random number that doesn't repeat?

how do i generate random #'s that do not repeat? below is some small code for generating 20 random #s btw 1 and 20...but i don't know how to make sure the sequence of picking the #s does not repeat...

Code:

---

// rgen.cpp : Defines the entry point for the console application.
//

#include "stdafx.h"
#include <ctime>
#include <cmath>
#include <cstdlib>
#include <algorithm>
#include <fstream>
#include <iostream>
#include <iomanip>
using namespace std;

int main()
{
        srand((unsigned)time(0));
        int random_integer;
        for(int index=0; index<20; index++)
        {
                random_integer = (rand()%20)+1;

                cout << random_integer << endl;
        }
}

---

you can do it if you will rand() the indexs look at that code:

Code:

---

  int arr[20]={0};

  for(int i=0; i<20; i++)
    arr&#091;i&#093;=i+1;
 
  srand((unsigned)time(0));

  int lastItem = 20;
  for(int index=0; index<20; index++)
  {
    int arrIndex = rand()%lastItem;
    lastItem--;
    int tmp = arr&#091;lastItem&#093;;
    arr&#091;lastItem&#093; = arr&#091;arrIndex&#093;;
    arr&#091;arrIndex&#093; = tmp;
  }

---

always you will get numbers between 1-20 shuffled and not repeated ;)

Cheers

you can also use random_shuffle in the C++ library.

Code:

---

int arr[20];

for (int i=0; i<20; ++i)
  arr[i] = i + 1;

std::random_shuffle(arr,arr+20);  // #include <algorithm>

---

Quote:

---

Originally Posted by golanshahar

you can do it if you will rand() the indexs look at that code:

Code:

---

  int arr[20]={0};

  for(int i=0; i<20; i++)
    arr[i]=i+1;
 
  srand((unsigned)time(0));

  int lastItem = 20;
  for(int index=0; index<20; index++)
  {
    int arrIndex = rand()%lastItem;
    lastItem--;
    int tmp = arr[lastItem];
    arr[lastItem] = arr[arrIndex];
    arr[arrIndex] = tmp;
  }

---

always you will get numbers between 1-20 shuffled and not repeated ;)

Cheers

---

thanks...but both

Code:

---

cout << arrIndex <<endl;

---

and

Code:

---

cout << tmp<<endl;

---

repeats the numbers....

how would I implement the swap() function? I think If I can swap the randomly generated number with its negative value, I'd never repeat the randomly generated code. Please help.

Thanks Philip,
But random_shuffle() will still repeat the numbers I think...It will just shuffle the generated random numbers. This helps so one doesn't go thro' the whole process of generating random numbers for say 1000 iterations...

how would one implement the random_shuffle code for x number of iterations?

You could always keep a list of numbers already generated. If rand() repeats a number just keep adding to it or subtracting from it until you find an unused number.

Make a new vector/list containing the indexe in your collection ( 0 to count-1). Use random_shuffle on this new collection than use it to iterate your data collection.

Quote:

---

Originally Posted by aaadetos

thanks...but both

Code:

---

cout << arrIndex <<endl;

---

and

Code:

---

cout << tmp<<endl;

---

repeats the numbers....

how would I implement the swap() function? I think If I can swap the randomly generated number with its negative value, I'd never repeat the randomly generated code. Please help.

---

try this

Code:

---

#include <ctime>
#include <cstdlib>
using namespace std;
int main()
{
 srand((unsigned)time(0));
 int random_integer;
 int lowest=1, highest=10;
  int range=(highest-lowest)+1;
  for(int index=0; index<20; index++)
  {
        random_integer = lowest+int(rand()+range*rand()/(RAND_MAX +1.0));
        cout << random_integer << endl;
  }
}

---

It is not clear to me what you want.

Do you want the numbers 1 thru 20 generated randomly ? What
do you want the next iteration ? If you still want the numbers
1 thru 20, just call random_shuffle again (in a loop)

Or do you not care what the numbers are ... you just want to
generate new ones each iteration (with the numbers in the current
iteration not equal to any of the numbers from previous iterations).

