Duplicates in an Array

Procedure to find and count duplicates in an array.

Finding Duplicates in a Sorted Array:

void findDuplicates(int a[], int n) {
​
int lastDuplicate = 0;
printf("Duplicates in the array :\n");
​
for (int i = 0; i < n; i++) {
// If two or more same elements appear consecutively
if (a[i] == a[i+1] && lastDuplicate != a[i]) {
printf("%d\t", a[i]);
// Keeping track of latest duplicate to avoid
// printing the same duplicate again
lastDuplicate = a[i];
}
}
printf("\n");
}

Counting Duplicates in a Sorted Array:

void countDuplicates(int a[], int n) {
​
int j;
printf("Duplicates in the array :\n");
​
for (int i = 0; i < n-1; i++) {
if (a[i] == a[i+1]) {
// Start counting duplicates from i + 1
j = i + 1;
while (a[i] == a[j]) {
// Finding the number of duplicates
j++;
}
printf("%d is appearing %d times\n", a[i], j-i);
// Start looking for duplicates again
i = j - 1;
}
}
​
}

Using Hash Array:

void findAndCountDuplicates(int a[], int n, int high) {
​
// Creating a hash array and initializing with 0
// Size of hash array = maximum element in input array
int h[high];
for (int i = 0; i < high; i++) {
h[i] = 0;
}
// Incrementing hash table values as per the numbers in
// original input array
for (int i = 0; i < n; i++) {
h[a[i]]++;
}
// If any element in hash table has value > 1,
// it means that the element is repeated
for (int i = 0; i <= high; i++) {
if (h[i] > 1) {
printf("%d appears %d times\n", i, h[i]);
}
}
​
}
Contributed by Nitin Ranganath