Unit 1.pptx

Design and Analysis of Algorithms
CSE 350
Unit 1

Syllabus
Unit 1 Introduction
A Introduction : Algorithms, Analyzing algorithms, Complexity of
algorithms, Growth of functions, Performance measurements
B Asymptotic Notations and their properties – Mathematical
analysis for Recursive and Non-recursive algorithms,
Recurrences relations, Master Method
C Divide-and-conquer: Analysis and Structure of divide-and-
conquer algorithms, Divide-and-conquer examples-Quick sort,
Merge sort,
Sorting in Linear Time, Heap Sort

What is Algorithm?
 A finite set of instruction that specifies a sequence of operations to be carried out
in order to solve a specific problem or class of problems is called an Algorithm.

Why study Algorithm?
 As the speed of processor increases, performance is frequently said to be less
central than other software quality characteristics (e.g. security, extensibility,
reusability etc.).
 However, large problem sizes are commonplace in the area of computational
science, which makes performance a very important factor.
 The study of Algorithm, therefore, gives us a language to express performance as
a function of problem size.

L1.3
Why study algorithms and performance?
• Algorithms help us to understand scalability.
• Performance often draws the line between what is feasible and what is
impossible.
• Algorithmic mathematics provides a language for talking about program
behavior.
• The lessons of program performance generalize to other computing
resources.
• Speed is fun!

Algorithm: The theoretical study of computer-program performance
and resource usage.
What’s more important than performance?
• Modularity
• Correctness
• Maintainability
• Functionality
• Robustness
• User-friendliness
• Programmer Time
• Simplicity
• Extensibility
• Reliability

Characteristics of Algorithms
 Input: It should externally supply zero or more quantities.
 Output: It results in at least one quantity.
 Definiteness: Each instruction should be clear and ambiguous.
 Finiteness: An algorithm should terminate after executing a finite number of steps.
 Effectiveness: Every instruction should be fundamental to be carried out, in
principle, by a person using only pen and paper.

 Feasible: It must be feasible enough to produce each instruction.
 Flexibility: It must be flexible enough to carry out desired changes with no efforts.
 Efficient: The term efficiency is measured in terms of time and space required by
an algorithm to implement. Thus, an algorithm must ensure that it takes little time
and less memory space meeting the acceptable limit of development time.
 Independent: An algorithm must be language independent, which means that it
should mainly focus on the input and the procedure required to derive the output
instead of depending upon the language.

Advantages of an Algorithm
 Effective Communication: Since it is written in a natural language like English, it
becomes easy to understand the step-by-step delineation of a solution to any
particular problem.
 Easy Debugging: A well-designed algorithm facilitates easy debugging to detect
the logical errors that occurred inside the program.
 Easy and Efficient Coding: An algorithm is nothing but a blueprint of a program
that helps develop a program.
 Independent of Programming Language: Since it is a language-independent, it
can be easily coded by incorporating any high-level language.

Pseudocode
Pseudocode refers to an informal high-level description of the operating principle of a
computer program or other algorithm. It uses structural conventions of a standard
programming language intended for human reading rather than the machine reading.
Advantages of Pseudocode:
 Since it is similar to a programming language, it can be quickly transformed into
the actual programming language than a flowchart.
 The layman can easily understand it.
 Easily modifiable as compared to the flowcharts.
 Its implementation is beneficial for structured, designed elements.
 It can easily detect an error before transforming it into a code.

Problem: Suppose there are 60 students in the class. How will you
calculate the number of absentees in the class?

Pseudo Approach:
 Initialize a variable called as Count to zero, absent to zero, total to 60
 FOR EACH Student PRESENT DO the following:
Increase the Count by One
 Then Subtract Count from total and store the result in absent
 Display the number of absent students

Algorithmic Approach:
 Count <- 0, absent <- 0, total <- 60
 REPEAT till all students counted
Count <- Count + 1
 absent <- total - Count
 Print "Number absent is:" , absent

L1.4
The problem of sorting
Input: sequence a1, a2, …, an of numbers.
Output: permutation a'1, a'2, …, a'n such
that a'1  a'2  …  a'n .
Example:
Input: 8 2 4 9 3 6
Output: 2 3 4 6 8 9

