Add last for vector (#519)

Author: kojix2

lib/daru/vector.rb

@@ -474,6 +474,11 @@ def tail q=10
       self[start..(size-1)]
     end
 
+    def last q=1
+      # The Enumerable mixin dose not provide the last method.
+      tail(q)
+    end
+
     def empty?
       @index.empty?
     end

spec/vector_spec.rb

@@ -850,6 +850,24 @@
         end
       end
 
+      context '#last' do
+        subject(:vector) do
+          Daru::Vector.new (1..20).to_a, dtype: dtype
+        end
+
+        it 'takes 1 by default' do
+          expect(vector.last).to eq 20
+        end
+
+        it 'takes num if provided' do
+          expect(vector.last(3)).to eq Daru::Vector.new (18..20).to_a, index: (17..19).to_a
+        end
+
+        it 'does not fail on too large num' do
+          expect(vector.last(3000)).to eq vector
+        end
+      end
+
       context "#concat" do
         before :each do
           @dv = Daru::Vector.new [1,2,3,4,5], name: :yoga,