Algorithm Design
CS105
Problem Solving
• Algorithm: set of unambiguous instructions to
solve a problem
– Breaking down a problem into a set of subproblems
– Example: Clean the house
• Without instructions – computers cannot do
anything at all!
Algorithm design
1. 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:
Making a perfect piece of toast
What
do we need: a loaf of bread, knife, toaster,
Variables
plate, butter, cutting board
Version 1
1. Move a loaf of bread on a cutting board
2. Cut a slice of bread with a knife
3. Move the slice of bread to the toaster
4. Turn toaster on
5. Wait for the toaster to finish
6. Move the toasted bread on a plate
7. Spread butter on the
with knife
Output
An example:
Making a perfect piece of toast
Variables: A= loaf of bread
K= knife
T= toaster
P= plate
B= butter
CB= cutting board
S= slice of bread
Version 2
1. move
move A to CB
2. cut S with K
3. move
move S to T
4. turn on T
5. wait for T
6. move
move S to P
7. spread B on S with K
An example:
Making toast for fussy friends: plain, butter, margarine
Variables: A= loaf of bread
K= knife
T= toaster
P= plate
B= butter
M= margarine
F= friend
CB= cutting board
S= slice of bread
X
Version 3
Move A to CB
FOR 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
}
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
INSTRUCTIONS
CONDITIONALS
LOOPS, FLOW OF CONTROL
Pseudocode: Flowchart
Fixing non functioning
lamp algorithm
Flowchart: Making toast for friends
Start
End
No
Variables:
A= loaf of bread
K= knife
T= toaster
P= plate
B= butter
M= margarine
F= friend
CB= cutting board
Cut S with K
Move S to T
Turn on T
Wait for T
Spread X on S
with K
Move S to P
Set X to B
S= slice of bread
X
Yes
Done
?
Move A to CB
Yes
Want
B?
No
No
Want
M?
Yes
Set X to M
Set X to
NONE
Add 30 to X
Start
Subtract 15 from X
Yes
Is X >
100?
No
Multiply X by 3
End
Is X <
80?
Yes
Print X
Yes
No
Divide X by 2
Is X an
integer
?
No
Round it to the nearest
integer
Add 30 to X
IF ( 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.
2.
3.
4.
5.
Add 30 to X
IF X >100, subtract 15
ELSE multiply X by 3
IF X<80, PRINT X END
ELSE Divide by 2
1. IF X is an integer, PRINT
X END
2. ELSE round X, then
PRINT X END