L1.5
Insertion Sort
INSERTION-SORT (A, n)
for j ← 1 to n
do key ← A[ j]
i ← j – 1
⊳ A[1 . . n]
while i > 0 and A[i] > key
do A[i+1] ← A[i]
i ← i – 1
A[i+1] = key
“pseudocode”
i j
key
sorted

L1.20
8 2 4 9 3 6
2 8 4 9 3 6
2 4 8 9 3 6

L1.21
8 2 4 9 3 6
2 8 4 9 3 6
2 4 8 9 3 6

L1.22
8 2 4 9 3 6
2 8 4 9 3 6
2 4 8 9 3 6
2 4 8 9 3 6

L1.23
8 2 4 9 3 6
2 8 4 9 3 6
2 4 8 9 3 6
2 4 8 9 3 6

L1.24
8 2 4 9 3 6
2 8 4 9 3 6
2 4 8 9 3 6
2 4 8 9 3 6
2 3 4 8 9 6

L1.25
8 2 4 9 3 6
2 8 4 9 3 6
2 4 8 9 3 6
2 4 8 9 3 6
2 3 4 8 9 6

L1.26
8 2 4 9 3 6
2 8 4 9 3 6
2 4 8 9 3 6
2 4 8 9 3 6
2 3 4 8 9 6
2 3 4 6 8 9 done

L1.27
Worst case: Input reverse sorted.
n
T(n)  ( j)  n2 
n
j2
Average case: All permutations equally likely.
T(n)  ( j /2)  n2 
j2
Is insertion sort a fast sorting algorithm?
• Moderately so, for small n.
• Not at all, for large n.
[arithmetic series]

 Insertion sort takes Q(n2) in the worst case, so sorting (as a problem)
is O(n2). Why?
 Any sort algorithm must look at each item, so sorting is W(n).
 In fact, using (e.g.) merge sort, sorting is Q(n lg n) in the worst case.
 Later, we will prove that we cannot hope that any comparison sort
to do better in the worst case.
Comp 122

Running time
L1.29
• The running time depends on the input: an already sorted sequence
is easier to sort.
• Parameterize the running time by the size of the input, since short
sequences are easier to sort than long ones.
• Generally, we seek upper bounds on the running time, because
everybody likes a guarantee.

L1.30
Worst-case: (usually)
• T(n) = maximum time of algorithm on any input of size n.
Average-case: (sometimes)
• T(n) = expected time of algorithm on any input of size n.
Best-case: (bogus)
• Cheat with a slow algorithm that works fast on some input.

L1.31
What is insertion sort’s worst-case time?
It depends on the speed of our computer:
• relative speed (on the same machine),
• absolute speed (on different machines).
BIG IDEA:
• Ignore machine-dependent constants.
• Look at growth of T(n) as n → ∞ .
“AsymptoticAnalysis”

Complexity of Algorithm
 Algorithm complexity measures how many steps are required by the algorithm to solve the
given problem.
 It evaluates the order of count of operations executed by an algorithm as a function of input
data size.
 O(f) notation represents the complexity of an algorithm, which is also termed as an
Asymptotic notation or "Big O" notation.
 Here the f corresponds to the function whose size is the same as that of the input data.
 The complexity of the asymptotic computation O(f) determines in which order the resources
such as CPU time, memory, etc. are consumed by the algorithm.
 The complexity can be found in any form such as constant, logarithmic, linear, n*log(n),
quadratic, cubic, exponential, etc.

Typical Complexities of an Algorithm
1. Constant Complexity:
It imposes a complexity of O(1). It undergoes an execution of a constant
number of steps like 1, 5, 10, etc. for solving a given problem. The count of
operations is independent of the input data size.

2. Logarithmic Complexity:
 It imposes a complexity of O(log(N)).
 It undergoes the execution of the order of log(N) steps.
 To perform operations on N elements, it often takes the logarithmic
base as 2.
 For N = 1,000,000, an algorithm that has a complexity of O(log(N))
would undergo 20 steps (with a constant precision).

3. Linear Complexity:
 It imposes a complexity of O(N).
 It encompasses the same number of steps as that of the total number
of elements to implement an operation on N elements.
 For example, if there exist 500 elements, then it will take about 500
steps. Basically, in linear complexity, the number of elements linearly
depends on the number of steps.

