Concepts of AlgorithmsCSC-244 Unit Zero

Pseudo code, Flowchart and Algorithm

Master PrinceComputer College

Qassim University K.S.A.

General Information• Instructor: Shahid Iqbal Lone• Tel: 0541-924839• E-mail:

loneshahid@yahoo.com• Discussion / Lectures/mailing GROUP:

CSC-244 at yahoo group• Text Books:

Introduction to Algorithm Second Edition by

Thomas H. CormenCharles E. Leiserson

Ronald L. RivestClifford Stein

ALGORITHMS AND FLOWCHARTS

• A typical programming task can be divided into two phases:

• Problem solving phase– produce an ordered sequence of steps that describe solution

of problem– this sequence of steps is called an algorithm

• Implementation phase – implement the program in some programming language

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Steps in Problem Solving• First produce a general algorithm (one can use

pseudocode) • Refine the algorithm successively to get step by

step detailed algorithm that is very close to a computer language.

• Pseudocode is an artificial and informal language that helps programmers develop algorithms. Pseudocode is very similar to everyday English.

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Pseudocode & Algorithm

• Example 1: Write an algorithm to determine a student’s final grade and indicate whether it is passing or failing. The final grade is calculated as the average of four marks.

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Pseudocode & AlgorithmPseudocode:• Input a set of 4 marks• Calculate their average by summing and dividing

by 4• if average is below 50

Print “FAIL”else

Print “PASS”

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Pseudocode & Algorithm• Detailed Algorithm • Step 1: Input M1,M2,M3,M4

Step 2: GRADE (M1+M2+M3+M4)/4

Step 3: if (GRADE < 50) thenPrint “FAIL”

elsePrint “PASS”

endif

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The Flowchart• (Dictionary) A schematic representation of a sequence of

operations, as in a manufacturing process or computer program.

• (Technical) A graphical representation of the sequence of operations in an information system or program. Information system flowcharts show how data flows from source documents through the computer to final distribution to users. Program flowcharts show the sequence of instructions in a single program or subroutine. Different symbols are used to draw each type of flowchart.

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The Flowchart

A Flowchart– shows logic of an algorithm– emphasizes individual steps and their

interconnections– e.g. control flow from one action to the next

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Flowchart Symbols

CENG 213 Data Structures 10

Example-1

Step 1: Input M1,M2,M3,M4Step 2: GRADE M1+M2+M3+M4)/4 Step 3: if (GRADE <50) then

Print “FAIL” else

Print “PASS” endifStep 4: End

START

InputM1,M2,M3,M4

GRADE(M1+M2+M3+M4)/4

ISGRADE<50

STOP

YN

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PRINT“FAIL”

PRINT“PASS”

Example 2• Write an algorithm and draw a flowchart to

convert the length in feet to centimeter.Pseudocode:• Input the length in feet (Lft)• Calculate the length in cm (Lcm) by

multiplying LFT with 30• Print length in cm (LCM)

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Example 2

Algorithm • Step 1: Input Lft• Step 2: Lcm Lft x 30 • Step 3: Print Lcm

START

InputLft

Lcm Lft x 30

PrintLcm

STOP

Flowchart

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Example 3

Write an algorithm and draw a flowchart that will read the two sides of a rectangle and calculate its area.

Pseudocode • Input the width (W) and Length (L) of a rectangle• Calculate the area (A) by multiplying L with W• Print A

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Example 3

Algorithm • Step 1: Input W,L• Step 2: A L x W • Step 3: Print A

START

InputW, L

A L x W

PrintA

STOP

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Example 4

• Write an algorithm and draw a flowchart that will calculate the roots of a quadratic equation

• Hint: d = sqrt ( ), and the roots are: • x1 = (–b + d)/2a and x2 = (–b – d)/2a

2 0ax bx c 2 4b ac

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Example 4

Pseudocode: • Input the coefficients (a, b, c) of the

quadratic equation• Calculate d• Calculate x1 • Calculate x2• Print x1 and x2

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Example 4

• Algorithm: • Step 1: Input a, b, c• Step 2: d sqrt ( )• Step 3: x1 (–b + d) / (2 x a)• Step 4: x2 (–b – d) / (2 x a)• Step 5: Print x1, x2

START

Inputa, b, c

d sqrt(b x b – 4 x a x c)

Printx1 ,x2

STOP

x1 (–b + d) / (2 x a)

X2 (–b – d) / (2 x a)

4b b a c

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DECISION STRUCTURES • The expression A>B is a logical expression• it describes a condition we want to test• if A>B is true (if A is greater than B) we take the

action on left• print the value of A • if A>B is false (if A is not greater than B) we take

the action on right• print the value of B

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DECISION STRUCTURES

isA>B

Print BPrint A

Y N

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IF–THEN–ELSE STRUCTURE

• The structure is as followsIf condition then

true alternative else

false alternativeendif

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IF–THEN–ELSE STRUCTURE• The algorithm for the flowchart is as

follows:If A>B then

print Aelse

print Bendif

isA>B

Print BPrint A

Y N

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Example 5 • Write an algorithm that reads two values, determines the

largest value and prints the largest value with an identifying message.

