Algorithm Class at KPHB (C, C++ Course Training Institute in KPHB, Kukatpally KPHB, Hyderabad.)

ALGORITHM CLASSS
Algorithm Analysis
DS C C++ JAVA TRAINING INSTITUTES IN KPHB HYDERABAD

Program=instructions + data
Description of algorithms and data structures to achieve a specific objective
Could be done in any language, even a natural language like Wnglish
Example add a and b
Read a ; read b; c=a+b; // Algorithm
A , b and c are data structures
Programming language: A standard notation for writing programs
Like english or telugu
Examples: C, Java, intel assemly language, machine language etc
Hardware understands machine language but we instructs using programming language.
Hence we need the program translators like compiler gcc
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What is a computer program
ALGORITHM CLASSS

ALGORITHM CLASSS
If you are strong with data structures, you can
 evaluate the quality of a program
(Analysis of Algorithms: Running time and memory space )
 write fast,efficient and quality programs with less memory usage
 solve new problems efficiently by choosing appropriate data structures and
algorithms
Why do we need this course

ALGORITHM CLASSS
 How smart you are to pick the appropriate data structure for a given problem(if you
know pros and cons of different DS only you can decide)
 How strong you are on programming basics (DS)
 Ability to decompose problems
 Ability to find solutions with a better logic
 Efficient programming skills
… etc.
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Why interviewer concentrates more on DS

ALGORITHM CLASSS
Data Structures:
A systematic way of organizing and accessing data
.
--No single data structure works well for ALL purposes.
Algorithm
is a step-by-step procedure for solving a problem in a finite amount of time.
Algorithm analysis
a process of determining the amount of time, resource, etc. required when executing an
algorithm
Estimate the running time
Estimate the memory space required.
Depends on the input size
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What is an algorithm analysis

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There are two aspects of algorithmic performance:
Time
Instructions take time.
How fast does the algorithm perform?
What affects its runtime?
Space
Data structures take space
What kind of data structures can be used?
How does choice of data structure affect the runtime?
We will focus on time:
How to estimate the time required for an algorithm
How to reduce the time required
Analysis of Algorithms
is the area of computer science that provides tools to analyze the efficiency of different methods of
solutions.
How do we compare the time efficiency of two algorithms that solve the same problem?
Naïve Approach: implement these algorithms in a programming language (C++), and run them to
compare their time requirements. Comparing the programs (instead of algorithms) has difficulties.
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ALGORITHM CLASSS
To analyze algorithms:
First, we start to count the number of significant operations in a particular solution to assess its
efficiency.
Then, we will express the efficiency of algorithms using growth functions.
Growth Rate
We measure an algorithm’s time requirement as a function of the problem size.
Problem size depends on the application: e.g. number of elements in a list for a sorting
algorithm, the number disks for towers of hanoi.
So, for instance, we say that (if the problem size is n)
Algorithm A requires 5*n2 time units to solve a problem of size n.
Algorithm B requires 7*n time units to solve a problem of size n.
The most important thing to learn is how quickly the algorithm’s time requirement grows as a
function of the problem size.
Algorithm A requires time proportional to n2.
Algorithm B requires time proportional to n.
An algorithm’s proportional time requirement is known as growth rate. We can compare the
efficiency of two algorithms by comparing their growth rates.
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ALGORITHM CLASSS
Function Growth Rate Name
c Constant
log N Logarithmic
log2N Log-squared
N Linear
N log N
N2 Quadratic
N3 Cubic
2N Exponential

ALGORITHM CLASSS
Experimental Studies
Write a program implementing the algorithm
Run the program with inputs of varying size and composition
Use a method like System.currentTimeMillis() to get an accurate measure of the actual
running time
Plot the results and compare
Limitations of the experiments
It is necessary to implement the algorithm, which may be difficult
Results may not be indicative of the running time on other inputs not included in the
experiment.
In order to compare two algorithms, the same hardware and software environments must
be used
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ALGORITHM CLASSS
Theoretical Analysis
Uses a high-level description of the algorithm instead of an implementation
Characterizes running time as a function of the input size, n.
Takes into account all possible inputs
Allows us to evaluate the speed of an algorithm independent of the hardware/software
environment
Asymptotic Algorithm Analysis
The asymptotic analysis of an algorithm determines the running time in big-Oh notation
To perform the asymptotic analysis
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

(Big-Oh)
T(n) is O(F(n)) if there are positive constants c and n0 such that
T(n)<= cF(n) when n >= n0
Provides an upper bound on the growth rate of the function.
(Big-Omega)
T(n) is Ω(F(n)) if there are positive constant c and n0 such that
T(n) >= cF(n) when n >= n0
we want to say that an algorithm takes at least a certain amount of time,
Def: (Big-Theta) T(n) is Θ(F(n)) if and only if
T(n) = O(F(n)) and T(n) = Ω(F(n)) => k1*F(n) <
Def: (Little-Oh) T(n) = o(F(n)) if and only if
T(n) = O(F(n)) and T(n) != Θ (F(n))
we cannot say here what the value of c_1c1 is, because it depends on the speed of the
computer, the programming language used, the compiler or interpreter that translates the
source program into runnable code, and other factors.
Algorithm analysis: other notations

