Display duplicates in array in C

Omg. Thanks! I knew I had to find out if my dupArray already contains a duplicate number but what I was doing to solve the problem did not work. Your logic makes perfect sense. Here's my quickly adjusted code (now working):

#include<stdio.h>



int main(void){

int dupArray[100], newArray[100], array[100], size = 0, number=0, i=0, j=0,   
x=0, y = 0, m = 0, n = 0, exists = 0;

printf("How many numbers do you want to compare? ");
scanf("%d", &size);

for(i = 0; i < size; i++){  
printf("Enter number %d", i+1);
printf("/%d: ", size);
scanf("%d", &array[i]);
}
for(y=0; y<size; y++){
for(j=0; j<size; j++){
exists = 0; 
for(m=0; m<=x; m++){
    if(array[j]==dupArray[m]){
    exists = 1;
    }   
}   
if(array[j]==array[y]&&y!=j && !exists)
{

    dupArray[x] = array[j];

    printf("duplicate: %d", dupArray[x]);
    printf("\n");
    x++;
}
}
}


return 0;
}

Now I'm going to make it look nice and use functions as a programmer should. :) Thanks again!

A simple and clean implementation of the problem

#include <stdio.h>

int exists(int arr[], int size, int num) {
    int i;
    for (i = 0; i < size; ++i) if (arr[i] == num) return 1;
    return 0;
}

int main(void) {
    int size, i, len = 0;
    scanf("%d", &size);
    int array[size], dups[size];
    for (i = 0; i < size; ++i) {
        scanf("%d", &array[i]);
        if (exists(array, i, array[i]) && !exists(dups, len, array[i]))
            printf("%d ", dups[len++] = array[i]);
    }
    return 0;
}

Input

10
5 5 6 3 6 7 9 0 0 4

5
5 1 5 6 5

Output

5 6 0

5

See DEMO.

You can try this code:

 #include <stdio.h>

int checkDuplicateInArray(int *s, int n){
    int number = 3;
    int found = 0, i = 0;

    while(i<n){
        if(s[i] == number){
            if(found == 0){
                printf("There was an\t\t%d\tFound at position\t%d\n", s[i], i+1);
                found++;
            }else{
                printf("There was another\t%d\tFound at position\t%d\n", s[i], i+1);
            }
        }
        i++;
    }

    if(found == 0){
        printf("There was no match Found\n");
        return 1;
    }
    return 0;
}

int main(void){
    int array[] = {10,2,65,3,5,19,7,8,3,10,55,12,99,3,17};
    int length = sizeof array / sizeof array[0];

    checkDuplicateInArray(array,length);

    return 0;
}

Output:

There was an      3   Found at position   4
There was another   3   Found at position   9
There was another   3   Found at position   14

This should be enough to give you an Idea.

First of all, your app might crash, if the user enters "101" when asked how many numbers he want to compare. That's because your reserve only space for 100 numbers in memory but you allow the user to enter as many numbers as he wants. You fix that by either limiting the size:

int size;
int array[100];
while (true) {
  printf("How many numbers do you want to compare? ");
  scanf("%d", &size);

  if (size <= 100) break;
  printf("At most 100 numbers are allowed!\n");
}

This will keep asking, until the user enters a numbers smaller or equal to 100 (or until he hits CTRL+C to terminate the program) or you allocate memory dynamically:

int size;
int * array = NULL;
while (true) {
  printf("How many numbers do you want to compare? ");
  scanf("%d", &size);

  if (size > 0) break;
  printf("You must enter a value > 0!\n");    
}
// calloc(x,y) => reserve memory for x elements,
//                each y bytes in size.
array = calloc(size, sizeof(int));

Then the most efficient way to check for duplicates in an array of numbers is to actually sort that array (or if you need to keep the original order, to sort a copy of it) and then duplicates will be directly next to each other. E.g. input is 3 5 7 5 9 2 7 4 8 3 and now you sort it, it will be 2 3 3 4 5 5 7 7 8 9 and now it's very easy to spot the duplicates. Finding duplicates in a sorted array is like that:

int i;
int lastValue = array[0];
// Of course the statement below is "a lie",
// I just need lastValueReported to be different to lastValue
int lastValueReported = lastValue - 1; 
for (i = 1; i < size; i++) {
  int currentValue = array[i];
  if (currentValue == lastValue) {
    // Wow, we have a duplicate!
    // But have we reported it already?
    if (currentValue != lastValueReported) {
      printf("duplicate: %d\n", currentValue);
      lastValueReported = currentValue;
    }
  }
  lastValue = currentValue; 
}

Now that was easy, wasn't it? This leaves you only with one last problem: How to sort an array of ints. Fortunately sorting is a very well known problem with a countless number of possible solutions and I leave this exercise to the reader. I you think about it, you may even come up with a simple sort scheme like selection sort or bubble sort all by yourself. These two are not very fast but sufficient for your use case.