2 chapter2 algorithm_analysispart1

Chapter 2: Algorithm Analysis - I
Text: Read Weiss, §2.1 – 2.4.2
1

Definition
• An Algorithm is a clearly specified set of
simple instructions to be followed to solve
a problem.
• Once an algorithm is given and decided
somehow to be correct, an important step
is to determine how much in the way of
resources, such as time or space, the
algorithm will require.
2

Mathematical Background
• Definition 2.1. T(N) = O(f(N)) if there are
c,n0 ≥0 such that T(N)≤cf(N) when N ≥ n0.
• Definition 2.2. T(N) = Ω(g(N)) if there are
c,n0 ≥0 such that T(N)≥cg(N) when N ≥ n0.
• Definition 2.3. T(N) = Ɵ(h(N)) iff
T(N)=O(h(N)) and T(N)=Ω(h(N)).
• Definition 2.4. T(N) = o(p(N)) if for all c,
there exists an n0 such that T(N)<cp(N)
when N > n0. (T(N) = o(p(N)) if
T(N)=O(p(N)) and T(N) <> Ɵ(p(N))
3

O (Big-Oh) Notation
• Definitions establish a relative order among
functions. We compare their relative rates of
growth.
• Example: For small values of N, T(N)=1000N
is larger than f(N)=N2. For N≥n0=1000 and c=
1, T(N) ≤ cf(N). Therefore, 1000N = O(N2)
(Big-Oh notation).
• Big-Oh notation says that the growth rate of
T(N) is less than or equal to that of f(N).
T(N)=O(f(N)) means f(N) is an upper bound
on T(N).
4

Ω and Ɵ notations
• T(N) = Ω(g(N)) (pronounced “omega”) says that
the growth rate of T(N) is greater than or equal
to that of g(N). g(N) is a lower bound on T(N).
• T(N) = Ɵ(h(N)) (pronounced “theta”) says that
the growth rate of T(N) equals the growth rate
h(N).
• T(N) = o(p(N)) (pronounced “little-oh”) says that
the growth rate of T(N) is less than that of p(N).
• Example: N2=O(N3), N3=Ω(N2)
5

Rules for Asymptotic Analysis-I
• Rule 1: If T1(N)=O(f(N)) and T2(N)=O(g(N)), then
a) T1(N)+T2(N)=O(f(N)+g(N))
(intuitevely, max(O(f(N)), O(g(N)))
b) T1(N)*T2(N)=O(f(N)*g(N))
• Rule 2: If T(N) is a polynomial
of degree k, then T(N)=Ɵ(Nk).
• Rule 3: logkN=O(N) for any
constant k.
6

Rules for Asymptotic Analysis-II
• It is very bad style to include constants or low-order
terms inside a Big-Oh. Do not say T(N)=O(2N2) or
T(N)=O(N2+N). The correct form is T(N)=O(N2).
• The relative growth rates of f(N) and g(N) can always be
determined by limN→∞(f(N)/g(N)): (using L’Hỏpital’s rule if
necessary)
1. The limit is 0: f(N)=o(g(N))
2. The limit is c≠0: f(N)=Ɵ(g(N))
3. The limit is ∞: g(N)=o(f(N))
4. The limit oscillates: no relation
7
)(
)(lim
)(
)(lim
)(lim,)(lim
Ng
Nf
NNg
Nf
N
Ng
N
Nf
N
′
′
∞→
=
∞→
∞=
∞→
∞=
∞→

Rules for Asymptotic Analysis-III
• Sometimes simple algebra is just sufficient.
• Example: f(N)=NlogN, g(N)=N1.5 are given.
Decide which of the two functions grows faster!
• This amounts to comparing logN and N0.5. This,
in turn, is equivalent to testing log2N and N. But
we already know that N grows faster than any
power of a log.
• It is bad to say f(N)≤O(g(N)), because the
inequality is implied by the definition.
f(N)≥O(g(N)) is incorrect since it does not make
sense.
8

Model of Computation
• Our model is a normal computer.
• Instructions are executed sequentially.
• It has a repertoire of simple instructions (addition,
multiplication, comparison, assignment).
• It takes one (1) time unit to execute these simple
instructions (assume our model has fixed size
integers and no fancy operations like matrix
inversion or sorting).
• It has infinite memory.
• Weaknesses: disk read vs addition, page faults
when memory is not infinite
9

What to Analyze
• The most important resource to analyze is generally
the running time of a program.
• Compiler and computer used affect it but are not
taken into consideration.
• The algorithm used and the input to it will be
considered.
• Tavg(N) (average running time: typical behavior) and
Tworst(N) (worst case: it is generally the required
quantity: guarantee for performance: bound for all
input) running times on input size N.
• The details of the programming language do not
affect a Big-Oh answer. It is the algorithm that is
analyzed not the program (implementation).
10