ALGORITHM CLASSS
Asymptotic Algorithm Analysis
The asymptotic analysis of an algorithm determines the running time in big-Oh notation
To perform the asymptotic analysis
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
Example:
We determine that algorithm arrayMax executes at most 6n  1 primitive
operations
We say that algorithm arrayMax “runs in O(n) time”
Since constant factors and lower-order terms are eventually dropped anyhow, we
can disregard them when counting primitive operations
Algorithm analysis: BIG-Oh notation

ALGORITHM CLASSS
Big O notation:
Big Oh notation is used to capture the most dominant term in a function, and to represent
the growth rate.
Also called asymptotic upper bound.
Ex: 100n3 + 30000n =>O(n3)
To simplify the running time estimation, for a function f(n), we ignore the constants and
lower order terms.
Example: 10n3+4n2-4n+5 is O(n3).
http://sites.google.com/site/algorithmclass
BIG-Oh notation

BIG-Oh notation

Theta
Definition: A theoretical measure of the execution of an algorithm, usually the time
or memory needed, given the problem size n, which is usually the number of items.
Informally, saying some equation f(n) = Θ (g(n)) means it is within a constant multiple
of g(n). The equation is read, "f of n is theta g of n".
Formal Definition: f(n) = Θ (g(n)) means there are positive constants c1, c2, and k,
such that 0 ≤ c1g(n) ≤ f(n) ≤ c2g(n) for all n ≥ k. The values of c1, c2, and k must be f
ixed for the function f and must not depend on n.

http://plus.google.com/+AlgorithmClass
Omege
(Big-Omega)
T(n) is Ω(F(n)) if there are positive constant c and n0 such that
T(n) >= cF(n) when n >= n0
we want to say that an algorithm takes at least a certain amount of time,

https://plus.google.com/+AlgorithmClass
BIG-Oh notation
Definition: A theoretical measure of the execution of an algorithm, usually the time or
memory needed, given the problem size n, which is usually the number of items. Informally,
saying some equation f(n) = O(g(n)) means it is less than some constant multiple of g(n).
The notation is read, "f of n is big oh of g of n".
Formal Definition: f(n) = O(g(n)) means there are positive constants c and k, such that 0 ≤
f(n) ≤ cg(n) for all n ≥ k. The values of c and k must be fixed for the function f and must not
depend on n.

ALGORITHM CLASSS
Primitive Operations
Basic computations performed by an algorithm
Largely independent from the programming language
Examples:
Evaluating an expression
Assigning a value to a variable
Indexing into an array
Calling a method
Returning from a method
Algorithm arrayMax(A, n)
currentMax=a[0]; // 2
for (i=1;i<n;i++) // 2n //(i=1 once, i<n n
times, i++ (n-1) times)
if (a[i] > currentMax) 2(n-1)
currentMax=a[i] 2(n-1)
return currentMax; 1
total time= 6n-1

Function Name
C Constant
LogN Logarithmic
Log2N Log-squared
N Linear
NlogN NlogN
N2 Quaratic
N3 Cubic
2n Exponential

Worst-case vs. Average-case
A worst-case bound is a guarantee over all inputs of size N.
In an average-case bound, the running time is measured as an average over all of the
possible inputs of size N.
We will mainly focus on worst-case analysis, but sometimes it is useful to do average
one.
Example:
Static Searching Problem
Given an integer X and an array A, return the position of X in A or an indication that it
is not present. If X occurs more than once, return any occurrence. The array A is never
altered.
Sequential search: =>O(n)
Binary search (sorted data): => O(logn)
BIG-Oh notation

Sequential search
A sequential search steps through the data sequentially until an match is found.
A sequential search is useful when the array is not sorted.
A sequential search is linear O(n) (i.e. proportional to the size of input)
 Unsuccessful search --- n times
 Successful search (worst) --- n times
 Successful search (average) --- n/2 times
Binary Search
If the array has been sorted, we can use binary search, which is performed from
the middle of the array rather than the end.
We keep track of low_end and high_end, which delimit the portion of the array in
which an item, if present, must reside.
If low_end is larger than high_end, we know the item is not present.
Algorithm analysis: Example

Algorithm Class
Email: algorithm.class@gmail.com
Website: http://sites.google.com/site/algorithmclass
G+ : http://plus.google.com/+AlgorithmClass/posts
Facebook : http://www.facebook.com/AlgorithmClassCCppDsJavaTrainingKphbHyderabad
ALGORITHM CLASSS
C C++ DS Data Structures CPP JAVA TRAINING INSTITUTE KPHB HYDERABAD
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Algorithm Class at KPHB (C, C++ Course Training Institute in KPHB, Kukatpally KPHB, Hyderabad.)

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