
The stack is a simple but important example of
an abstract data type.
It is used to store elements where the
Last element In
is the First one Out (LIFO).
Anyone with a cluttered desk or an overfull sink of washingup
will be familiar with a stack.
New elements are added or pushed onto the top of the stack.
The first element to be removed or popped
is taken from the top  the last one in.
All recursive programming languages use an implicit systemstack
to allocate space for local variables of routines as they are called.
Some algorithms require the use of an explicit stack.
Type:
stack = empty_stack  push Element_Type × stack
Operations:
empty :stack > boolean
top :stack >Element_Type
pop :stack>stack
push:Element_Type × stack > stack
Rules:
empty(empty_stack) = true
empty(push(e,s)) = false
top(push(e,s)) = e
top(empty_stack) = error
pop(push(e,s)) = s
pop(empty_stack) = error
The Stack Abstract Data Type.
The operations above are pure or free of sideeffects.
It is frequently the case that stacks do not share components
and then it is usual to write
pop and push procedures ((sub)routines, methods, ...)
that change a stack as a sideeffect.
In addition, top and pop operations are often used in sequence to get the
top value and then remove it.
In this case the pop procedure is usually written to
perform both operations.
Many programs use a single stack.
In this case just one instance of a stack may be declared
as a global variable (dangerous) or as part
of a stack module (safer).
The operations can then act on this instance implicitly.
Example: Reverse Polish Calculator
A small "pocketcalculator" program is given here.
The input language is
reversePolish (after Jan Lukasiewicz) or postfix notation which has
the advantage that no brackets are needed.
At least one commercially available pocketcalculator
uses this notation.
infix reversePolish
 
a+b*c a b c * +
(a+b)*c a b + c *
The program uses a single stack which
is implemented as a module in the following sections.
The module specifies a clean interface between the use of the stack
and the implementation of the stack.
The latter can be changed so long as the interface is adhered to and
the calculator program will still work.
The implementation(s) can be verified and tested to ensure
compliance with the interface.
Use of modules in this way is a useful technique in the management of
large programming projects.
Each operand is pushed onto the stack.
Each operator takes two operands from the top of the stack
and pushes its result onto the stack
[C/Stack/calculator.c]
.so stack/stack.calc.t
See the later section on the shunting algorithm
and the section on tree traversal in a later chapter for methods
of generating reversePolish notation from infix notation.
Implementation
There are two natural ways to implement a stack depending on
the circumstances.
A stack can be implemented by an array of elements.
An integer "pointer" top
indicates the element currently at the top of the stack.
Initially top is zero for an empty stack.
The array must be declared to have some fixed size.
This is acceptable if
an accurate estimate of the stack's maximum size can be made
before the program runs.
The other implementation uses a linked list.
This is more suitable if an accurate estimate of the
stack's maximum size cannot be made in advance.
Both methods of implementing a single stack are given here as modules.
The modules export only the abstract stack operations.
Either module can be used with the previous calculator program.
This illustrates that modules protect other parts of the program
from information that they do not need to know.
This design principle may not seem very convincing from
the small examples that must of necessity be given in this book,
but it is very important indeed in large programming projects.
Note that if multiple stacks are required
an opaque stack type must be exported from the module
so that separate stack instances can be declared;
the operations must then have a stack parameter to indicate which
stack is to be manipulated.
Stacks by Array
If a stack is known to be small or
if an accurate estimate of its maximum size is known then
an array gives an efficient implementation.
On the other hand,
if several stacks are used then enough space is reserved
for them all to be full simultaneously
even if it is known that this event can never occur.
.so stack/stack.mod.arr.t
Stacks by Lists
There is a very close correspondence between
lists
and stacks.
The empty stack corresponds to the empty or null list
and push corresponds to constructing a list cell.
This makes an implementation by lists attractive
if a good prior estimate of the stack's size is not available.
.so stack/stack.mod.list.t
Example: Shunting Algorithm
Dijkstra's shunting algorithm
is a method of generating reversePolish
notation from conventional infix notation.
infix reversePolish
 
a+b*c = a+(b*c) abc*+
(a+b)*c ab+c*
a*b+c ab*c+
a*(b+c) abc+*
Infix and reversePolish.
We say that infix multiplication and division have a higher priority
than addition and subtraction.
The former bind more tightly to their operands than the latter.
The shunting algorithm uses a stack of operators.
<output< <input<
a*b+c*d

. .
. .
. .
. .