4. Quadratic Complexity:
 It imposes a complexity of O(n2).
 For N input data size, it undergoes the order of N2 count of operations
on N number of elements for solving a given problem.
 If N = 100, it will endure 10,000 steps.
 In other words, whenever the order of operation tends to have a
quadratic relation with the input data size, it results in quadratic
complexity.

5. Cubic Complexity:
 It imposes a complexity of O(n3).
 For N input data size, it executes the order of N3 steps on N elements to solve a
given problem.
 For example, if there exist 100 elements, it is going to execute 1,000,000 steps.

6. Exponential Complexity
 It imposes a complexity of O(2n), O(N!), O(nk), ….
 For N elements, it will execute the order of count of operations that is
exponentially dependable on the input data size.
 For example, if N = 10, then the exponential function 2N will result in 1024.
Similarly, if N = 20, it will result in 1048 576, and if N = 100, it will result in a
number having 30 digits.
 The exponential function N! grows even faster; for example, if N = 5 will
result in 120. Likewise, if N = 10, it will result in 3,628,800 and so on.

 How to approximate the time taken by the Algorithm?
 There are two types of algorithms:
 Iterative Algorithm: In the iterative approach, the function repeatedly
runs until the condition is met or it fails. It involves the looping
construct.
 Recursive Algorithm: In the recursive approach, the function calls
itself until the condition is met. It integrates the branching structure.

Asymptotic Notations
 Asymptotic Notation is a way of comparing function that ignores
constant factors and small input sizes. Three notations are used to
calculate the running time complexity of an algorithm:

 Defined for functions over the natural numbers.
 Ex: f(n) = Q(n2).
 Describes how f(n) grows in comparison to n2.
 Define a set of functions; in practice used to compare two function sizes.
 The notations describe different rate-of-growth relations between the
defining function and the defined set of functions.

-notation
(g(n)) = {f(n) :
 positive constants c1, c2, and
n0, such that n  n0,
we have 0  c1g(n)  f(n)  c2g(n)
}
For function g(n), we define (g(n)), big-Theta
of n, as the set:
g(n) is an asymptotically tight bound for f(n).
Intuitively: Set of all functions that
have the same rate of growth as g(n).

O-notation
O(g(n)) = {f(n) :
 positive constants c and n0,
such that n  n0,
we have 0  f(n)  cg(n) }
For function g(n), we define O(g(n)), big-O of n,
as the set:
g(n) is an asymptotic upper bound for f(n).
Intuitively: Set of all functions whose
rate of growth is the same as or lower
than that of g(n).
f(n) = (g(n))  f(n) = O(g(n)).
(g(n))  O(g(n)).

 -notation
g(n) is an asymptotic lower bound for f(n).
Intuitively: Set of all functions whose
rate of growth is the same as or higher
than that of g(n).
f(n) = (g(n))  f(n) = (g(n)).
(g(n))  (g(n)).
(g(n)) = {f(n) :
 positive constants c and n0,
such that n  n0,
we have 0  cg(n)  f(n)}
For function g(n), we define (g(n)), big-
Omega of n, as the set:

 OR (g(n)) = O(g(n)) Ç W(g(n))
 In practice, asymptotically tight bounds are obtained from asymptotic upper
and lower bounds.
Theorem : For any two functions g(n) and f(n),
f(n) = (g(n)) iff
f(n) = O(g(n)) and f(n) = (g(n)).

Asymptotic Notation in Equations
 Can use asymptotic notation in equations to replace expressions
containing lower-order terms.
 For example,
4n3 + 3n2 + 2n + 1 = 4n3 + 3n2 + (n)
= 4n3 + (n2) = (n3). How to interpret?
 In equations, (f(n)) always stands for an anonymous function g(n) Î
(f(n))
 In the example above, (n2) stands for
3n2 + 2n + 1.

o-notation
f(n) becomes insignificant relative to g(n) as n approaches infinity:
lim [f(n) / g(n)] = 0
n
g(n) is an upper bound for f(n) that is not asymptotically tight.
o(g(n)) = {f(n):  c > 0,  n0 > 0 such that
 n  n0, we have 0  f(n) < cg(n)}.
For a given function g(n), the set little-o:

w -notation
f(n) becomes arbitrarily large relative to g(n) as n approaches infinity:
lim [f(n) / g(n)] = .
n
g(n) is a lower bound for f(n) that is not asymptotically tight.
w(g(n)) = {f(n):  c > 0,  n0 > 0 such that
 n  n0, we have 0  cg(n) < f(n)}.
For a given function g(n), the set little-omega:

