Generating unique random numbers except from a spe

Question

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• Unique (non-repeating) random numbers in O(1)? 21 answers

I was wondering, how can I generate unique random numbers except from a specific one. For example, if I want to generate numbers in range 1 to 10 except from 3, the output should be something like this:

7 6 1 2 4 9 5 8 10

Answer 1

Shuffle the numbers 1 - 10 and remove 3.

It doesn't matter if you remove the 3 before or after shuffling.

Alternatively, shuffle the numbers 1 - 9 and relabel 3 as 10...

For shuffling without bias you can use for example the Fisher-Yates algorithm. http://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle

Answer 2

This is my answer - returns random value in [min, max), except "except".

int myrand(int min, int max, int except) {
   int rc;
   do {
      rc = min + rand() % (max - min); 
   } while(rc == except);
   return rc;
}

Answer 3

This code will generate unique random numbers from minimum to maximum of a given range.

#include<stdio.h>
#include <stdbool.h>
#include <stdlib.h>
#include <time.h>

int main()
{

    int max_range, min_range, i = 0, rand_num;

    srand((unsigned)time(NULL));
    printf("Enter your maximum of range: ");
    scanf("%d", &max_range);
    printf("Enter your minimum of range: ");
    scanf("%d", &min_range);
    bool digit_seen[max_range + 1]; // VLAs For C99 only 

    for (int i = min_range; i <= max_range; i++)
        digit_seen[i] = false;  

    for (;;)
    {  
         rand_num = rand() % max_range + min_range;
         if(rand_num !=3)
             if(!digit_seen[rand_num])
             {
                printf("%d ", rand_num);
                digit_seen[rand_num] = true;
                i++;
             }
         if( i == (max_range - 1) )
             exit(0);
    }    
    return 0;
}

Answer 4

Generate random number in the range 1..9 and add one if the number is greater than or equal to 3.

Answer 5

You may try like this:-

unsigned int
randomnumber(unsigned int min, unsigned int max)
{
       double scaled = (double)rand()/RAND_MAX;

       return (max - min +1)*scaled + min;
}

then later you can do this:-

 x = randomnumber(1,10);

if (x==3)
{ x = x+1;}

or

if (x!=3)
{ printf("%d",x)}

Answer 6

Generate a number. Check its value, if the number is 3 generate another one. If it isn't 3 then use it.

EDIT: Thinking before coffee is a terrible plan. If you want to get every number in the range in a random order then I agree with the others talking about shuffling lists. If however you want some random subset of the range I would store a list of forbidden values. Shuffling and only taking the first n numbers would also be suitable if the range isn't very large (e.g. not something like 0<x<INT_MAX).

Every time you generate a number check if the generated number is on the forbidden list and if it is, generate another number. Every time you generate a valid number you add it to the list to ensure generated numbers are unique. The list should also be initialised with your unwanted numbers (3 in the example given).