Chapter 1 Data structure.pptx

1.1. Introduction to Data Structures and Algorithms Analysis
 A program is written in order to solve a problem. A solution to a problem
actually consists of two things:
I. A way to organize the data
II. Sequence of steps to solve the problem
 The way data are organized in a computer’s memory is said to be data
structure and
 The sequence of computational steps to solve a problem is said to be an
algorithm.
 Therefore, a program is nothing but data structures plus algorithms.
 A data structure is a systematic way of organizing and accessing data and
 an algorithm is a step-by-step procedure for performing some task in a
finite amount of time.

Introduction to Data Structures
 The first step to solve the problem is obtaining one’s own
abstract view, or model, of the problem.
 This process of modeling is called abstraction.
 The model defines an abstract view to the problem.
 This implies that the model focuses only on problem related
stuff and
 A programmer tries to define the properties of the problem.
3

Cont.
 These properties Abstract data include
 The data which are affected and
 The operations that are involved in the problem.
 With abstraction you create a well-defined entity that can be
properly handled.
 These entities define the data structure of the program.
 An entity with the properties just described is called an abstract
data type (ADT).
4

Abstract Data Types
 An abstract data type (ADT) is a set of objects together with a set of
operations.
 An ADT is a mathematical model of a data structure that specifies the
type of the data stored, the operations supported on them
 Objects such as lists, sets, and graphs, along with their operations, can
be viewed as ADTs, just as integers, real’s, and booleans are data types.
 The ADT specifies:
1. What can be stored in the Abstract Data Type?
2. What operations can be done on/by the Abstract Data Type?
5

Abstraction
 Abstraction is a process of classifying characteristics as
relevant and irrelevant for the particular purpose at hand and
ignoring the irrelevant ones.
 The abstraction is to distill a complicated system down to its
most fundamental parts and
 describe these parts in a simple, precise language.
 Typically, describing the parts of a system involves naming
them and explaining their functionality.
6

Algorithms
 An algorithm is a well-defined computational procedure that
takes :
 some value or a set of values as input and
 produces some value or a set of values as output.
 Data structures model the static part of the world.
 They are unchanging while the world is changing.
 In order to model the dynamic part of the world we need to
work with algorithms.
 Algorithms are the dynamic part of a program’s world model.
 An algorithm transforms data structures from one state to
another state in two ways:
 An algorithm may change the value held by a data structure
 An algorithm may change the data structure itself 7

Properties of an algorithm
What makes an algorithm good?
 Finiteness: Algorithm must complete after a finite number of steps.
 Definiteness: Each step must be clearly defined, having one and only
one interpretation. At each point in computation, one should be able
to tell exactly what happens next.
 Sequence: Each step must have a unique defined preceding and
succeeding step. The first step (start step) and last step (halt step)
must be clearly noted.
 Feasibility: It must be possible to perform each instruction.
 Correctness: It must compute correct answer for all possible legal
inputs.
8

Cont.
 Language Independence: It must not depend on any one
programming language.
 Completeness: It must solve the problem completely.
 Effectiveness: It must be possible to perform each step exactly and in
a finite amount of time.
 Efficiency: It must solve with the least amount of computational
resources such as time and space.
 Generality: Algorithm should be valid on all possible inputs.
 Input/Output: There must be a specified number of input values, and
one or more result values.
9

Algorithm Analysis Concepts
 Algorithm analysis refers to the process of determining the
amount of computing time and storage space required by
different algorithms.
 In other words, it’s a process of predicting the resource
requirement of algorithms in a given environment.
 It is the study of efficiency of programs.
 Input size of the program, machine type used, implementation
quality, running time of algorithm and others affect the
efficiency of a program.
10

… Cont.
 In order to solve a problem, there are many possible algorithms.
 One has to be able to choose the best algorithm for the problem
at hand using some scientific method.
 To classify some data structures and algorithms as good, we
need precise ways of analyzing them in terms of resource
requirement. The main resources are:
 Running Time
 Memory Usage
 Communication Bandwidth
 Algorithm analysis tries to estimate these resources required to
solve a problem at hand.
 Running time is usually treated as the most important since
computational time is the most precious resource in most
problem domains.
11

