#! /usr/bin/python3.4

## a common use of classes is to implement data structures
## this is an example of a stack,
## which is a LIFO - last in first out - structure
## it is a collection.
## items are added to the stack with push and removed with pop
## -----------------------------------------------------------
## The python virtual machine for interpreting
## byte code is based on a stack architecture.
## -----------------------------------------------------------
class stack:

    def __init__(self):
        self.items = []
        self.size = 0

    def __repr__(self):
        return "stack({})".format(list(self.items))

    def isEmpty(self):
        return self.items == []

    def push(self, item):
        self.items.append(item)
        self.size += 1

    def peek(self):
        if self.isEmpty():
            print ("Error: stack is empty")
        else:
            return self.items[-1]

    def pop(self):
        if self.isEmpty():
            print ("Error: stack is empty")
        else:
            self.size -= 1
            return self.items.pop()

    ## swap the top two items in the stack
    def rotate(self):
        if self.size < 2:
            print ("Error: stack has fewer than 2 elements")
        else:
            self.items[-1], self.items[-2] = self.items[-2], self.items[-1]

    ## define the iterator for stack.  Used in for or list comprehension
    def __iter__(self):
        """Return iterator for the stack."""
        if self.isEmpty():
            return None
        else:
            index = self.size -1
            while index >= 0:
                yield self.items[index]
                index -= 1

s = stack()
s.push(1)
s.push(2)
s.push(3)
s.push(4)

## test the iterator
def test(s):
    for i in s:
        print (i)
    return [x for x in s]

## revstr uses a stack to reverse a string
## ----------------------------------------
def revstr(str):
    s = stack()
    for c in str:
        s.push(c)
    result = []
    while (not s.isEmpty()):
        result.append(s.pop())
    return ''.join(result)