Today I’m going to do a quick entry about prepending a linked list (adding something at the beginning). You’re probably familiar with how I’ve been going through this, so tests first!
describe 'prepend' do
it 'adds something at the beginning' do
list = LinkedList.new
list.append("B")
list.append("C")
list.append("D")
expect(list.stringify).to eq("B, C, D")
end
end
The idea is, “Oops, I forgot that A is at the beginning of the alphabet”. This test should pass with no changes. But, I will add this to test prepend
list.prepend("A")
expect(list.stringify).to eq("A, B, C, D")
So now the full test is
describe 'prepend' do
it 'adds something at the beginning' do
list = LinkedList.new
list.append("B")
list.append("C")
list.append("D")
expect(list.stringify).to eq("B, C, D")
list.prepend("A")
expect(list.stringify).to eq("A, B, C, D")
end
end
And that fails, because prepend is an undefined method. So we will go over to that class and make it a method.
First, I need a new node.
def prepend(data)
new_node = Node.new(data)
end
Next, I’m going to need to take the current head, and make it the next_node of this new_node, and immediately after that, set the new_node to be the head.
def prepend(data)
new_node = Node.new(data)
new_node.next_node = head
@head = new_node
end
And that is it, the test passes. Nice and simple!
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