Thanks Philip,
Yes, calling random_shuffle in a loop will work for x # of iterations. Here is what I want to do for now:

I want to randomly pick 20 #'s from an array of x elements (say x = 20), in a manner that i don't pick any number from the array of 20 elements more than once. Let the elements in the source array be numbered 1 - 20 for instance.

Now, If I can randomly pick from this array, I need to implement the swap() function such that it replaces the the number I have picked with its negative conterpart...say i pick #2; it replaces it with -2, so that since the random # generator goes only from 1 - 20, via:

Code:

---

rand()%20+1

---

, i 'll never pick the number again.

Apparently, with the code below, I am not:
1) selecting from the array layer_array[]
2) correctly implementing swap()

Please help.

Code:

---

int main()
{
        const int size = 20;

        //initialize all elements of the layer array to zero.
        int layer_array[size]={0};

        //re-initialize the array elements by incrementing...good if u got a million variables...
        for(int i=0; i<size; i++)
                layer_array[i]=i+1;

        //seed the random # generator using the system time
        srand((unsigned)time(0));

        int random_integer[size] ={0};//initialize the array of random numbers

        for(int index=0; index<size; index++)
        {
                random_integer[index] = (rand()%size)+1;
                swap(&layer_array[index],&layer_array[index*-1]);

                cout << random_integer[index] << endl;
        }

        return 0;


}

---

Quote:

---

Originally Posted by aaadetos

Thanks Philip,
Yes, calling random_shuffle in a loop will work for x # of iterations. Here is what I want to do for now:

I want to randomly pick 20 #'s from an array of x elements (say x = 20), in a manner that i don't pick any number from the array of 20 elements more than once. Let the elements in the source array be numbered 1 - 20 for instance.

Now, If I can randomly pick from this array, I need to implement the swap() function such that it replaces the the number I have picked with its negative conterpart...say i pick #2; it replaces it with -2, so that since the random # generator goes only from 1 - 20, via:

Code:

---

rand()%20+1

---

, i 'll never pick the number again.

Apparently, with the code below, I am not:
1) selecting from the array layer_array[]
2) correctly implementing swap()

Please help.

Code:

---

int main()
{
        const int size = 20;

        //initialize all elements of the layer array to zero.
        int layer_array[size]={0};

        //re-initialize the array elements by incrementing...good if u got a million variables...
        for(int i=0; i<size; i++)
                layer_array[i]=i+1;

        //seed the random # generator using the system time
        srand((unsigned)time(0));

        int random_integer[size] ={0};//initialize the array of random numbers

        for(int index=0; index<size; index++)
        {
                random_integer[index] = (rand()%size)+1;
                swap(&layer_array[index],&layer_array[index*-1]);

                cout << random_integer[index] << endl;
        }

        return 0;


}

---

---

That seems like it's more complicated than it needs to be. I don't see in the code where you're ignoring negative numbers either. As I mentioned earlier, if rand results in you picking a number that's already used, just pick the next available number that hasn't been used yet. Alternatively, you could use a dynamic array, vector or list and remove an item entirely once it's selected. Then just use the modulus operator using the current size of the list of available numbers.

i'm not ignoring -ve numbers in the code....
the swap() is to replace the randomly picked number with a negative one. I'm doing this incorrectly, cos the line:

Code:

---

swap(&layer_array[index],&layer_array[index*-1]);

---

gives error c2664....

What im trying to do forms the backbone of latin hypercube sampling...and things may get sloppy or really slow down system resources if i add and subtract from the array in a piecewise manner...think abt 10,000 iterations....
Thanks again.

Quote:

---

Originally Posted by aaadetos

i'm not ignoring -ve numbers in the code....
the swap() is to replace the randomly picked number with a negative one. I'm doing this incorrectly, cos the line:

Code:

---

swap(&layer_array[index],&layer_array[index*-1]);

---

gives error c2664....

What im trying to do forms the backbone of latin hypercube sampling...and things may get sloppy or really slow down system resources if i add and subtract from the array in a piecewise manner...think abt 10,000 iterations....
Thanks again.

