Running Time (§1.1)
w Most algorithms transform
Analysis of Algorithms
Input
Algorithm
Output
n
An algorithm is a step-by-step procedure for
solving a problem in a finite amount of time.
n
best case
average case
worst case
120
100
Running Time
input objects into output
objects.
w The running time of an
algorithm typically grows
with the input size.
w Average case time is often
difficult to determine.
w We focus on the worst case
running time.
80
60
40
20
Easier to analyze
Crucial to applications such as
games, finance and robotics
0
1000
2000
3000
Analysis of Algorithms
Experimental Studies (§ 1.6)
w Write a program
w It is necessary to implement the
8000
algorithm, which may be difficult
7000
Time (ms)
2
Limitations of Experiments
9000
implementing the
algorithm
w Run the program with
inputs of varying size and
composition
w Use a method like
System.currentTimeMillis() to
get an accurate measure
of the actual running time
w Plot the results
4000
Input Size
w Results may not be indicative of the
6000
running time on other inputs not included
in the experiment.
w In order to compare two algorithms, the
same hardware and software
environments must be used
5000
4000
3000
2000
1000
0
0
50
100
Input Size
Analysis of Algorithms
3
Theoretical Analysis
4
Pseudocode (§1.1)
Example: find max
element of an array
of an algorithm
More structured than Algorithm arrayMax(A, n)
English prose
Input array A of n integers
Less detailed than a
Output maximum element of A
program
currentMax ← A[0]
Preferred notation for
for i ← 1 to n − 1 do
describing algorithms
if A[i] > currentMax then
Hides program design
currentMax ← A[i]
issues
return currentMax
w High-level description
w Uses a high-level description of the
algorithm instead of an implementation
w Characterizes running time as a
function of the input size, n.
w Takes into account all possible inputs
w Allows us to evaluate the speed of an
algorithm independent of the
hardware/software environment
Analysis of Algorithms
Analysis of Algorithms
5
w
w
w
w
Analysis of Algorithms
6
The Random Access Machine
(RAM) Model
Pseudocode Details
w Method call
w Control flow
n
n
n
n
n
if … then … [else …]
while … do …
repeat … until …
for … do …
Indentation replaces braces
w Method declaration
Algorithm method (arg [, arg…])
Input …
Output …
w A CPU
var.method (arg [, arg…])
w Return value
w An potentially unbounded
return expression
w Expressions
bank of memory cells,
each of which can hold an
arbitrary number or
character
← Assignment
(like = in Java)
= Equality testing
(like == in Java)
n 2 Superscripts and other
mathematical
formatting allowed
w Memory cells are numbered and accessing
any cell in memory takes unit time.
Analysis of Algorithms
7
w
w
w
w
performed by an algorithm
Identifiable in pseudocode
Largely independent from the
programming language
Exact definition not important
(we will see why later)
Assumed to take a constant
amount of time in the RAM
model
8
w By inspecting the pseudocode, we can determine the
w Examples:
n
n
n
n
n
maximum number of primitive operations executed by
an algorithm, as a function of the input size
Evaluating an
expression
Assigning a value
to a variable
Indexing into an
array
Calling a method
Returning from a
method
Analysis of Algorithms
Algorithm arrayMax(A, n)
currentMax ← A[0]
for i ← 1 to n − 1 do
if A[i] > currentMax then
currentMax ← A[i]
{ increment counter i }
return currentMax
# operations
2
2 +n
2(n − 1)
2(n − 1)
2(n − 1)
1
Total
9
Estimating Running Time
7n − 1
Analysis of Algorithms
10
Growth Rate of Running Time
w Algorithm arrayMax executes 7n − 1 primitive
w Changing the hardware/ software
