Lecture1

CSE 215: Algorithms

Introduction
Proof By Induction
Asymptotic notation

The Course

• Purpose: a rigorous introduction to the design
and analysis of algorithms
 Not a lab or programming course
 Not a math course, either
• Textbook: Introduction to Algorithms,
Cormen, Leiserson, Rivest, Stein
 The “Big White Book”
 Second edition: now “Smaller Green Book”
 An excellent reference you should own

The Course

• Instructor: Sukarna Barua
 Lecturer, Dept. of CSE, BUET
 Mail me @ sukarna_barua@yahoo.com
 Office: Room No. 209, Dept. of CSE, BUET
 Call me @ 01674 069126 (in emergency cases,
usually you should mail me)

The Course

• Grading policy:
 Class Test: 20%
 Mid: 30%
 Final: 40%
 Participation: 10%

The Course

• Prerequisites:
 Basic Programming Skills
 Basic Mathematics Skills
 Knowledge of programming (C/Java/Anyother)
and Data Structure is required

Review: Induction

• Suppose
 S(k) is true for fixed constant k
o Often k = 0
 S(n) S(n+1) for all n >= k
• Then S(n) is true for all n >= k

Proof By Induction

• Claim:S(n) is true for all n >= k
• Basis:
 Show formula is true when n = k
• Inductive hypothesis:
 Assume formula is true for an arbitrary n
• Step:
 Show that formula is then true for n+1

Induction Example:
Gaussian Closed Form
• Prove 1 + 2 + 3 + … + n = n(n+1) / 2
 Basis:
o If n = 0, then 0 = 0(0+1) / 2
 Inductive hypothesis:
o Assume 1 + 2 + 3 + … + n = n(n+1) / 2
 Step (show true for n+1):
1 + 2 + … + n + n+1 = (1 + 2 + …+ n) + (n+1)
= n(n+1)/2 + n+1 = [n(n+1) + 2(n+1)]/2
= (n+1)(n+2)/2 = (n+1)(n+1 + 1) / 2

Induction Example:
Geometric Closed Form
• Prove a0 + a1 + … + an = (an+1 - 1)/(a - 1) for all a
≠1
 Basis: show that a0 = (a0+1 - 1)/(a - 1)
a0 = 1 = (a1 - 1)/(a - 1)
 Inductive hypothesis:
o Assume a0 + a1 + … + an = (an+1 - 1)/(a - 1)
 Step (show true for n+1):
a0 + a1 + … + an+1 = a0 + a1 + … + an + an+1
= (an+1 - 1)/(a - 1) + an+1 = (an+1+1 - 1)/(a - 1)

Induction
• We’ve been using weak induction
• Strong induction also holds
 Basis: show S(0)
 Hypothesis: assume S(k) holds for arbitrary k <= n
 Step: Show S(n+1) follows
• Another variation:
 Basis: show S(0), S(1)
 Hypothesis: assume S(n) and S(n+1) are true
 Step: show S(n+2) follows

Asymptotic Performance

• In this course, we care most about asymptotic
performance
 How does the algorithm behave as the problem
size gets very large?
o Running time
o Memory/storage requirements
o Bandwidth/power requirements/logic gates/etc.

Asymptotic Notation

• By now you should have an intuitive feel for
asymptotic (big-O) notation:
 What does O(n) running time mean? O(n2)?
O(n lg n)?
 How does asymptotic running time relate to
asymptotic memory usage?
• Our first task is to define this notation more
formally and completely

Analysis of Algorithms

• Analysis is performed with respect to a
computational model
• We will usually use a generic uniprocessor
random-access machine (RAM)
 All memory equally expensive to access
 No concurrent operations
 All reasonable instructions take unit time
o Except, of course, function calls
 Constant word size
o Unless we are explicitly manipulating bits

Input Size

• Time and space complexity
 This is generally a function of the input size
o E.g., sorting, multiplication
 How we characterize input size depends:
o Sorting: number of input items
o Multiplication: total number of bits
o Graph algorithms: number of nodes & edges
o Etc

Running Time

• Number of primitive steps that are executed
 Except for time of executing a function call most
statements roughly require the same amount of
time
oy=m*x+b
o c = 5 / 9 * (t - 32 )
o z = f(x) + g(y)
• We can be more exact if need be

Analysis

• Worst case
 Provides an upper bound on running time
 An absolute guarantee
• Average case
 Provides the expected running time
 Very useful, but treat with care: what is “average”?
o Random (equally likely) inputs
o Real-life inputs

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