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Accepted
Ricardo Jesus Jarquin Perez
Asked: 2020-09-26 10:31:49 +0800 CST 2020-09-26 10:31:49 +0800 CST

C++ random number generator

  • 772

the problem is the following I have my code in which 10 random numbers are generated in an array, then it shows them and then it orders them, what I want is that when re-executing the "fill" function, which is the one that generates the numbers an array is generated with new numbers and they are not the same as the previous time.

#include<iostream>
#include <stdlib.h>
#include <time.h>
using namespace std;
int llenar();
int mostrar();
int array[10];
int burbuja();
int main(){
 llenar();
 mostrar();
 burbuja();
 llenar();
 mostrar();
};

int burbuja(){
  int i,j,aux;

  for(i=0;i<10;i++){
      for(j=0;j<10;j++){
          if(array[j] > array[j+1]){
              aux = array[j];
              array[j] = array[j+1];
              array[j+1] = aux;
          }
      }
  }

  for(i=1;i<11;i++){
      cout<<array[i]<<" ";
  }
  cout<<endl;
}

int mostrar(){
    for(int i=0; i<10; i++){
        cout << array[i] << " ";
   }
cout<<endl;
};

int llenar(){
  array[0]='NULL';

  int num, c;
  srand(time_t());

  for(c = 1; c <= 10; c++)
  {
      array[c-1] = 1 + rand() % (100 - 1);
     /* cout << array[c-1]<< " ";*/
  }

  return 0;
}
c++

3 Answers

  1. Best Answer

    You have a few bugs :

    int llenar( ) {
  array[0]='NULL';

    If it is int array[10], surely what you want to do is

    array[0] = 0;

    Changing the above, and also srand( time( NULL ) );, the numbers start to come out random.

    Apart from that, you declare

    int burbuja()

    Y

    int mostrar( )

    However, in none of them do you return anything , and the compiler warns you.

    And the last:

    int llenar( ) {
  array[0] = 0;

  int num, c;

    You have another notice that you don't use the variable num.

    edit

    It repeats the numbers because, between successive calls to srand( time( NULL ) ), too little time passes .

    To make the effect more noticeable, remove it srand( )from the function llenar( )and put it at the beginning ofmain( )

    int main( ) {
  srand( time( NULL ) );
  ... resto del código ...
    • 2

  2. If you want " C++ Random Number Generator " completely forget about C utilities like C randheaders <stdlib.h>and<time.h> .

    Starting from the C++11 standard, the C++ language offers a complete pseudo-random number generation library that allows you to choose the probability distribution (uniform, Bernoulli, Poisson, normal, discrete, constant, linear...), the underlying type of the generated value and even the algorithm to use (minstd, mt19937, ranlux, knuth...).

    From your question I deduce that you want a uniform distribution (which is what rand()1 offers ) between 1 and 100. With the C++ pseudo-random number library it can be done like this:

    // Este arreglo tiene todos los valores a 0 por defecto. El compilador se encarga de ello.
int array[10]{};

//vv <---- devuelve void, no int.
void llenar(){
    // El random device gestiona las semillas de generacion
    std::random_device rd;
    // Usamos el algoritmo mt19937
    std::mt19937 gen(rd());
    // Obtendremos una distribucion uniforme entre 1 y 100
    std::uniform_int_distribution<> dis(1, 100);

    for (auto &valor : array){
        valor = dis(gen);
    }

    // La funcion no devuelve nada porque no tiene nada que devolver.
}

    Each time it is called llenar, a new one will be created std::random_devicewhich will have a different seed and consequently give different numbers.

    1 But its uniformity is destroyed by using the modulo operation, as it is in your code.

    • 2

  3. Try including the ctime library :

    #include <ctime>

    And then in the fill function , change:

    srand(time_t());

    By:

    srand(time(NULL));

    The srand function defines a seed that changes the pseudorandom sequence that rand generates . Using time(NULL) the current value of time is used as a seed, which will make the sequence generated by rand different each time the program is executed . Your code is already doing this, but (without testing) I think this way should work.

    • 0