operations in the worst case. Define:
environment
a = Time taken by the fastest primitive operation
b = Time taken by the slowest primitive operation
n
w Let T(n) be worst-case time of arrayMax. Then
a (7n − 1) ≤ T(n) ≤ b(7n − 1)
n
Affects T(n) by a constant factor, but
Does not alter the growth rate of T(n)
w The linear growth rate of the running
w Hence, the running time T(n) is bounded by two
linear functions
Analysis of Algorithms
Analysis of Algorithms
Counting Primitive
Operations (§1.1)
Primitive Operations
w Basic computations
2
1
0
11
time T(n) is an intrinsic property of
algorithm arrayMax
Analysis of Algorithms
12
Growth Rates
Constant Factors
functions:
n
n
Linear ≈ n
Quadratic ≈ n2
Cubic ≈ n 3
T (n )
n
w In a log-log chart,
the slope of the line
corresponds to the
growth rate of the
function
1E+30
1E+28
1E+26
1E+24
1E+22
1E+20
1E+18
1E+16
1E+14
1E+12
1E+10
1E+8
1E+6
1E+4
1E+2
1E+0
1E+0
w The growth rate is
Cubic
not affected by
Quadratic
constant factors or
lower-order terms
n
Linear
n
w Examples
T ( n)
w Growth rates of
102 n + 105 is a linear
function
105 n2 + 108 n is a
quadratic function
n
n
1E+26
1E+24
1E+22
1E+20
1E+18
1E+16
1E+14
1E+12
1E+10
1E+8
1E+6
1E+4
1E+2
1E+0
Quadratic
Quadratic
Linear
Linear
1E+0
1E+2
1E+4
1E+6
1E+8
1E+2
1E+4
1E+10
Analysis of Algorithms
13
f(n) ≤ cg(n) for n ≥ n0
w Example: 2n + 10 is O(n)
n
n
n
n
2n + 10 ≤ cn
(c − 2) n ≥ 10
n ≥ 10/(c − 2)
Pick c = 3 and n 0 = 10
1,000,000
3n
w Example: the function
2n+10
n2
n
n
n
n
10
10
100
n 2 ≤ cn
n≤c
The above inequality
cannot be satisfied
since c must be a
constant
100n
10n
n
10,000
1,000
1,000
100
1
10
100
1,000
n
15
Analysis of Algorithms
16
Big-Oh and Growth Rate
w The big-Oh notation gives an upper bound on the
7n-2 is O(n)
need c > 0 and n 0 ≥ 1 such that 7n-2 ≤ c•n for n ≥ n 0
this is true for c = 7 and n 0 = 1
growth rate of a function
w The statement “f(n) is O(g(n))” means that the growth
rate of f(n) is no more than the growth rate of g(n)
+ 20n2 + 5
3n 3 + 20n2 + 5 is O(n3 )
need c > 0 and n 0 ≥ 1 such that 3n3 + 20n 2 + 5 ≤ c•n3 for n ≥ n 0
this is true for c = 4 and n 0 = 21
log n + log log n
3 log n + log log n is O(log n)
need c > 0 and n 0 ≥ 1 such that 3 log n + log log n ≤ c•log n for n ≥ n 0
this is true for c = 4 and n 0 = 2
Analysis of Algorithms
1
n
7n-2
n3
is not O(n)
10
1
More Big-Oh Examples
n 3n 3
14
n^2
100,000
1
Analysis of Algorithms
n
1E+10
Big-Oh Example
10,000
g(n), we say that f(n) is 1,000
O(g(n)) if there are
positive constants
100
c and n0 such that
1E+8
Analysis of Algorithms
Big-Oh Notation (§1.2)
w Given functions f(n) and
1E+6
n
n
17
w We can use the big-Oh notation to rank functions
according to their growth rate
g(n) grows more
f(n) grows more
Same growth
f(n) is O(g(n))
g(n) is O(f(n))
Yes
No
Yes
No
Yes
Yes
Analysis of Algorithms
18
Big-Oh Rules
Asymptotic Algorithm Analysis
w The asymptotic analysis of an algorithm determines
w If is f(n) a polynomial of degree d, then f(n) is
O(n d), i.e.,
n
n
n
Drop lower-order terms
Drop constant factors
n
Say “2n is O(n)” instead of “2n is
n
O(n2 )”
n
w Use the simplest expression of the class
n
Analysis of Algorithms
eventually dropped anyhow, we can disregard them
when counting primitive operations
19
Computing Prefix Averages
asymptotic analysis with
two algorithms for prefix
averages
w The i-th prefix average of
an array X is average of the
first (i + 1) elements of X:
A[i] = (X[0] + X[1] + … + X[i])/(i+1)
w Computing the array A of
35
quadratic time by applying the definition
X
A
30
Algorithm prefixAverages1(X, n)
Input array X of n integers
Output array A of prefix averages of X #operations
A ← new array of n integers