ALGORITHMStep 1: Input VALUE1, VALUE2Step 2: if (VALUE1 > VALUE2) then

MAX VALUE1else

MAX VALUE2endif

Step 3: Print “The largest value is”, MAX

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Example 5

MAX VALUE1

Print“The largest value is”, MAX

STOP

Y N

START

InputVALUE1,VALUE2

MAX VALUE2

isVALUE1>VALUE2

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NESTED IF’S

• One of the alternatives within an IF–THEN–ELSE statement– may involve further IF–THEN–ELSE statement

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Example 6

• Write an algorithm that reads three numbers and prints the value of the largest number.

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Example 6Step 1: Input N1, N2, N3Step 2: if (N1>N2) then

if (N1>N3) then MAX N1 [N1>N2, N1>N3]

else MAX N3 [N3>N1>N2]

endifelse

if (N2>N3) then MAX N2 [N2>N1, N2>N3]

else MAX N3 [N3>N2>N1]

endifendif

Step 3: Print “The largest number is”, MAX

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Example 6

• Flowchart: Draw the flowchart of the above Algorithm.

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Example 7

• Write and algorithm and draw a flowchart to a) read an employee name (NAME), overtime

hours worked (OVERTIME), hours absent (ABSENT) and

b) determine the bonus payment (PAYMENT).

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Example 7

Bonus ScheduleOVERTIME – (2/3)*ABSENT Bonus Paid

>40 hours>30 but 40 hours>20 but 30 hours>10 but 20 hours 10 hours

S.R. 50S.R. 40S.R. 30S.R. 20S.R. 10

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Step 1: Input NAME,OVERTIME,ABSENTStep 2: if (OVERTIME–(2/3)*ABSENT > 40) then PAYMENT 50 else if (OVERTIME–(2/3)*ABSENT > 30) then

PAYMENT 40 else if (OVERTIME–(2/3)*ABSENT > 20) then PAYMENT 30 else if (OVERTIME–(2/3)*ABSENT > 10) then PAYMENT 20 else PAYMENT 10 endif

Step 3: Print “Bonus for”, NAME “is S.R.”, PAYMENT

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Example 7

• Flowchart: Draw the flowchart of the above algorithm?

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33

Problem Statement

Convert a decimal number into binary

Convert 75 to Binary

75237 1218 129 024 122 021 020 1

1001011

remainder

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We will write down the pseudo code for this problem in a formal way.

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36

Solution in Pseudo Code

1. Let the decimal number be an integer x, x > 02. Let the binary equivalent be an empty string y3. Repeat while x > 0 {

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

}4. Print y5. Stop

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Q: Is this the only possible algorithm for converting a decimal number into a binary representation?

If not, then is this the best?In terms of speed?In terms of memory requirement?In terms of ease of implementation?

You must ask these questions after writing any algorithm!

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Tips on Writing Good Pseudo Code• Use indention for improved clarity

• Do not put “code” in pseudo code – make your pseudo code computer language independent

• Don’t write pseudo code for yourself – write it in an clear fashion so that anyone of the team member with a reasonable knowledge can understand and implement it.

• Prefer formulas over English language descriptions

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Start

Find quotient& remainder

of x ÷ 2

INPUT x

x>0 ?

Print y

Stop

y = CONC(remainder, x)x = quotient

x is the decimal numbery is the binary equivalent

Flowchart of Decimalto Binary Conversion

Yes

No

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Start

Find quotient & remainder of x ÷ 2

Get x

Print y

Stop

y = CONC(remainder, x)x = quotient

x is the decimal numbery is the binary equivalent

Another Flowchart of Decimalto Binary Conversion

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While x > 0

Example 8Write a program that asks the user to enter a temperature reading in centigrade and then prints the equivalent Fahrenheit value.

fahrenheitPrompt for centigrade valueRead centigrade valueCompute fahrenheit valueDisplay fahrenheit value

centigrade

OutputProcess InputSTOP

inputCentcentigrade

calcFahfahrenheit = (1.8*centigrade) + 32

outputFahfahrenheit

STARTConvert centigrade to fahrenheit

PSEUDOCODE:Display "Enter Centigrade: "Input centigradeFahrenheit = 1.8 * centigrade + 32Display "Centigrade to Fahrenheit = " fahrenheit

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END

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