Algorithms II: Building Blocks, Pseudo Code, and Flowcharts, Slides of Introduction to Computing

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CS101 Introduction to Computing

Lecture 17

Algorithms II

Focus of the last lecture was on Algorithms

Became familiar with the concept of algorithms:

• What they are? (SEQUENCE OF STEPS)
• What is their use?
• What are their types?
• What are the techniques used for representing them? - Pseudo code - Flowcharts - Actual code

Algorithm Building Blocks

All problems can be solved by employing any one of the following building blocks or their combinations

1. Sequences
2. Conditionals
3. Loops

5

Start or stop

Process

Input or output

Connector

Decision

Flow line

Off-page connector

Review of

Flowchart

Elements

Sequences

A sequence of instructions that are executed in the precise order they are written in:

statement block 1 statement block 2 statement block 3

7

statement block 1

statement block 2

statement block 3

Conditionals

Select between alternate courses of action depending upon the evaluation of a condition

If ( condition = true )

statement block 1

Else

statement block 2

End if

8

statement block 1

condition

True False

statement block 2

We will now present the algorithm for a problem whose solution is familiar to us

We will first go through the problem statement and then present the algorithm in three different formats:

1. Pseudo code
2. Flowchart
3. Actual code

Problem Statement

Convert a decimal number into binary

We did write down the pseudo code for this problem last time

Lets do it again, and in a slightly more formal way

Solution in Pseudo Code

1. Let the decimal number be an integer x, x > 0
2. Let the binary equivalent be an empty string y
3. Repeat while x > 0 {

Determine the quotient & remainder of x ÷ 2 y = CONCATENATE( remainder, y ) x = quotient }

1. Print y
2. Stop

Tips on Writing Good Pseudo Code

• Use indention for improved clarity
• Do not put “code” in pseudo code – make your pseudo code language independent
• Don’t write pseudo code for yourself – write it in an unambiguous fashion so that anyone with a reasonable knowledge can understand and implement it
• Be consistent
• Prefer formulas over English language descriptions

17

Start

Find quotient & remainder of x ÷ 2

Get x

x >0?

Print y

Stop

y = CONC(remainder, x ) x = quotient

x is the decimal number y is the binary equivalent

Flowchart of Decimal to Binary Conversion

Yes

No

(^19)

Decimal to Binary Conversion in JavaScript

NOTE: Don’t worry if you don’t understand this code for now; you will - later!Docsity.com

Another Example: Sorting

Sort the following objects w.r.t. their heights