--- Cont
 There are two approaches to measure the efficiency of algorithms:
 Empirical (Experimental) method: Programming competing
algorithms and trying them on different instances. This method is
used for absolute time measurement.
 Analytical (Theoretical) Method: Determining the quantity of
resources required mathematically (like execution time, memory
space, etc.) needed by each algorithm.
 However, it is difficult to use actual clock-time as a consistent
measure of an algorithm’s efficiency, because clock-time can vary
based on many things. For example,
 Specific processor speed
 Current processor load
 Specific data for a particular run of the program
 Input Size
 Input Properties
 Operating Environment
12

Complexity Analysis
 Complexity Analysis is the systematic study of the cost of
computation, measured either in time units or in operations
performed, or in the amount of storage space required.
 The goal is to have a meaningful measure that permits
comparison of algorithms independent of operating platform.
 There are two things to consider:
 Time Complexity: Determine the approximate number of
operations and time required to solve a problem of size n.
 The running times of operations on the data structure should
be as small as possible.
 Space Complexity: Determine the approximate memory
required to solve a problem of size n.
 The data structure should use as little memory as possible.
13

Running time
 The critical resource for a program is most often its running
time.
 Running time is the amount of time that any algorithm takes to
run.
 However, you cannot pay attention to running time alone.
 You must also be concerned with other factors such as the space
required to run the program (main memory and disk space).
 The primary analysis tool we use in this course involves
characterizing the running times of algorithms and data
structure operations.
 Running time is a natural measure of “goodness,” since time is
a precious resource
- computer solutions should run as fast as possible.
14

1.2.2. Complexity of Algorithms
 There is no generally accepted set of rules for algorithm
analysis.
 However, an exact count of operations is commonly used.
 Arbitrary time unit is assumed for analyzing the algorithm,
and we have the following set of analysis rules.
Rule 1: Basic operations
 The execution of the following operations takes one (1) time
unit.
 Assignment Operation
 Single Input/ Output Operation
 Single Boolean Operations
 Single Arithmetic Operations
 Function Return 15

… Cont.
 Rule 2: Selection statements
 Running time of a selection statement (if, switch) is the time for the
condition evaluation + the maximum of the running times for the individual
clauses.
 Rule 3: Loops
 Running time for a loop is equal to the running time for the statements inside
the loop body multiplied (*) by number of iterations of the loop.
 Rule 4: Nested Loops
 The total running time of a statement inside a group of nested loops is the
running time of the statements multiplied by the product of the sizes of all
the loops.
 Rule 5: Function call
 Running time of a function call is 1 for setup + the time for any parameter
calculations + the time required for the execution of the function body.
 Rule 6: Consecutive statements
 For consecutive statements, the running time will be computed as the sum of
the running time of the separate blocks of code.
16

Example 1
The following examples show us how running time of the code fragments is computed.
Examples 1:
int count ( )
{
int k=0;
cout<< “Enter an integer”;
cin>>n;
for (i=0; i<n; i++)
k=k+1;
return 0;
}
Time Units to Compute
1 for the assignment statement: int k=0
1 for the output statement.
1 for the input statement.
For the loop (for):
1 assignment (i = 0), n+1 tests, and n increments.
n loops of 2 units for an assignment and addition.
1 for the return statement.
-------------------------------------------------------------------
T (n)= 1+1+1+(1+n+1+n)+2n+1 = 4n+6 = O(n)
17

Example 2:
int total(int n)
{
int sum=0;
for (int i=1;i<=n;i++)
sum=sum+1;
return sum;
}
1 for the assignment statement: int sum=0
In the for loop:
1 assignment, n+1 tests, and n increments.
-------------------------------------------------------------------
T (n)= 1+ (1+n+1+n)+2n+1 = 4n+4 = O(n)
18

