ALGORITHM-ANALYSIS.ppt

1
Data Structures
Performance Analysis

2
Fundamental Concepts
Some fundamental concepts that you should
know:
– Dynamic memory allocation.
– Recursion.
– Performance analysis.

3
• There are problems and algorithms to solve them.
• Problems and problem instances.
• Example: Sorting data in ascending order.
– Problem: Sorting
– Problem Instance: e.g. sorting data (2 3 9 5 6 8)
– Algorithms: Bubble sort, Merge sort, Quick sort,
Selection sort, etc.
• Which is the best algorithm for the problem? How
do we judge?

4
• Two criteria are used to judge algorithms:
(i) time complexity (ii) space complexity.
• Space Complexity of an algorithm is the
amount of memory it needs to run to
completion.
• Time Complexity of an algorithm is the
amount of CPU time it needs to run to
completion.

5
Space Complexity
• Memory space S(P) needed by a program P,
consists of two components:
– A fixed part: needed for instruction space (byte
code), simple variable space, constants space
etc.  c
– A variable part: dependent on a particular
instance of input and output data. 
Sp(instance)
• S(P) = c + Sp(instance)

6
Space Complexity: Example 1
1. Algorithm abc (a, b, c)
2. {
3. return a+b+b*c+(a+b-c)/(a+b)+4.0;
4. }
For every instance 3 computer words
required to store variables: a, b, and c.
Therefore Sp()= 3. S(P) = 3.

7
Space Complexity: Example 2
1. Algorithm Sum(a[], n)
2. {
3. s:= 0.0;
4. for i = 1 to n do
5. s := s + a[i];
6. return s;
7. }

8
Space Complexity: Example 2.
• Every instance needs to store array a[] & n.
– Space needed to store n = 1 word.
– Space needed to store a[ ] = n floating point
words (or at least n
words)
– Space needed to store i and s = 2 words
• Sp(n) = (n + 3). Hence S(P) = (n + 3).

9
Time Complexity
• Time required T(P) to run a program P also
consists of two components:
– A fixed part: compile time which is
independent of the problem instance  c.
– A variable part: run time which depends on the
problem instance  tp(instance)
• T(P) = c + tp(instance)

10
Time Complexity
• How to measure T(P)?
– Measure experimentally, using a “stop watch”
 T(P) obtained in secs, msecs.
– Count program steps  T(P) obtained as a step
count.
• Fixed part is usually ignored; only the
variable part tp() is measured.

11
Time Complexity
• What is a program step?
– a+b+b*c+(a+b)/(a-b)  one step;
– comments  zero steps;
–while (<expr>) do  step count equal to
the number of times <expr> is executed.
–for i=<expr> to <expr1> do  step count
equal to number of times <expr1> is checked.

12
Time Complexity: Example 1
Statements S/E Freq. Total
1 Algorithm Sum(a[],n) 0 - 0
2 { 0 - 0
3 S = 0.0; 1 1 1
4 for i=1 to n do 1 n+1 n+1
5 s = s+a[i]; 1 n n
6 return s; 1 1 1
7 } 0 - 0
2n+3

13
Time Complexity: Example 2
Statements S/E Freq. Total
1 Algorithm Sum(a[],n,m) 0 - 0
2 { 0 - 0
3 for i=1 to n do; 1 n+1 n+1
4 for j=1 to m do 1 n(m+1) n(m+1)
5 s = s+a[i][j]; 1 nm nm
6 return s; 1 1 1
2nm+2n+2

14
Performance Measurement
• Which is better?
– T(P1) = (n+1) or T(P2) = (n2 + 5).
– T(P1) = log (n2 + 1)/n! or T(P2) = nn(nlogn)/n2.
• Complex step count functions are difficult
to compare.
• For comparing, ‘rate of growth’ of time and
space complexity functions is easy and
sufficient.

15
Big O Notation
• Big O of a function gives us ‘rate of growth’ of the
step count function f(n), in terms of a simple
function g(n), which is easy to compare.
• Definition: [Big O] The function f(n) = O(g(n))
(big ‘oh’ of g of n) iff there exist positive
constants c and n0 such that f(n) <= c*g(n) for all
n, n>=n0. See graph on next slide.
• Example: f(n) = 3n+2 is O(n) because 3n+2 <= 4n
for all n >= 2. c = 4, n0 = 2. Here g(n) = n.

17
Big O Notation
• Example: f(n) = 10n2+4n+2 is O(n2)
because 10n2+4n+2 <= 11n2 for all n >=5.
• Example: f(n) = 6*2n+n2 is O(2n) because
6*2n+n2 <=7*2n for all n>=4.
• Algorithms can be: O(1)  constant; O(log
n)  logrithmic; O(nlogn); O(n) linear;
O(n2)  quadratic; O(n3)  cubic; O(2n) 
exponential.

18
Big O Notation
• Now it is easy to compare time or space
complexities of algorithms. Which
algorithm complexity is better?
– T(P1) = O(n) or T(P2) = O(n2)
– T(P1) = O(1) or T(P2) = O(log n)
– T(P1) = O(2n) or T(P2) = O(n10)

19
Some Results
Sum of two functions: If f(n) = f1(n) + f2(n), and
f1(n) is O(g1(n)) and f2(n) is O(g2(n)), then f(n) =
O(max(|g1(n)|, |g2(n)|)).
Product of two functions: If f(n) = f1(n)* f2(n), and
f1(n) is O(g1(n)) and f2(n) is O(g2(n)), then f(n) =
O(g1(n)* g2(n)).

ALGORITHM-ANALYSIS.ppt

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