How does this loop work (Check Duplicate in array loop)

Question

Beginner programmer here can someone please explain to me how this loop works. How can the loop detect the duplicate element in the array? sorry for the simple question.

#include <iostream>


using namespace std;
int main()
{
    int num[5];
    int numSize = sizeof(num) / sizeof(num[0]);


    for(int i = 0; i < numSize; i++)
    {
        cout << "Enter number : ";
        cin >> num[i];

        if(i >= 0)
        {

            for(int j = 0 ; j < numSize; j++)
            {
                if(num[i] == num[j] && (i != j))
                {
                    i--;
                    j++;
                }
            }
        }
    }

    for(int p = 0; p < numSize; p++)
        cout << num[p] << " ";
}

This code seems to cause undefined behavior. Namely, `num[j]` for `j>i` is read before it was written. — Daniel Langr, Aug 04 '20 at 06:09
Why don't you put print statements or debug it and see what is going on? — karastojko, Aug 04 '20 at 06:09
While it sometimes can be useful to read other peoples code to learn, reading undocumented and uncommented code is *hard*, and sometimes impossible to learn something useful from. Before attempting to check other peoples code (especially people who you can't communicate with) I suggest you learn the basics some other way. Perhaps get and read [some good books](https://stackoverflow.com/questions/388242/the-definitive-c-book-guide-and-list/388282#388282), or take a couple of classes. — Some programmer dude, Aug 04 '20 at 06:14

score 0 · Accepted Answer · answered Aug 04 '20 at 07:40

Avoid indexing when it is possible. It is not final solution but it may show you right direction.

#include <iostream>
#include <algorithm>

int main()
{
    int num[] = { 0, 5, 6, 8, 0, 2, 5, 8 };
    std::sort(std::begin(num), std::end(num));   
    
    auto it = std::begin(num);        
    while(1) {
        it = std::adjacent_find(it, std::end(num));
        if(it != std::end(num)) {
            std::cout << "Double pairs " << *it << " and " << *(it+1) << std::endl;
            ++it;
        }
        else {
            break;
        }        
    }
    
    return 0;
}

Output:
Double pairs 0 and 0                                                                                                                                                               
Double pairs 5 and 5                                                                                                                                                               
Double pairs 8 and 8

How does this loop work (Check Duplicate in array loop)

1 Answers1