Why only two operators?
The theme of "Freedom of Constraints" is important in software design.
(Also in any design or engineering context. And anything creative or artistic.)
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.
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.
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.
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.
Stacks are useful in many scenarios
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