cs200: stacks - colorado state universitycs200/fall16/slides/02-stacks.pdf · cs200 - stacks 15 ....
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CS200: Stacks
n Prichard Ch. 7
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Linear, time-ordered structures
n Two data structures that reflect a temporal relationship q order of removal based on order of insertion
n We will consider: q “first come,first serve”
n first in first out - FIFO (queue) q “take from the top of the pile”
n last in first out - LIFO (stack)
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3 CS200 - Stacks
Stacks or queues?
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What can we do with coin dispenser?
n “push” a coin into the dispenser. n “pop” a coin from the dispenser.
n “peek” at the coin on top, but don’t pop it. n “isEmpty” check whether this dispenser is
empty or not.
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Stacks
n Last In First Out (LIFO) structure q A stack of dishes in a cafe
n Add/Remove done from same end: the top 5
4 3 2 1
top
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Possible Stack Operations
n isEmpty(): determine whether stack is empty
n push(): add a new item to the stack n pop(): remove the item added most recently n peek(): retrieve, but don’t remove, the item
added most recently
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Checking for balanced braces
n How can we use a stack to determine whether the braces in a string are balanced?
abc{defg{ijk}{l{mn}}op}qr
abc{def}}{ghij{kl}m
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Pseudocode
while ( not at the end of the string){ if (the next character is a “{“){ aStack.push(“{“) } else if (the character is a “}”) { if(aStack.isEmpty()) ERROR!!! else aStack.pop() }}if(!aStack.isEmpty()) ERROR!!!
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question
n Could you use a single int to do the same job?
n How?
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Try it on abc{defg{ijk}{l{mn}}op}qr {st{uvw}xyz}
abc{def}}{ghij{kl}m
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Expressions n Types of Algebraic Expressions
q Prefix q Postfix q Infix
n Prefix and postfix are easier to parse. No ambiguity. Infix requires extra rules: precedence and associativity. What are these?
n Postfix: operator applies to the operands that immediately precede it.
n Examples: 1. - 5 * 4 3 2. 5 - 4 * 3 3. 5 4 3 * -
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What type of expression is “5 4 3 – *”? A. Prefix B. Infix C. Postfix D. None of the above (i.e., illegal)
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What type of expression is “5 * 4 3 –”? A. Prefix B. Infix C. Postfix D. None of the above (i.e., illegal)
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Evaluating a Postfix Expression
while there are input tokens left read the next token if the token is a value push it onto the stack. else //the token is a operator taking n arguments pop the top n values from the stack and perform the operation push the result on the stack
If there is only one value in the stack return it as the result else throw an exception
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Quick check
n If the input string is “5 3 + 2 *”, which of the following could be what the stack looks like when trying to parse it?
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2 3 5
+ 3 5
2 8
A B C
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Stack Interface push(StackItemType newItem)
q adds a new item to the top of the stack
StackItemType pop() throws StackException q deletes the item at the top of the stack and returns it q Exception when deletion fails
StackItemType peek() throws StackException q returns the top item from the stack, but does not remove it q Exception when retrieval fails
boolean isEmpty() q returns true if stack empty, false otherwise
Preconditions? Postconditions?
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Comparison of Implementations
n Options for Implementation: q Array based implementation q Array List based implementation q Reference based implementation
n What are the advantages and disadvantages of each implementation?
n Let’s look at a Linked List based implementation n In P1 you program an Array List based
implementation
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Stacks and Recursion
n Most implementations of recursion maintain a stack of activation records, called
the Run Time Stack
n Activation records, or Stack Frames, contain (amongst other things) parameters and local variables of the method called
n Within recursive calls, the most recently executed activation record is stored at the top of the stack.
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Applications - the run-time stack n Nested method calls tracked on
call stack (aka run-time stack) q First method that returns is the last one
invoked n Element of call stack - activation
record or stack frame q parameters q local variables q return address: pointer to next
instruction to be executed in calling method
http://en.wikipedia.org/wiki/Image:Call_stack_layout.svg CS200 - Stacks 18
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Factorial example
int factorial(n){ // pre n>=0 // post return n! if(n==0) { r=1; return r;} else {r=n*factorial(n-1); return r;} }
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RTS factorial(3): wind phase
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n=3, r=? n=3, r=?
n=2, r=?
n=3, r=?
n=2, r=?
n=1, r=?
n=3, r=?
n=2, r=?
n=1, r=?
n=0, r=1
only active frame: top of the run time stack
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RTS factorial(3): unwind phase
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n=3, r=6 n=3, r=?
n=2, r=2
n=3, r=?
n=2, r=?
n=1, r=1
return 6
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More complex example: The Towers of Hanoi n Move pile of disks from source to destination n Only one disk may be moved at a time. n No disk may be placed on top of a smaller disk.
CS200 - Recursion
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Moves in the Towers of Hanoi
Source Destination Spare CS200 - Recursion
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Recursive Solution
CS200 - Recursion
// pegs are numbers, via is computed // f: source peg, t: dest peg, v: via peg // state corresponds to return address public void hanoi(int n, int f, int t){ if (n>0) { // state 0 int v = 6 - f - t; hanoi(n-1,f, v); // state 1 System.out.println("move disk " + n + " from " + f + " to " + t); hanoi(n-1,v,t); // state 2 } }
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Run time stack for hanoi(3,1,3)
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0:n=3,f=1,t=3 1:n=3,f=1,t=3 0:n=2,f=1,t=2
1:n=3,f=1,t=3 1:n=2,f=1,t=2 0:n=1,f=1,t=3
1:n=3,f=1,t=3 1:n=2,f=1,t=2 1:n=1,f=1,t=3 0:n=0,f=1,t=2
if (n>0) { // state 0 int v = 6 - f - t; hanoi(n-1,f, v); // state 1 System.out.println("move disk " + n + “ from" + f + " to" + t); hanoi(n-1,v,t); // state 2 }
only active frame: top of the run time stack
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Run time stack for hanoi(3,1,3)
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1:n=3,f=1,t=3 1:n=2,f=1,t=2 1:n=1,f=1,t=3
if (n>0) { // state 0 int v = 6 - f - t; hanoi(n-1,f, v); // state 1 System.out.println("move disk " + n + “ from" + f + " to" + t); hanoi(n-1,v,t); // state 2 }
1:n=3,f=1,t=3 1:n=2,f=1,t=2 2:n=1,f=1,t=3
1:n=3,f=1,t=3 1:n=2,f=1,t=2 2:n=1,f=1,t=3
0:n=0,f=2,t=3
System.out:
1:n=3,f=1,t=3 1:n=2,f=1,t=2
“move disk 1 from 1 to 3” “move disk 2 from 1 to 2”
etcetera
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Hanoi with explicit run time stack n In Programming Assignment 1 you will create a
Hanoi program with an explicit run time stack rts.
n The main loop of the program is: while(rts not empty){ pop frame check frame state perform appropriate actions, including pushing frames }
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