Maximum Subsequence Sum
Problem - I
• Definition: Given (possibly negative) integers
A1, A2, ..., AN, find the maximum value of .
(For convenience, the maximum subsequence
sum is 0 if all integers are negative)
• Example: For input -2, 11, -4, 13, -5, -2, the
answer is 20 (A2 through A4).
• There are many algorithms to solve this
problem. We will discuss 4 of these.
11
∑
=
j
ik
k
A

Problem - II
• For a small amount of input, they all run in a
blink of the eye.
• Algorithms should not form bottlenecks.Times do
not include the time to read.
12

Problem - III
-O(NlogN)
Algorithm is
not linear.
Verify it by a
straight-edge.
-Relative
growth rates
are evident.
13

Problem - IV
• Illustrates
how useless
inefficient
algorithms
are for even
moderately
large
amounts of
input.
14

Running Time Calculations
• Several ways to estimate the running time of a program (empirical vs
analytical)
• Big-Oh running times. Here is a simple program fragment to
calculate
- The declarations count for no time.
- Lines 1 and 4 count for 1 unit each
- Line 3 counts for 4 units per time
executed (2 multiplications, 1 addition,
1 assignment) and is executed N
times for a total of 4N units.
- Line 2 costs 2N+2 units (1 unit for initial
assignment, N+1 units for comparison tests
N units for all increments).
- Total time T(N) is 6N+4 which is O(N).
15
∑ =
N
i
i
1
3
unsigned int sum( int n ) {
unsigned int i, partial_sum;
/*1*/ partial_sum = 0;
/*2*/ for( i=1; i<=n; i++ )
/*3*/ partial_sum += i*i*i;
/*4*/ return( partial_sum );
}
Some shortcuts could be taken
without affecting the final answer
(Line 3 is an O(1) statement, Line 1
is insignificant compared to for loop.

General Rules - I
• RULE 1-FOR LOOPS:The running time of a for loop is at
most the running time of the statements inside the for
loop (including tests) times the number of iterations.
• RULE 2-NESTED FOR LOOPS: Analyze these inside
out. The total running time of a statement inside a group
of nested for loops is the running time of the statement
multiplied by the product of the sizes of all the for loops.
Example: the following program fragment is O(n2):
16
for( i = 0; i < n; i++ )
for( j=0; j < n; j++ )
k++;

General Rules - II
• RULE 3-CONSECUTIVE STATEMENTS: These just
add (which means that the maximum is the one that
counts – Rule 1(a) on page 6).
Example: the following program fragment, which has O(n)
work followed by O (n2) work, is also O (n2):
17
for( i = 0; i < n; i++)
a[i] = 0;
for( i = 0; i < n; i++ )
for( j = 0; j < n; j++ )
a[i] += a[j] + i + j;

General Rules - III
• RULE 4-lF/ELSE: For the fragment
• the running time of an if/else statement is never more
than the running time of the test plus the larger of the
running times of S1 and S2.
• Clearly, this can be an over-estimate in some cases, but
it is never an under-estimate.
18
if( condition )
S1
else
S2

General Rules - IV
• Other rules are obvious, but a basic strategy of analyzing
from the inside (or deepest part) out works. If there are
function calls, obviously these must be analyzed first.
• If there are recursive procedures, there are several
options. If it is really just a thinly veiled for loop, the
analysis is usually trivial. Example: The following function
is really just a simple loop and is obviously O (n):
19
unsigned int factorial( unsigned int n ) {
if( n <= 1 )
return 1;
else
return( n * factorial(n-1) );
}

General Rules - V
• When recursion is properly used, it is difficult to convert the
recursion into a simple loop structure. In this case, the analysis
will involve a recurrence relation that needs to be solved.
• Example: Consider the following program:
- If the program is run for values of n
around 40, it becomes terribly inefficient.
Let T(n) be the running time for the
Function fib(n). T(0)=T(1)=1 (time to do
the test at Line 1 and return). For n≥2,
the total time required is then
T(n ) = T(n - 1) + T(n - 2) + 2
(where the 2 accounts for the work at Line 1 plus the addition at
Line 3).
T(n) ≥ fib(n)=fib(n-1)+fib(n-2) ≥ (3/2)n (for n > 4, by induction)
• Huge amount of redundant work (violates 4th rule of recursion-
compound interest rule. fib(n-1) has already computed fib(n-2)
20
unsigned int fib( unsigned int n ) {
/*1*/ if( n <= 1 )
/*2*/ return 1;
else
/*3*/ return( fib(n-1) + fib(n-2) );
}

2 chapter2 algorithm_analysispart1

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