 Operator Stack


Shunting Algorithm.
It is named after a way of visualising its operation
in terms of a small railway system.
Imagine a mainline with the input coming from the right and going to the
left.
A siding is connected to the mainline by a triangle.
Such a triangle and siding can be used to turn a train around
by driving into the
siding and reversing out again on the other branch  LIFO.
Such triangles are used in real railway systems in the absence of a turntable.
In fact operands go straight through on the mainline
but operators go onto the siding.
<output< <input<
ab c*d

. .
. .
. .
. . +

 Operator Stack

 *
There is a rule that an operator of a given priority, such as `+',
may not sit in the siding/stack immediately above an operator of higher
or equal priority, such as `*'.
Therefore, the `*' must be popped onto the mainline before `+'
can be pushed onto the stack.
The c and d pass through on the mainline and the second `*' can be pushed
above the lower priority `+'.
<output< <input<
ab*cd

. .
. .
. .
. .

 Operator Stack
 *
 +
At the end of input, those operators that are left on the stack are popped
to give the complete output
ab*cd*+
Parentheses have still be considered.
They are used to change the binding of infix operators to operands.
eg. a*b+c*d = (a*b)+(c*d) != a*(b+c)*d
Parentheses isolate a subexpression.
The algorithm therefore treats the subexpression as a piece of subinput.
<output< <input<
a*(b+c)*d

. .
. .
. .
. .

 Operator Stack


When an opening `(' is encountered,
it is pushed onto the stack and hides everything beneath.
A low priority operator, such as `+' can sit above `('
while it cannot sit directly above `*'.
<output< <input<
abc )*d

. .
. .
. .
. .

 + Operator Stack
 (
 *
The closing `)' is treated as the end of the subexpression
and pops any operators of the subexpression 
up to the opening `('.
Both parentheses are then discarded.
<output< <input<
abc+ *d

. .
. .
. .
. .

 Operator Stack

 *
The algorithm then continues as before until
the complete output
abc+*d*
is produced.
The basic shunting algorithm can easily be extended to check
that the input expression is syntactically correct.
Exercises
 Modify the stackbylist implementation to
free cells when they are popped.
 Implement the shunting algorithm in Turing.
 Modify the calculator program so that it uses the shunting algorithm to
let it do calculations given in infix notation.
 Use a stack to
convert the following procedure to nonrecursive, iterative form:
procedure r(x:T)
if p(x) then
a(x)
else
b(x)
r(f(x))
c(x)
end if
end p
where T is an arbitrary type,
p is an arbitrary predicate, a, b and c are arbitrary procedures
and f is an arbitrary function.
 Convert quick sort from recursive to a nonrecursive, iterative form.
To do this introduce a stack of (bounds of) array segments.
Initially push the (bounds of) the whole array onto the stack.
Repeatedly pop an array segment and,
if its size is bigger than 1, partition it
and push first the larger (why?) and then the smaller partition onto the stack.
Terminate when the stack is empty.
 Hard:
Write a prettyprinter for some programming language.
e.g. There are various "bracket structures" in Turing:
begin ... end
if ... then ... else ... end if
case ... label ... end case
loop ... end loop
for ... end for
record ... end record
union ... end union
procedure <ident> ... end <ident>
function <ident> ... end <ident>
module <ident> ... end <ident>
The prettyprinter should only adjust the indentation of lines;
it should not break lines.
Text within a bracket should be indented uniformly relative to the bracket.
If the closing bracket begins a line,
it should match the indentation of the opening bracket.
eg. if ...
 ...
then ...
 ...
else ...
 ...
end if ...
Use a stack of symbols to match opening and closing brackets.
Beware of forward procedure and function declarations
and of procedure and function parameters.
Keywords within comments and strings should not affect indentation.
Assume that the programs being prettyprinted are syntactically correct.
© L_Allison, 198486,
Department of Computer Science, U.W.A.,
Perth, W.A..

