# Count how many pairs of a number appear in an array

Lets say with the array:

array = {1,2,1,2,1,3,2,1};

I want the output to be:

2 pairs of number 1, 1 pair of number 2

For that I created a Hash table. The code :

class Trail{
static void countFreq(int arr[], int n)
{
Map<Integer, Integer> mp = new HashMap<>();

// Insert elements into HashTable while avoiding overwriting:
for (int i = 0; i < n; i++)
{
// This part is to avoid overwriting:
if (mp.containsKey(arr[i]))
{
mp.put(arr[i], mp.get(arr[i]) + 1);
}
else
{
mp.put(arr[i], 1);
}
}
// Traverse through map and print frequencies
for (Map.Entry<Integer, Integer> entry : mp.entrySet())
{
System.out.println(entry.getKey() + " " + entry.getValue());

}

}

public static void main(String[] args) {

int arr[] = {1,2,1,2,1,3,2,1};
int n = arr.length;
countFreq(arr, n);
}
}

Not sure what to do next to print out the desired output. Been stuck in this simple part for a long time.

The calculation of frequencies seems to be fine, only printing part needs to be addressed. To get the number of pairs, divide the frequency by 2 (or shift right by 1), and skip if the pair count is 0 (according to the expected output).

Printing should be moved into a separate method:

static void printFreq(Map<Integer, Integer> mp) {
for (Map.Entry<Integer, Integer> entry : mp.entrySet()) {
int pairs = entry.getValue() / 2;
if (pairs < 1) {
continue; // skip 0 pairs
}
System.out.print(", ");
}
String p = pairs > 1 ? " pairs " : " pair ";
System.out.print(pairs + p + "of number " + entry.getKey());
}
System.out.println();
}

However, Stream API may be used for such tasks:

• use vararg int ... arr to pass the array of integer values in more convenient way (n as the array length is redundant)
• use Collectors.groupingBy and Collectors.summingInt (or Collectors.counting) to calculate raw frequency
• calculate the number of pairs
• map each key-value pair into String
• join the strings using Collectors.joining
static void countFreq(int ... arr) {
String message = Arrays.stream(arr)
.boxed()
.collect(Collectors.groupingBy(
x -> x,
Collectors.summingInt(x -> 1)
)) // Map<Integer, Integer>
.entrySet()
.stream()
.peek(e -> e.setValue(e.getValue() / 2))
.filter(e -> e.getValue() > 0)
.map(e -> String.format("%d %s of number %d",
e.getValue(), e.getValue() > 1 ? "pairs" : "pair", e.getKey()
))
.collect(Collectors.joining(", "));
System.out.println(message);
}

Output (in both cases):

2 pairs of number 1, 1 pair of number 2