---

But you're never checking for a negative number when setting your random_integer value. Why use swap at all? If you're really set on this procedure, and it really isn't a good approach, why not just code layer_array[index] *= -1? What do you plan on doing when you do detect a negative number in layer_array?

The Boost Random Number Library provides a vast variety of generators and distributions to produce random numbers having useful properties, such as uniform distribution.

http://www.boost.org/libs/random/index.html

Quote:

---

Originally Posted by aaadetos

thanks...but both

Code:

---

cout << arrIndex <<endl;

---

and

Code:

---

cout << tmp<<endl;

---

repeats the numbers....

how would I implement the swap() function? I think If I can swap the randomly generated number with its negative value, I'd never repeat the randomly generated code. Please help.

---

you should check the arr in the end! it will always be shuffled after my code add :

Code:

---

  for(int index=0; index<20; index++)
    cout << arr[index] << endl;

---

and you will see each time randomized unrepeated numbers from the range 1-20

Cheers

Hi golanshahar, your code totally nailed it: killed 2 birds with one stone without the need for random_shuffle(), etc...how do you do it? I reposted the code, but can't really figure out how it works....please explain what happens at the 2 statements below:

Code:

---

                arr[lastItem] = arr[arrIndex];
                arr[arrIndex] = tmp;

---

Also am I on the right track with understanding the rest of the program?

Thanks a lot.

Code:

---

int main()
{
        //Initialize the array to be sampled: 1-20
        int arr[20]={0};

    for(int i=0; i<20; i++)
                arr[i]=i+1;

        //Provide seed of system time to the random # generator.
        srand((unsigned)time(0));

        int lastItem = 20;

        //For each instance of the random number generator.....
        for(int index=0; index<20; index++)
        {
                //name the random number generated.
                int arrIndex = rand()%lastItem;
                //Reduce the number of items from which sampling is to be done, by 1
                lastItem--;

                //call the new last item: 19, tmp.
                int tmp = arr[lastItem];

                //je n'est pas comprendre!
                arr[lastItem] = arr[arrIndex];
                arr[arrIndex] = tmp;

                //traverse the array and write out each value to the screen
                for(int index=0; index<20; index++)
                    cout << arr[index] << endl;
        }

        return 0;
}

---

Quote:

---

Originally Posted by aaadetos

Hi golanshahar, your code totally nailed it: killed 2 birds with one stone without the need for random_shuffle(), etc...

---

Glad i could help :wave:

Quote:

---

Originally Posted by aaadetos

how do you do it? I reposted the code, but can't really figure out how it works....please explain what happens at the 2 statements below:

Code:

---

                arr[lastItem] = arr[arrIndex];
                arr[arrIndex] = tmp;

---

---

well the basic idea is to rand the indexs of the array instead of the data item themselves.
lets look at simple sample
say we have this array:
[1,2,3,4,5]
and we want to shuffle ( or rand() ) the items so each time we will get different permotaion of the 1...5 numbers.

now if you will shuffle the data itself ( what you tried doing in your first post ) of course you can rand() the number lets say 4 more then once! and this is not what we want.

so better idea is to rand() the indexs and each rand() iteration we reduce the rand() limit. meaning:
first time we getting a number from rand between 0-4 (n-1)
lets say 3

the item in the 3rd index is 4 so what we are doing is swapping between the last item and the item we got from rand so our array will look like this:
[1,2,3,5,4]
now we reducing the rand() to rand()%n-1 so we will rand number bewteen 0-3
lets say we rand 1
so again we will swap the item we rand with the pointer to the n-1 so array will look like this:
[1,5,3,2,4]
and so on.....

in that way of action we will always have shuffled array without repeats of the same item.

this is VERY basic trick it is used in games ( like shuffling cards etc ), and images displaying ( When you want to fade in an image but in a way each time you will display different pixel so the result will be like million of pieces of the image building it step by step ( power point for instace have this effect ;) )

anyway glad i could help and hope the idea is more clear now :D

Cheers