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
4.
Steps in ProblemSolving
• 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.
5.
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.
6.
Pseudocode & Algorithm
Pseudocode:
•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”
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.
9.
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
10.
Flowchart Symbols
Basic
Oval
Parallelogram
Rectangle
Diamond
Hybrid
Name SymbolUse in Flowchart
Denotes the beginning or end of the program
Denotes an input operation
Denotes an output operation
Denotes a decision (or branch) to be made.
The program should continue along one of
two routes. (e.g. IF/THEN/ELSE)
Denotes a process to be carried out
e.g. addition, subtraction, division etc.
Flow line Denotes the direction of logic flow in the program
11.
Example
PRINT
“PASS”
Step 1: InputM1,M2,M3,M4
Step 2: GRADE (M1+M2+M3+M4)/4
Step 3: if (GRADE <50) then
Print “FAIL”
else
Print “PASS”
endif
START
Input
M1,M2,M3,M4
GRADE(M1+M2+M3+M4)/4
IS
GRADE<5
0
PRINT
“FAIL”
STOP
Y
N
12.
Example 2
• Writean 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)
Example 3
Write analgorithm 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
15.
Example 3
Algorithm
• Step1: Input W,L
• Step 2: A L x W
• Step 3: Print A
START
Input
W, L
A L x W
Print
A
STOP
16.
Example 4
• Writean 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
0
ax bx c
2
4
b ac
17.
Example 4
Pseudocode:
• Inputthe coefficients (a, b, c) of the quadratic equation
• Calculate d
• Calculate x1
• Calculate x2
• Print x1 and x2
18.
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
Input
a, b, c
d sqrt(b x b – 4 x a x c)
Print
x1 ,x2
STOP
x1 (–b + d) / (2 x a)
X2 (–b – d) / (2 x a)
4
b b a c
19.
DECISION STRUCTURES
• Theexpression 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
Example 5
• Writean algorithm that reads two values, determines the largest value and prints
the largest value with an identifying message.
ALGORITHM
Step 1: Input VALUE1, VALUE2
Step 2: if (VALUE1 > VALUE2) then
MAX VALUE1
else
MAX VALUE2
endif
Step 3: Print “The largest value is”, MAX
25.
Example 5
MAX VALUE1
Print
“The largest value is”,
MAX
STOP
Y N
START
Input
VALUE1,VALUE2
MAX VALUE2
is
VALUE1>VALUE2
26.
NESTED IFS
• Oneof the alternatives within an IF–THEN–ELSE statement
• may involve further IF–THEN–ELSE statement
27.
Example 6
• Writean algorithm that reads three numbers and prints the value of
the largest number.
28.
Example 6
Step 1:Input N1, N2, N3
Step 2: if (N1>N2) then
if (N1>N3) then
MAX N1 [N1>N2, N1>N3]
else
MAX N3 [N3>N1>N2]
endif
else
if (N2>N3) then
MAX N2 [N2>N1, N2>N3]
else
MAX N3 [N3>N2>N1]
endif
endif
Step 3: Print “The largest number is”, MAX
Example 7
• Writeand 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).
31.
Example 7
Bonus Schedule
OVERTIME– (2/3)*ABSENT Bonus Paid
>40 hours
>30 but 40 hours
>20 but 30 hours
>10 but 20 hours
10 hours
$50
$40
$30
$20
$10
32.
Step 1: InputNAME,OVERTIME,ABSENT
Step 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 $”, PAYMENT