Algorithm Design

CS105

Problem Solving• Algorithm: set of unambiguous instructions to

solve a problem– Breaking down a problem into a set of sub-

problems– Example: Clean the house

• Without instructions – computers cannot do anything at all!

Algorithm design1. Analysis and specification

- Analyze: Understand/define the problem- Specify: Specify particulars

2. Algorithm development phase- Develop: Logical sequence of steps- Test: Follow outline, test cases

3. Implementation phase- Code: The steps into a programming language- Test: Debug

4. Maintenance phase- Use the program- Maintain: Correct errors, meet changing requirements

An example:

Variables

Output

An example:

Variables: A= loaf of bread K= knife T= toaster P= plate B= butter CB= cutting board

Making a perfect piece of toast

Version 21. move A to CB 2. cut S with K 3. move S to T 4. turn on T 5. wait for T 6. move S to P 7. spread B on S with K

S= slice of bread

move

move

move

An example:

Variables: A= loaf of bread K= knife T= toaster P= plate B= butter M= margarine F= friend CB= cutting board

Making toast for fussy friends: plain, butter, margarineVersion 3Move A to CBFOR every F eating toast{ Cut S with K Move S to T Turn on T Wait for T Move S to P IF F likes B { X=B } ELSE IF F likes M { X=M } ELSE { X= NOTHING } Spread X on S with K}

S= slice of breadX

Basic concepts• Example contains concepts used in most

algorithms• Instructions – simple and unambiguous• Variables – input and temporary• Subprocedures – smaller tasks• Looping: FOR each variable, WHILE– Act of repeating tasks

• Conditional statements: IF ELSE– Selectively execute instructions

Pseudocode: Flowchart• Flowchart: diagram that represents an

algorithm• Symbols:

START , END LOOPS, FLOW OF CONTROL

INSTRUCTIONS

CONDITIONALS

Pseudocode: FlowchartFixing non functioning lamp algorithm

Flowchart: Making toast for friends

Variables: A= loaf of breadK= knifeT= toasterP= plateB= butterM= margarineF= friend CB= cutting board

S= slice of breadX

Start Move A to CB Done?

Cut S with K

Move S to T

Turn on T

End

Wait for T

Move S to P

Want B?

Want M?

Set X to B

Set X to M

Set X to NONE

Spread X on S with K

Yes

NoYes

No

Yes

No

Start

End

Add 30 to X

Multiply X by 3

Is X > 100?

Is X < 80?

Subtract 15 from X

Print XYes

No

No

Yes

Divide X by 2Is X an integer

?

Yes

No Round it to the nearest integer

Add 30 to XIF ( x>100){

Subtract 15 from X}ELSE

{Multiply X by 3

}IF ( x<80){

PRINT X}ELSE

{ Divide X by 2 IF ( X is an integer) {

PRINT X } ELSE

{ Round X to the nearest integer

PRINT X

}

}

1. Add 30 to X2. IF X >100, subtract 153. ELSE multiply X by 34. IF X<80, PRINT X END5. ELSE Divide by 2

1. IF X is an integer, PRINT X END

2. ELSE round X, then PRINT X END