n
for i ← 0 to n − 1 do
n
s ← X[0]
n
for j ← 1 to i do
1 + 2 + …+ (n − 1)
s ← s + X[j]
1 + 2 + …+ (n − 1)
A[i] ← s / (i + 1)
n
return A
1
25
20
15
10
0
1
2
3
4
5
6
Analysis of Algorithms
7
21
Arithmetic Progression
There is a simple visual
proof of this fact
w Thus, algorithm
prefixAverages1 runs in
O(n 2) time
20
Prefix Averages (Quadratic)
5
prefix averages of another
array X has applications to
financial analysis
n
Analysis of Algorithms
w The following algorithm computes prefix averages in
w We further illustrate
prefixAverages1 is
O(1 + 2 + …+ n)
w The sum of the first n
integers is n(n + 1) / 2
We determine that algorithm arrayMax executes at most
7n − 1 primitive operations
We say that algorithm arrayMax “runs in O(n) time”
w Since constant factors and lower-order terms are
Say “3n + 5 is O(n)” instead of “3n + 5 is O(3n)”
w The running time of
We find the worst-case number of primitive operations
executed as a function of the input size
We express this function with big-Oh notation
w Example:
w Use the smallest possible class of functions
n
the running time in big-Oh notation
w To perform the asymptotic analysis
22
Prefix Averages (Linear)
w The following algorithm computes prefix averages in
7
linear time by keeping a running sum
6
5
4
3
2
1
0
Analysis of Algorithms
Analysis of Algorithms
1
2
3
4
5
6
23
Algorithm prefixAverages2(X, n)
Input array X of n integers
Output array A of prefix averages of X
A ← new array of n integers
s←0
for i ← 0 to n − 1 do
s ← s + X[i]
A[i] ← s / (i + 1)
return A
#operations
n
1
n
n
n
1
w Algorithm prefixAverages2 runs in O(n) time
Analysis of Algorithms
24
Relatives of Big-Oh
Math you need to Review
w Summations (Sec. 1.3.1)
w Logarithms and Exponents (Sec. 1.3.2)
w big-Omega
w
w
log b(xy) = logbx + logby
log b (x/y) = log bx - log by
log bxa = alogbx
log ba = logx a/log x b
w properties of exponentials:
a(b+c) = aba c
abc = (ab)c
ab /ac = a(b-c)
Proof techniques (Sec. 1.3.3) b = a loga b
b c = a c*log a b
Basic probability (Sec. 1.3.4)
Analysis of Algorithms
25
Analysis of Algorithms
26
Example Uses of the
Relatives of Big-Oh
Intuition for Asymptotic
Notation
Big-Oh
n f(n) is O(g(n)) if f(n) is asymptotically less than or equal to g(n)
big-Omega
n f(n) is Ω(g(n)) if f(n) is asymptotically greater than or equal to g(n)
big-Theta
n f(n) is Θ(g(n)) if f(n) is asymptotically equal to g(n)
little-oh
n f(n) is o(g(n)) if f(n) is asymptotically strictly less than g(n)
little-omega
n f(n) is ω(g(n)) if is asymptotically strictly greater than g(n)
Analysis of Algorithms
f(n) is Ω(g(n)) if there is a constant c > 0
and an integer constant n0 ≥ 1 such that
f(n) ≥ c•g(n) for n ≥ n0
w big-Theta
n f(n) is Θ(g(n)) if there are constants c ’ > 0 and c’’ > 0 and an
integer constant n 0 ≥ 1 such that c’•g(n) ≤ f(n) ≤ c’’•g(n) for n ≥ n0
w little-oh
n f(n) is o(g(n)) if, for any constant c > 0, there is an integer
constant n 0 ≥ 0 such that f(n) ≤ c•g(n) for n ≥ n 0
w little-omega
n f(n) is ω(g(n)) if, for any constant c > 0, there is an integer
constant n 0 ≥ 0 such that f(n) ≥ c•g(n) for n ≥ n 0
n
w properties of logarithms:
27
n
5n 2 is Ω(n2 )
n
f(n) is Ω(g(n)) if there is a constant c > 0 and an integer constant n 0 ≥ 1
such that f(n) ≥ c•g(n) for n ≥ n0
let c = 5 and n0 = 1
5n 2 is Ω(n)
n
f(n) is Ω(g(n)) if there is a constant c > 0 and an integer constant n 0 ≥ 1
such that f(n) ≥ c•g(n) for n ≥ n0
let c = 1 and n0 = 1
2
5n is ω(n)
f(n) is ω(g(n)) if, for any constant c > 0, there is an integer constant n0 ≥
0 such that f(n) ≥ c•g(n) for n ≥ n0
need 5n02 ≥ c•n0 → given c, the n0 that satifies this is n0 ≥ c/5 ≥ 0
Analysis of Algorithms
28