Comparison of Functions
f  g  a  b
f (n) = O(g(n))  a  b
f (n) = (g(n))  a  b
f (n) = (g(n))  a = b
f (n) = o(g(n))  a < b
f (n) = w (g(n))  a > b

Properties
 Transitivity
f(n) = (g(n)) & g(n) = (h(n))  f(n) = (h(n))
f(n) = O(g(n)) & g(n) = O(h(n))  f(n) = O(h(n))
f(n) = (g(n)) & g(n) = (h(n))  f(n) = (h(n))
f(n) = o (g(n)) & g(n) = o (h(n))  f(n) = o (h(n))
f(n) = w(g(n)) & g(n) = w(h(n))  f(n) = w(h(n))
 Reflexivity
f(n) = (f(n))
f(n) = O(f(n))
f(n) = (f(n))

 Symmetry
f(n) = (g(n)) iff g(n) = (f(n))
 Complementarity
f(n) = O(g(n)) iff g(n) = (f(n))
f(n) = o(g(n)) iff g(n) = w((f(n))
Comp 122

Live Class (PDF 1)
 Substitution Method
 Recursion
 Master’s theorem

Merge sort
L1.54
MERGE-SORT A[1 . . n]
To sort n numbers:
1. If n = 1, done.
2. Recursively sort A[ 1 . . n/2 ] and
A[ n/2+1 . . n ] .
3. “Merge” the 2 sorted lists.
Key subroutine: MERGE

Merging two sorted arrays:
L1.55
20 12
13 11
7 9
2 1

L1.57
20 12 20 12
13 11 13 11
7 9 7 9
2 1 2
1

L1.58
20 12 20 12
13 11 13 11
7 9 7 9
2 1 2
1 2

L1.59
20 12 20 12 20 12
13 11 13 11 13 11
7 9 7 9 7 9
2 1 2
1 2

L1.60
20 12 20 12 20 12
13 11 13 11 13 11
7 9 7 9 7 9
2 1 2
1 2 7

L1.61
20 12 20 12 20 12 20 12
13 11 13 11 13 11 13 11
7 9 7 9 7 9 9
2 1 2
1 2 7

L1.62
20 12 20 12 20 12 20 12
13 11 13 11 13 11 13 11
7 9 7 9 7 9 9
2 1 2
1 2 7 9

L1.63
20 12 20 12 20 12 20 12 20 12
13 11 13 11 13 11 13 11 13 11
7 9 7 9 7 9 9
2 1 2
1 2 7 9

L1.64
20 12 20 12 20 12 20 12 20 12
13 11 13 11 13 11 13 11 13 11
7 9 7 9 7 9 9
2 1 2
1 2 7 9 11

L1.65
20 12 20 12 20 12 20 12 20 12 20 12
13 11 13 11 13 11 13 11 13 11 13
7 9 7 9 7 9 9
2 1 2
1 2 7 9 11

L1.66
20 12 20 12 20 12 20 12 20 12 20 12
13 11 13 11 13 11 13 11 13 11 13
7 9 7 9 7 9 9
2 1 2
1 2 7 9 11 12

L1.67
20 12 20 12 20 12 20 12 20 12 20 12
13 11 13 11 13 11 13 11 13 11 13
7 9 7 9 7 9 9
2 1 2
1 2 7 9 11 12
Time = (n) to merge a total of n elements (linear time).

Analyzing merge sort
L1.68
MERGE-SORT (A, n) ⊳ A[1 . . n]
To sort n numbers:
1. If n = 1, done.
2. Recursively sort A[ 1 . . n/2 ]
and A[ n/2+1 . . n ] .
3. “Merge” the 2 sorted lists
T(n)
(1)
2T(n/2)
(n)
Abuse
Sloppiness: Should be T( n/2 ) + T( n/2 ) ,
but it turns out not to matter asymptotically.

Recurrence for merge sort
L1.69
T(n) =
(1) if n = 1;
2T(n/2) + (n) if n > 1.
• We shall usually omit stating the base case when T(n) = (1) for
sufficiently small n (and when it has no effect on the solution to
the recurrence).
• Further slides provide several ways to find a good upper bound
on T(n).

