same values but with a small change in the decimal field. ex : 27.xxx.

Heres my code, any help would be appreciated.

#include <iostream>
#include <ctime>
#include <cstdlib>

using namespace std;

int main()
{
    srand( time(NULL) );
    double r = (68.556*rand()/RAND_MAX - 32.768);
    cout << r << endl;
    return 0;
}

I should mention if you're using a C++11 compiler, you can use something like this, which is actually easier to read and harder to mess up:

#include <random>
#include <iostream>
#include <ctime>


int main()
{
    //Type of random number distribution
    std::uniform_real_distribution<double> dist(-32.768, 32.768);  //(min, max)

    //Mersenne Twister: Good quality random number generator
    std::mt19937 rng; 
    //Initialize with non-deterministic seeds
    rng.seed(std::random_device{}()); 

    // generate 10 random numbers.
    for (int i=0; i<10; i++)
    {
      std::cout << dist(rng) << std::endl;
    }
    return 0;
}

As bames53 pointed out, the above code can be made even shorter if you make full use of c++11:

#include <random>
#include <iostream>
#include <ctime>
#include <algorithm>
#include <iterator>

int main()
{
    std::mt19937 rng; 
    std::uniform_real_distribution<double> dist(-32.768, 32.768);  //(min, max)
    rng.seed(std::random_device{}()); //non-deterministic seed
    std::generate_n( 
         std::ostream_iterator<double>(std::cout, "\n"),
         10, 
         [&]{ return dist(rng);} ); 
    return 0;
}

How to generate random numbers in Java, To generate random numbers using the class ThreadLocalRandom , follow the steps below: Import the class java.util.concurrent.ThreadLocalRandom. Call the method. To generate random number of type int ThreadLocalRandom.current().nextInt() To generate random number of type double ThreadLocalRandom.current().nextDouble() In order to generate Random double type numbers in Java, we use the nextDouble () method of the java.util.Random class. This returns the next random double value between 0.0 (inclusive) and 1.0 (exclusive) from the random generator sequence. Declaration - The java.util.Random.nextDouble () method is declared as follows −

Also, If you are not using c++ 11 you can use the following function instead:

double randDouble(double precision, double lowerBound, double upperBound) {
 double random;
 random = static_cast<double>(((rand()%(static_cast<int>(std::pow(10,precision)*(upperBound - lowerBound) + 1))) + lowerBound*std::pow(10,precision)))/std::pow(10,precision);
 return random;
}

Java, Learn how to generate random numbers in Java - both unbounded a random bounded Long – that is, a Long value within a given range or� To generate random integers between two numbers, you can use the RANDBETWEEN function. In the example shown, the formula in B5 is: = RANDBETWEEN(1,100) This formula is then copied down from B5 to B11.

So, I think this is a typical case of "using time(NULL) isn't a great way of seeding random numbers for runs that start close together". There isn't that many bits that change in time(NULL) from one call to the next, so random numbers are fairly similar. This is not a new phenomena - if you google "my random numbers aren't very random", you'll find LOTS of this.

There are a few different solutions - getting a microsecond or nanosecond time would be the simplest choice - in Linux gettimeofday will give you a microsecond time as part of the struct.

Generate Random double type number in Java, In order to generate Random double type numbers in Java, we use the nextDouble() method of the java. util. Random class. This returns the next random double value between 0.0 (inclusive) and 1.0 (exclusive) from the random generator sequence. As well as a random double value with the Apache Commons Math library: @Test public void givenUsingApache_whenGeneratingRandomDoubleUnbounded_thenCorrect() { double generatedDouble = new RandomDataGenerator().getRandomGenerator().nextDouble(); } 8. Generate a Double Within a Range

It seams to be plainly obvious but some of the examples say otherwise... but i thought when you divide 1 int with another you always get an int? and you need to type cast each int to double/float before you divide them.

ie: double r = (68.556* (double)rand()/(double)RAND_MAX - 32.768);

also if you call srand() every time you call rand() you reset the seed which results in similar values returned every time instead of ''random'' ones.

Java: generating random integer and double in a range, Random class provides method nextDouble() which can return uniformly distributed pseudo random double values between 0.0 and 1.0. static� You can create random Double values: // returns a value between 0.0 and 1.0 scala> r.nextDouble res3: Double = 0.6946000981900997. You can set the seed value using an Int or Long when creating the Random object: scala> val r = new scala.util.Random(100) r: scala.util.Random = scala.util.Random@bbf4061. You can also set the seed value after a

I've added a for loop to your program:

#include <iostream>
#include <ctime>
#include <cstdlib>

using namespace std;

int main () {
    srand(time (NULL));

    for (int i = 0; i < 10; ++i) {
        double r = ((68.556 * rand () / RAND_MAX) - 32.768);

        cout << r << endl;
    }

    return 0;
}

Example output:

 31.6779 
-28.2096
 31.5672
 18.9916 
-1.57149 
-0.993889
-32.4737
 24.6982
 25.936 
 26.4152

It seems Okay to me. I've added the code on Ideone for you.

