Given an array of integers representing the color of each sock, determine how many pairs of socks with matching colors there are. For example, there are 7 socks with colors [1, 2, 1, 2, 1, 3, 2]. There is one pair of color 1 and one of color 2. There are three odd socks left, one of each color. The number of pairs is 2.
Create a function that returns an integer representing the number of matching pairs of socks that are available.
sock_merchant([10, 20, 20, 10, 10, 30, 50, 10, 20]) ➞ 3
sock_merchant([50, 20, 30, 90, 30, 20, 50, 20, 90]) ➞ 4
sock_merchant([]) ➞ 0N/A