Example 3:
void func( )
{ int x=0, i=0, j=1;
cout<< “Enter an Integer value”;
cin>>n;
while (i<n)
{ x++;
i++; }
while (j<n)
j++; }
1 for the first assignment statement: x=0;
1 for the second assignment statement: i=0;
1 for the third assignment statement: j=1;
1 for the output statement.
1 for the input statement.
In the first while loop: n+1 tests
n loops of 2 units for the two increment (addition) operations
In the second while loop:
n tests
n-1 increments
-------------------------------------------------------------------
T (n)= 1+1+1+1+1+(n+1+2n)+(n+n-1) = 5n+5 = O(n) 19

Example 4:
int sum (int n)
{
int partial_sum = 0;
for (int i = 1; i <= n; i++)
partial_sum = partial_sum +(i * i * i);
return partial_sum;
}
1 for the assignment.
1 assignment, n+1 tests, and n increments for the loop expression (for).
n loops of 4 units for an assignment, addition, and two multiplications.
-------------------------------------------------------------------
T (n) = 1+(1+n+1+n)+4n+1 = 6n+4 = O(n)
20

Example 5:
void func ( )
{
int i =1, sum = 0;
while (i < = n)
{
for (int j =0;j<n; j++)
{ sum = i + j; }
i ++;
}}
1 for the first assignment (i = 1).
1 for the second assignment (sum = 0).
In the while loop
n+1 tests
n loops of the following
For the for loop
1 for the increment operation.
-------------------------------------------------------------------
T (n) = 1+1+(n+1)+n[(1+n+1+n+2n)+1] = 4n2+4n+3 = O(n2) 21

Example 6:
int k=0;
for (int i=1; i<n; i*=2)
for(int j=1;j<=n;j++)
k++;
1 for the first assignment (k = 0).
For the first loop (for)
1 assignment, 1+ tests, and multiplication of i*=2.
iterations of the following
For the second loop (for)
n loops of one unit (increment operation)
-------------------------------------------------------------------
T (n) = 1+1+1+ ++[(1+n+1+n)+n] = 3n+4+3= O(n)
22

1.2.3. Formal Approach to Analysis
 In the above examples we have seen that analysis is a bit
complex.
 However, it can be simplified by using some formal approach in
which case we can ignore initializations, loop control, and book
keeping.
 For Loops: Formally
 In general, a for loop translates to a summation.
 The index and bounds of the summation are the same as the
index and bounds of the for loop.
 Suppose we count the number of additions that are done. There is 1
addition per iteration of the loop, hence N additions in total.
23
for (int i = 1; i <= N; i++) {
sum = sum+i;
}
N
N
i


1
1

Nested Loops: Formally
 Nested for loops translate into multiple summations, one for each for loop.
 Again, count the number of additions. The outer summation is for the outer for loop.
24
for (int i = 1; i <= N; i++) {
for (int j = 1; j <= M; j++) {
sum = sum+i+j;
}
}
MN
M
N
i
N
i
M
j
2
2
2
1
1 1

 
 
 
Consecutive Statements: Formally
 Add the running times of the separate blocks of your code
for (int i = 1; i <= N; i++) {
sum = sum+i;
}
for (int i = 1; i <= N; i++) {
for (int j = 1; j <= N; j++) {
sum = sum+i+j;
}
}
2
1 1
1
2
2
1 N
N
N
i
N
j
N
i
















  


Compute Running time with given input
 Example:
 Suppose we have hardware capable of executing 106
instructions per second. How long would it take to execute an
algorithm whose complexity function was T (n) = 2n2 on an
input size of n=108?
 Solution
 The total number of operations to be performed would be T
(108):
T(108) = 2*(108)2 =2*1016
The required number of seconds required would be given by
T(108)/106 so:
Running time =2*1016/106 = 2*1010
 The number of seconds per day is 86,400 so this is about
231,480 days (634 years).
25

