Set

In Ruby, sets have only unique elements. Like a hash, a set provides fast lookups. But it stores no values alongside the keys—it only stores keys.

Sets provide many logical methods for adding, removing and testing elements. We can use operations like subset for easy analysis.

First example

This example creates a new Set with the Set.new syntax. The argument to new() is an array of 2 elements—each element is added to the set.

Start We call the add() method twice. One string, "socrates," is added but already exists. The duplicate is not stored.

Next We call the size() method on the set, which returns 3. The set has 3 elements because the duplicate was not retained.

Finally We invoke the "include" method. This returns true because the argument exists within the set.

Info When we call "include," a hashed lookup occurs. In large sets, this is much faster than a linear search through the elements.

require "set"

# Create new Set.
s = Set.new(["plato", "socrates"])

# Add two more elements.
s.add("cebes")
s.add("socrates")

# Get size.
puts s.size()

# See if Set includes this element.
puts s.include?("socrates")3
true

Merge, each

Next, this program uses the merge() method and the each() iterator. With merge(), we pass in a collection and the set adds all of its elements.

Detail With each, we iterate over the elements in the set. This uses the same syntax forms available on the hash and array in Ruby.

Tip This is the simplest way to loop over a set. In this example, we use the identifier "n."

require "set"

# Create set.
s = Set.new(["x", "y"])

# Merge in these two elements.
s.merge(["y", "z"])

# Iterate the set.
s.each do |n|
    # Display the element.
    puts n
endx
y
z

Subset, operators

One common operation with a set is the "subset" test. We can determine if all elements from one set are contained within another.

require "set"

# Contains three elements.
set1 = Set.new [10, 20, 30]

# Is a superset of set1.
set2 = Set.new [10, 20, 30, 40, 50]

# This is true.
if set1 <= set2
    puts true
end

# This is not evaluated to true.
if set2 <= set1
    puts false
end

# This is true.
# ... Subset is the same as the <= operator.
if set1.subset?(set2)
    puts true
endtrue
true

A set stores just keys, not values. In this way, it is like a hash with no values. In places where no values are required, this leads to clearer code.

Set benefits

Performance with sets is good—hashed lookups make searches faster. But the greater benefit with a set is its logical methods, which simplify code.