remove duplicate numbers from randomly generated array

Question

I am learning C++, and came across an idea about how to remove duplicates from a randomly generated integer array with values from 0 to 100. I coded it like this, but I see that some numbers are repeating. I really can't figure out the bug. Please help.

#include <iostream>
#include <ctime>

using namespace std;
int main()
{
  srand(time(NULL));
  int item[100];
  int z;
  int count = 0;

  for (z = 0; z < 100; z++) {
    item[z] = rand() % (z + 1);
    count++;
    cout << item[z] << endl;
  }

  cout << "Total = ";
  cout << count << endl;
  int i, j;
  int NewLength = 1;

  for (i = 1; i < 100; i++) {
    for (j = 0; j < NewLength; j++) {
      if (item[i] == item[j])
        break;
    }

    if (j == NewLength)
      item[NewLength++] = item[i];
  }

  for (int x = 0; x < 100; x++) {
    cout << item[x] << endl;
  }
}

Answer 1

You start with an array of 100 integers. Assuming some of the integers are duplicates, there are less than 100 unique integers. So, since there are less than 100 unique integers in your array, unless your algorithm generates new integer values which were not already in the original sequence, you can be certain you will see duplicates if you print out all 100 values of the array, as you do at the end or your program.

If you only print up to NewLength, you shouldn't see any duplicates (assuming your algorithm is correct, I didn't check it rigorously).

for (int x = 0; x < NewLength; x++) {
    cout << item[x] << endl;
}

Answer 2

If you can use C++11 and do NOT mind doing so, what you can do is store numbers 0 to 100 in an array.

Next you can shuffle them randomly and voila.. Array of unique numbers from 0 to 100 in a random order. Otherwise if you can't, ignore this solution.

#include <random>
#include <numeric>
#include <algorithm>
#include <iostream>


int main()
{
    int arr[100] = {0};

    std::iota(&arr[0], &arr[100], 0);

    std::random_device rd;
    std::mt19937 mt(rd());

    std::shuffle(&arr[0], &arr[100], mt);

    for (int i = 0; i < 100; ++i)
        std::cout<<arr[i]<<" ";
}

Another option is to generate an array of 0 to 100 and swap indexes at random (assuming you cannot use C++11 and random_shuffle).

Answer 3

#include <ctime>

int main()
{


      srand(time(NULL));
      int item[100];
      int z;
      int count = 0;

      for (z = 0; z < 100; z++) {
        item[z] = rand() % (z + 1);
        count++;
        cout << item[z] << endl;
      }

      cout << "Total = ";
      cout << count << endl;

      //Assume if duplicate found assign -1 in the place of duplicate

      for(int i = 0;i<100;++i)
      {
          if(item[i] == -1) //If this number already checked & it is a duplicate
              continue;

          for(int j=i+1;j<99;++j)
          {
              if(item[i] == item[j])
              {
                  //Duplicate found.
                  item[j] = -1; //Assign -1 in the place of duplicate
              }
          }
      }

         for (z = 0; z < 100; z++) {
             if(item[z] != -1)
                cout << item[z] << endl;
      }


        char c;
        cin >> c;
    return 0;
}

Answer 4

Here is yet another "remove duplicate from a randomly generated integer array".

This line of code below reverses one iteration if a duplicate random number is encountered.

for(int j=0;j<i;j++) if (random_once[j]==random_once[i]) i--;

Entire code:

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


int main()
{

      int size=100 ;
      int random_once[100 ];

      srand(time(0));

      for (int i=0;i<size;i++)  
      {
          random_once[i]=rand() % 100;

          // generate unique random number only once
          for(int j=0;j<i;j++) if (random_once[j]==random_once[i]) i--;  

      }
      cout<<" "<< i <<"\n\n\n ";
      for ( i=0;i<size;i++) cout<<" "<<random_once[i]<<"\t";

  return 0;
}

Output:

  60     61      96      77      89      94      31      87      19      24
 27      99      64      7       8       88      62      75      37      97
 28      55      92      48      91      47      50      13      36      85
 21      34      82      23      40      4       79      90      46      10
 18      0       2       74      56      33      70      84      49      73
 69      57      32      41      83      67      76      43      35      44
 80      14      25      93      11      15      63      9       26      16
 5       98      17      38      51      30      53      58      3       65
 66      52      95      86      78      29      42      12      45      59
 81      71      39      6       1       68      20      72      54      22

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