Exercises
Determine the run time equation and complexity of each of the following code segments.
1. for (i=0;i<n;i++)
for (j=0;j<n; j++)
sum=sum+i+j;
2. for(int i=1; i<=n; i++)
for (int j=1; j<=i; j++)
sum++;
What is the value of the sum if n=20?
3. int k=0;
for (int i=0; i<n; i++)
for (int j=i; j<n; j++)
k++;
What is the value of k when n is equal to 20?
4. int k=0;
for (int i=1; i<n; i*=2)
for(int j=1; j<n; j++)
k++;
What is the value of k when n is equal to 20?
26

Home Work
5. int x=0;
for(int i=1;i<n;i=i+5)
x++;
What is the value of x when n=25?
6. int x=0;
for(int k=n;k>=n/3;k=k-5)
x++;
7. int x=0;
for (int i=1; i<n;i=i+5)
for (int k=n;k>=n/3;k=k-5)
x++;
8. int x=0;
for(int i=1;i<n;i=i+5)
for(int j=0;j<i;j++)
for(int k=n;k>=n/2;k=k-3)
x++;
What is the correct big-Oh Notation for the above code segment?
27

1.3. Measures of Times
 The running time of an algorithm can be define into three
possible functions Tbest(n), Tavg(n) and Tworst(n)
 Average Case (Tavg): The amount of time the algorithm takes
on an "average" set of inputs.
 Worst Case (Tworst): The amount of time the algorithm takes on
the worst possible set of inputs.
 Best Case (Tbest): The amount of time the algorithm takes on
the smallest possible set of inputs.
28

1.4. Asymptotic Analysis
 Asymptotic analysis is concerned with how the running time of an
algorithm increases with the size of the input in the limit, as the size of
the input increases without bound.
 We will see the most important functions used in the analysis of
algorithms.
O(1) – Constant Time
 Pronounced: "Order 1", "O of 1", "big O of 1"
 The runtime is constant, i.e., independent of the number of input
elements n.
 When evaluating overall running time, we typically ignore these
statements since they don’t factor into the complexity.
 This is the function, f (n) = c.
29

O(n) – Linear Time
 Pronounced: "Order n", "O of n", "big O of n"
 The time grows linearly with the number of input elements n:
If n doubles, then the time approximately doubles, too.
 O(n²) – Quadratic Time(square time)
 Pronounced: "Order n squared", "O of n squared", "big O of n squared"
 The time grows linearly to the square of the number of input elements:
 If the number of input elements n doubles, then the time roughly
quadruples.
30

Cont.
 O(n²) Examples
 Examples of quadratic time are simple sorting
algorithms like Insertion Sort, Selection Sort,
and Bubble Sort.
 O(log n) – Logarithmic Time
 Pronounced: "Order log n", "O of log n", "big O of log
n"
 The effort increases approximately by a constant
amount when the number of input elements doubles.
 An example of logarithmic growth is the binary
search for a specific element in a sorted array of size n.
31

Asymptotic Analysis known function
32

Cont .
 There are five notations used to describe a running time
function. These are:
 Big-Oh Notation (O)
 Big-Omega Notation (W)
 Theta Notation (Q)
 Little-o Notation (o)
 Little-Omega Notation (w)
33

The Big-Oh Notation
 Big-Oh notation is a way of comparing algorithms and
 It is used for computing the complexity of algorithms; i.e.,
 the amount of time that it takes for computer program to run.
 It’s only concerned with what happens for very a large value of n.
 Amount of work the CPU has to do (time complexity)
as the input size grows (towards infinity).
 Formal Definition: f (n)= O (g (n)) if there exist c, k ∊ ℛ+ such that for all n≥
k, f (n) ≤ c.g (n).
 Examples: The following points are facts that you can use for Big-Oh
problems:
 1<=n for all n>=1
 n<=n2 for all n>=1
 2n <=n! for all n>=4
 log2n<=n for all n>=2
 n<=nlog2n for all n>=2 34

 Read The remaining notations of running time
35

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