introduction stacks in data structures and algorithms

CSE 373
Data Structures and Algorithms
Lecture 1: Introduction; ADTs; Stacks; Eclipse

Course objectives
 Learn basic data structures and algorithms
 data structures – how data is organized
 algorithms – unambiguous sequence of steps to compute
something
 algorithm analysis – determining how long an algorithm will
take to solve a problem
 Become a better software developer
 "Data Structures + Algorithms = Programs"
-- Niklaus Wirth, author of Pascal language
2

Abstract Data Types
 abstract data type (ADT): A specification of a collection of
data and the operations that can be performed on it.
 Describes what a collection does, not how it does it
 Described in Java with interfaces (e.g., List, Map, Set)
 Separate from implementation
 ADTs can be implemented in multiple ways by classes:
 ArrayList and LinkedList implement List
 HashSet and TreeSet implement Set
 LinkedList , ArrayDeque, etc. implement
Queue
 Java messed up on Stack—there's no Stack interface, just a class.
3

List ADT
 An ordered collection the form A0, A1, ..., AN-1, where N is the
size of the list
 Operations described in Java's List interface (subset):
 ArrayList and LinkedList are implementations
4
add(elt, index) inserts the element at the specified position
in the list
remove(index) removes the element at the specified position
get(index) returns the element at the specified position
set(index, elt) replaces the element at the specified position
with the specified element
contains(elt) returns true if the list contains the element
size() returns the number of elements in the list

Stack ADT
5
 stack: a list with the restriction that insertions/deletions
can only be performed at the top/end of the list
 Last-In, First-Out ("LIFO")
 The elements are stored in order of insertion,
but we do not think of them as having indexes.
 The client can only add/remove/examine
the last element added (the "top").
 basic stack operations:
 push: Add an element to the top.
 pop: Remove the top element.
 peek: Examine the top element.

Applications of Stacks
 Programming languages:
 method calls are placed onto a stack (call=push, return=pop)
 Matching up related pairs of things:
 find out whether a string is a palindrome
 examine a file to see if its braces { } and other operators
match
 Sophisticated algorithms:
 searching through a maze with "backtracking"
 many programs use an "undo stack" of previous operations
6
method
3
return var
local vars
parameters
method
2
return var
local vars
parameters
method
1
return var
local vars
parameters

Class Stack
7
Stack<Integer> s = new Stack<Integer>();
s.push(42);
s.push(-3);
s.push(17); // bottom [42, -3, 17] top
System.out.println(s.pop()); // 17
Stack<E>() constructs a new stack with elements of type E
push(value) places given value on top of stack
pop() removes top value from stack and returns it;
throws EmptyStackException if stack is
empty
peek() returns top value from stack without removing
it;
throws EmptyStackException if stack is
empty
size() returns number of elements in stack
isEmpty() returns true if stack has no elements

Stack limitations/idioms
 Remember: You can’t loop over a stack like you do a list.
Stack<Integer> s = new Stack<Integer>();
...
for (int i = 0; i < s.size(); i++) {
do something with s.get(i);
}
 Instead, you pull contents out of the stack to view them.
 Idiom: Remove each element until the stack is empty.
while (!s.isEmpty()) {
do something with s.pop();
}
8

Exercise
9
 Write a method symbolsBalanced that accepts a
String as a parameter and returns whether or not the
parentheses and the curly brackets in that String are
balanced as they would have to be in a valid Java
program.
 Use a Stack to solve this problem.

Eclipse concepts
10
 workspace: a collection of projects
 stored as a directory
 project: a Java program
 must have your files in a project in order to be able to
compile, debug and run them
 by default stored in a directory in your workspace
 perspective: a view of your current project using a set
of pre-laid-out windows and menus
 Java perspective
 debugging perspective

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