Here are four runs:

 Run 1:
    -29.0863
    -22.3973
     34.1034
    -1.41155
    -2.60232
    -30.5257
     31.9254
    -17.0673
     31.7522
     28.227

Run 2:
    -14.2872
    -0.185124
    -27.3674
     8.12921
     22.4611
    -0.414546
    -21.4944
    -11.0871
     4.87673
     5.4545

Run 3:
    -23.9083
    -6.04738
    -6.54314
     30.1767
    -16.2224
    -19.4619
     3.37444
     9.28014
     25.9318
    -22.8807

Run 4:
     25.1364
     16.3011
     0.596151
     5.3953
    -25.2851
     10.7301
     18.4541
    -18.8511
    -0.828694
     22.8335

Perhaps you're not waiting at least a second between runs?

Random Number Generation in Java, The random() method returns a double value with a positive sign, For generating random numbers between a given a range, you need to� Use randint () when you want to generate a random number from an inclusive range. Use randrange () when you want to generate a random number within a range by specifying step value. It produces a random number from an exclusive range. You should be aware of some value constraints of a randrange () function.

Generate Random Numbers between Specified Range in Java, In this post, we will see how to generate random integers between specified range in Java. 1. Random.nextInt() 2. Math.random() 3. ThreadLocalRandom class.. Generate random integers (maximum 10,000). Each integer should have a value between and (both inclusive; limits ±1,000,000,000). Format in column(s). Part 2: Go! Be patient! It may take a little while to generate your numbers

Java Random doubles() method example, The doubles() method of Random class returns an effectively unlimited stream of pseudorandom double values, each between zero (inclusive) and one (exclusive ). A pseudorandom double value is generated as if it's the result of method of DoubleStream class 10 random double values from range 0 to 1� By default, rand returns normalized values (between 0 and 1) that are drawn from a uniform distribution. To change the range of the distribution to a new range, (a, b), multiply each value by the width of the new range, (b – a) and then shift every value by a.

nextDouble, Gets the next random Double from the random number generator in the specified range. Generates a Double random value uniformly distributed between the� Generating random numbers within a range The rand () function generates random numbers that can be any integer value. But, to generate random numbers within a specific range, we have a formula that returns a random number between given ranges.

Comments
  • Have you tried using a loop. Since time(NULL) doesn't change that much from one second to the next, chances are that you get the same number each time. gettimeofday()'s microsecond count may also work better.
  • @MatsPetersson Passing consecutive values to srand should not and usually does not result in nearby results from even the first call to rand. (Though since there are no actual requirements on the quality of rand, I guess YMMV.)
  • I needed the range i specified. I would have used (int) if i wanted that range
  • Np, although you should also change the name of normal_dist since that should only be used with std::normal_distribution or similar. (normal distributions are also called Gaussian.)
  • Lol. Can't believe I missed that. Updated again :)
  • note that uniform_real_distribution generates numbers in the range [min,max) ie will not generate max.
  • The code doesn't do integer division; 68.556*rand()/RAND_MAX is like: (68.556*rand())/RAND_MAX, which means the numerator is a double.
  • ah, left to right, i see. I would still put it there to avoid unforeseen issues when i later change the code.
  • I know that works, but when you run the program again, it keeps giving the same exact loop, but with small changes in the decimals. It gives the same exact loop
  • Does my example output above look like the output on ideone? I ran it again and got, "30.3961 -13.9189 14.837 25.0323 -7.88294 24.9879 3.77392 18.4251 -8.11763 35.4337" then "25.0197 31.0387 -10.2659 -8.66796 -28.1792 -6.34876 7.51343 -32.6934 -23.4206 -16.8222". I'm not sure what you're doing wrong (unless you're waiting less than a second between runs), but the issue isn't in your code.
  • Yea i copied your code from ideone. The only problem i have is that the first value in the loop is always the same (small change in decimal). The rest is random.
  • Sounds like your PRNG is very low quality. Try adding an extra rand() call to the beginning of your program. (But, honestly, if you have any requirements all, don't use rand(). It has no quality guarantees.)
  • yep ! it works for some reason. In my main program, i have to put a useless rand() floating to make everything work, kinda weird. Thanks alot !