Remove duplicates

In an array, any element value may occur. But often we want just unique, non-duplicate elements. We invoke the uniq method to correct this problem.

Method info

Uniq and "uniq!" remove all except the first element with a value. But often programs are more efficient (and clearer) when values are kept in a hash, which eliminates dupes.

First example

We create 2 arrays. We first create an array containing integers. The array has two duplicated numbers: a 2 and a 3. We invoke the "uniq!" method, which operates in-place.

# An input array.
values = Array[4, 1, 2, 2, 3, 3]
print values, "\n"

# Convert the array with uniq.
values.uniq!
print values, "\n"

# A string array.
names = Array["cat", "dog", "dog", "dog", "mouse"]
print names, "\n"

# Create array copy with only unique values.
unique = names.uniq
print unique, "\n"
[4, 1, 2, 2, 3, 3]
[4, 1, 2, 3]
["cat", "dog", "dog", "dog", "mouse"]
["cat", "dog", "mouse"]

Find duplicates

This method uses the each iterator over on an array, and uses a hash to record which elements have been encountered. It then reports duplicates as it locates them.

def find_duplicates(elements)
    encountered = {}

    # Examine all elements in the array.
    elements.each do |e|
        # If the element is in the hash, it is a duplicate.
        if encountered[e]
            puts "Dupe exists for: " << e
        else
            # Record that the element was encountered.
            encountered[e] = 1
        end
    end
end

# Use the example method.
elements = ["bird", "dog", "bird", "cat"]
find_duplicates(elements)
Dupe exists for: bird

A summary

In Ruby, we find many convenience methods—uniq is one. In many languages, such functions must written from scratch. Ruby makes this unnecessary.

With a hash, we can eliminate duplicates automatically—this makes more sense for many programs. Ordering is lost in this way. Uniq meanwhile retains ordering.