Stack

• a stack is a metaphor for a physical stack

photo by Michael Stern CC-BY-SA

What makes a stack a stack

• a stack is an abstract data type with only two operators
  • push adds an item to the top
  • pop removes an item from the top
• a stack is a LIFO (last in, first out) structure

Pushing and Popping

https://en.wikipedia.org/wiki/Stack_(abstract_data_type)

The Freedom of Constraints

Why only two operators?

• it is powerful because of its limitations
  • easier to optimize
  • easier to debug
  • easier to understand code that uses it

The theme of "Freedom of Constraints" is important in software design.

(Also in any design or engineering context. And anything creative or artistic.)

Arrays vs. Stacks

In Ruby, the easiest way to implement a stack is by using an array.

In fact, every array already knows how to push and pop.

Try this in IRB:

fruitStack = []
fruitStack.push("apple")
fruitStack.push("banana")
fruitStack                    #  [ 'apple', 'banana' ]
fruitStack.push("cherry")
fruitStack                    #  [ 'apple', 'banana', 'cherry' ]
fruit = fruitStack.pop()
fruit                         #  'cherry'
fruitStack                    #  [ 'apple', 'banana' ]

Note that after a pop, the stack's contents are changed. Pop removes and returns the final value from the array.

Stack Trace

You may have heard the term "stack trace". A stack trace is part of most error messages, e.g.:

fizz.rb:7:in `fizz': undefined local variable or method `buzz' for main:Object (NameError)
        from fizz.rb:2:in `fizzbuzz'
        from fizz.rb:17:in `<main>'```

In this context the term "stack" refers to the call stack.

The Ruby interpreter is a program, and that program uses a stack internally to keep track of the list of functions that call functions that call functions that call...

For instance, in the above stack trace, you can see that the function <main> called the function fizzbuzz, which tried to call the function buzz (but couldn't find it).

Now you know why a stack trace is upside down! It's because a stack is LIFO.

Lab: Fibonacci Stack

Using a stack, put the following program into a file called fib.rb...

series = [0, 1];
while (series.length < 10) do



end
p series

...and complete it so that running ruby fib.rb prints

[ 0, 1, 1, 2, 3, 5, 8, 13, 21, 34 ]

which are the first 10 elements of the Fibonacci sequence.

Although series is an array, please treat it like a stack -- that is, you can only use series.push and series.pop, not any other array operations.

Please split into pairs and do this right now. A solution is on the next slide.

Solution: Fibonacci Stack

series = [0, 1]
while (series.length < 10) do
    b = series.pop
    a = series.pop
    c = a + b
    series.push(a)
    series.push(b)
    series.push(c)
end
p series

Note that we had to pop a and b in reverse order because it's a stack.

Note also that we had to push a and b back on to the stack after adding them to get c.

Uses for stacks

Stacks are useful in many scenarios

• function call stack
• reverse-polish calculator
• backtracking, e.g. chess AI
• recursion

Lab: reverse-polish calculator