L1.70
Recursion tree
Solve T(n) = 2T(n/2) + cn, where c > 0 is constant.

L1.71
T(n)

L1.72
T(n/2) T(n/2)
cn

L1.73
cn
T(n/4) T(n/4) T(n/4) T(n/4)
cn/2 cn/2

L1.74
cn
cn/4 cn/4 cn/4 cn/4
cn/2 cn/2
(1)

L1.75
cn
cn/4 cn/4 cn/4 cn/4
cn/2 cn/2
(1)
h = lg n

L1.76
cn/4 cn/4 cn/4 cn/4
cn/2 cn/2
(1)
h = lg n
cn cn

L1.77
cn/4 cn/4 cn/4 cn/4
cn/2
(1)
h = lg n
cn cn
cn/2 cn

L1.78
cn/4 cn/4
cn/2
(1)
h = lg n
cn cn
cn/2 cn
cn/4 cn/4 cn
…

L1.79
cn/4 cn/4
cn/2
h = lg n
cn cn
cn/2 cn
cn/4 cn/4 cn
(1) #leaves = n (n)
…

L1.80
cn/4 cn/4
cn/2
h = lg n
cn cn
cn/2 cn
cn/4 cn/4 cn
(1) #leaves = n (n)
Total  (n lg n)
…

Conclusions
L1.81
• (n lg n) grows more slowly than (n2).
• Therefore, merge sort asymptotically beats insertion sort in
the worst case.
• In practice, merge sort beats insertion sort for n > 30 or so.
• Go test it out for yourself!

Divide And Conquer
This technique can be divided into the following three parts:
1. Divide: This involves dividing the problem into some sub problem.
2. Conquer: Sub problem by calling recursively until sub problem solved.
3. Combine: The Sub problem Solved so that we will get find problem
solution.

L3.3
Example:
(merge sort)
1. Divide: Trivial.
2. Conquer: Recursively sort 2 subarrays.
3. Combine: Linear-time merge.
T(n) = 2T(n/2) + O(n)
# subproblems
subproblem size
work dividing
and combining

Standard algorithms that follows Divide and Conquer algorithm
1. Quicksort is a sorting algorithm. The algorithm picks a pivot element, rearranges the
array elements in such a way that all elements smaller than the picked pivot element
move to left side of pivot, and all greater elements move to right side. Finally, the
algorithm recursively sorts the subarrays on left and right of pivot element.
2. Merge Sort is also a sorting algorithm. The algorithm divides the array in two halves,
recursively sorts them and finally merges the two sorted halves.
3. Closest Pair of Points The problem is to find the closest pair of points in a set of points
in x-y plane. The problem can be solved in O(n^2) time by calculating distances of every
pair of points and comparing the distances to find the minimum. The Divide and
Conquer algorithm solves the problem in O(nLogn) time.

4. Strassen’s Algorithm is an efficient algorithm to multiply two matrices. A
simple method to multiply two matrices need 3 nested loops and is O(n^3).
Strassen’s algorithm multiplies two matrices in O(n^2.8974) time.
5. Cooley–Tukey Fast Fourier Transform (FFT) algorithm is the most
common algorithm for FFT. It is a divide and conquer algorithm which works
in O(nlogn) time.

Live Class (PDF 2)
 Quick sort
 Sorting in Linear Time
 Heap Sort

Heap Sort
 If the parent node is stored at index I, the left child can be calculated by 2
* I + 1 and the right child by 2 * I + 2 (assuming the indexing starts at 0).
 Heap Sort Algorithm for sorting in increasing order:
1. Build a max heap from the input data.
2. At this point, the largest item is stored at the root of the heap. Replace
it with the last item of the heap followed by reducing the size of heap by
1. Finally, heapify the root of the tree.
3. Repeat step 2 while the size of the heap is greater than 1.

 Working of Heap Sort
1. Since the tree satisfies Max-Heap property, then the largest item is stored
at the root node.
2. Swap: Remove the root element and put at the end of the array (nth
position) Put the last item of the tree (heap) at the vacant place.
3. Remove: Reduce the size of the heap by 1.
4. Heapify: Heapify the root element again so that we have the highest
element at root.
5. The process is repeated until all the items of the list are sorted.

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