Download Data Structures and Algorithm Analysis

Data Structures and Algorithm
Analysis
(Or how to design efficient code that
works)
Why Learn Data Structures?
• Write faster code.
• I’ll prove to you that a faster computer cannot fix a bad/slow
algorithm.
• Write code that saves memory.
• Efficiency – save computer resources.
reduce
in essence…
Always Efficient Code?
•
Efficiency versus clarity
–
–
–
–
–
•
Simplest code is easier to write.
Simplest code is easier to understand.
Simplest code is easier to debug.
Simplest code may not be fast/efficient.
But simplest code may be sufficient!
Choose simpler code…
– When 100% sure that speed is not critical
•
Choose efficiency…
– when necessary
– when in doubt
Example: Slow Code is Ok
• Example: Direct deposit of paycheck.
– time: 1 hour (from time check is entered)
– result: customer just thinks check hasn’t
arrived or cleared yet
– solution: slower code just fine
Direct
deposit sure
is nice.
Example: Need Fast Code
• Example: Customer transfers money from
online account.
– time: takes 15 minutes
– result: customer panics
– solution: faster code
My money
is gone!
What Are Data?
• Definition: Data are any collection of objects.
• Examples:
–
–
–
–
–
Satellite measurements of sea-level rise.
Colors and sizes of Imelda Marcos’ shoes.
Numbers of bad hair days for each president.
Sets of Java classes.
Complete works of Shakespeare.
What Are structures?
• Definition: A structure is a particular
organization (or representation) of data.
data: your classes
• Examples:
–
–
–
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Arrays
Lists
Hashes
Trees (branches with a data
“leaf” at the end of each branch)
structure: Nearby classrooms are close by on tree
What Are Operations?
• Definition: An operation is any function
that manipulates or uses the data.
•
•
•
•
•
•
alphabetize
count
insert
delete
sum
etc.
What’s a Data Structure?
• Definition: Any data representation (i.e.,
organization) and it’s associated
operations.
• Example: An array of integers, with
operations that (a) sort in order, (b) find the
nth largest, (c) insert and delete new
numbers.
Data Structure Example
operation
•
data
structure
Sort N numbers in an array.
–
Solution (Bubblesort):
1. Read into array.
2. Scan pairs from left to right, and swap if out of order.
8, 6, 2, 3
6, 8, 2, 3
6, 2, 8, 3
6, 2, 3, 8
3. Repeat scans until no more changes.
6, 2, 3, 8
–
2, 6, 3, 8
2, 3, 6, 8
2, 3, 6, 8
Time:
•
•
Seconds with 10 numbers.
Days with 1 million numbers.
(Just fine.)
(No good!)
Data Structure Comparison
• Heapsort and Bubblesort are both operations on data.
• But the heap uses a different structure – a tree instead of an
array.
Bubblesort
100 seconds
1,000,000 seconds
300 years
If it takes this long for bubblesort…
Heapsort (fancy, learn later)
1 second
3000 seconds
6 days
Then heapsort takes this long.
(So a heap data structure is far superior!)