Transcript Course_2_Alg_Design_and_Control_Structures

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Algorithms and Structured Program
Design and Development
Outline
0 Introduction
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Top-down design technique
Algorithms
Control Structures - Flowcharts
Control Structures - Pseudocode
The If Selection Structure
The If/Else Selection Structure
The While Repetition Structure
Formulating Algorithms:
Case Study 1 (Counter-Controlled Repetition)
9 Formulating Algorithms with Top-down, Stepwise Refinement:
Case Study 2 (Sentinel-Controlled Repetition)
10 Formulating Algorithms with Top-down, Stepwise Refinement:
Case Study 3 (Nested Control Structures)
11 Assignment Operators
12 Increment and Decrement Operators
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Introduction
• What is a program?
– A set of step-by-step instructions that directs the computer to
perform tasks and produce results.
• Programming Languages
– A programming language is a set of rules that instructs a
computer what operations to perform.
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Introduction
• Before writing a program:
– Have a thorough understanding of the problem
– Carefully plan an approach for solving it
“Top-Down design technique”
• While writing a program:
– Know what “building blocks” are available
– Use good programming principles
“Structured programs”
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Introduction
• What Can a Program Do?
• A program can only instruct a
computer to:
–
–
–
–
–
–
–
Read Input
Sequence
Calculate
Store data
Compare and branch
Iterate or Loop
Write Output
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1 Top-Down Design
• If we look at a problem as a whole, it
may seem impossible to solve because it
is so complex.
Examples:
– writing a game
– writing a word processor
• Complex problems can be solved using
top-down design, also known as stepwise
refinement, where
– We break the problem into parts
– Then break the parts into parts
– Soon, each of the parts will be easy to do
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1 Top-Down Design
1. Determine overall goal.
2. Break that goal into two, three or more detailed
parts.
3. Put all details. Keep repeating steps 1 and 2
until you cannot reasonably break down the
problem any further.
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1 Advantages of Top-Down Design
• Breaking the problem into parts helps us
to clarify what needs to be done.
• At each step of refinement, the new
parts become less complicated and,
therefore, easier to figure out.
• Parts of the solution may turn out to be
reusable.
• Breaking the problem into parts allows
more than one person to work on the
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Simplified Development Cycle
Analyze the problem
Design the solution algorithm
Design the user interface
Write the code
Test and debug the program
Complete the documentation
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1 Example_1 :Top-Down Design
• Problem:
– We own a home improvement company.
– We do painting, roofing, and basement
waterproofing.
– A section of town has recently flooded (zip code
21222).
– We want to send out pamphlets to our customers in
that area.
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1 Example_1 : The Top Level
• Get the customer list from a file.
• Sort the list according to zip code.
• Make a new file of only the customers with the
zip code 21222 from the sorted customer list.
• Print an envelope for each of these customers.
Main
Read
Sort
Select
Print
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1 Example_2 : Another Example
• Problem: Write a program that draws this
picture of a house.
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1 Example_2 : The Top Level
•
•
•
•
Draw
Draw
Draw
Draw
the
the
the
the
outline of the house
chimney
door
windows
Main
Draw
Outline
Draw
Chimney
Draw
Door
Draw
Windows
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1 Example_2 : Pseudocode for Main
Call Draw Outline
Call Draw Chimney
Call Draw Door
Call Draw Windows
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1 Example_2 : Observation
• The door has both a frame and knob.
break this into two steps.
We could
Main
Draw
Outline
Draw
Chimney
Draw
Door Frame
Draw
Door
Draw
Windows
Draw
Knob
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1 Example_2 : Pseudocode for Draw Door
Call Draw Door Frame
Call Draw Knob
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1 Example_2 : Another Observation
• There are three windows to be drawn.
Main
Draw
Outline
Draw
Windows
. . .
Draw
Window 1
Draw
Window 2
Draw
Window 3
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1 Example_2 : One Last Observation
• But don’t the windows look the same?
They
just have different locations.
• So, we can reuse the code that draws a
window.
– Simply copy the code three times and edit it to place the window in the
correct location, or
– Use the code three times, “sending it” the correct location each time (we
will see how to do this later).
• This is an example of code reuse.
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1 Example_2 : Reusing the Window Code
Main
Draw
Outline
. . .
Draw
Windows
Draw a
Window
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1 Example_2 : Pseudocode for Draw Windows
Call Draw a Window, sending in Location 1
Call Draw a Window, sending in Location 2
Call Draw a Window, sending in Location 3
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Algorithms
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• Computing problems
– All can be solved by executing a series of actions in a
specific order
• Algorithm: procedure in terms of
– Actions to be executed
– The order in which these actions are to be executed
• Program control
– Specify order in which statements are to executed
3 Control Structures
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• Sequential execution
– Statements executed one after the other in the order written
• Transfer of control
– When the next statement executed is not the next one in
sequence
– Overuse of goto statements led to many problems
• All programs written in terms of 3 control
structures
– Sequence structures : Programs executed sequentially by
default
– Selection structures : if, if/else, and switch
– Repetition structures: while, do/while and for
3 Control Structures
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• Flowchart
– Graphical representation of an algorithm
– Drawn using certain special-purpose symbols connected by
arrows called flowlines
– Rectangle symbol (action symbol):
• Indicates any type of action
– Oval symbol:
• Indicates the beginning or end of a program or a section of code
• Single-entry/single-exit control structures
– Connect exit point of one control structure to entry point of
the next (control-structure stacking)
– Makes programs easy to build
3 Flowcharts
Process
I/O
Decision
Flow
Connector
Terminal
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3 Flowchart Rules
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1. Use standard symbols.
2. The flowchart’s logic should generally
flow from top of the page to the bottom
and from left to right.
3. The decision symbol always has two exit
point and should always ask a yes or no
question.
4. Instruction inside the symbols should be
clear English description.
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3 Flowchart Structures - IF-THEN
Entry
false
Test condition
true
True
statement
Exit
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3 Flowchart Structures - IF-THEN-ELSE
Entry
false
Test condition
False
statement
true
True
statement
Exit
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3 Flowchart Structures - Iterate
•A program loop is a form of
iteration.
•A computer can be instructed to
repeat instructions under certain
conditions.
No
3 Flowchart Structures –
DO WHILE Loop
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Entry
Exit
Test condition
Yes
True statement
No
3 Flowchart Structures –
DO UNTIL Loop
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Entry
True
statement
Exit
No
Test condition
Yes
3 Flow Chart - Example
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• “find the greatest number
in 3 numbers”..
– Output = ?
• Input = ?
An integer value which is the greatest one.
Read three integer values (a, b and c) from screen.
• The Logic steps =
1. Compare a and b and find the big one.
?2. Compare the big value and c and decide
the biggest value.
3. Show the user this biggest value.
3 Flow Chart - Example 31
“find the greatest number in 3 numbers”..
An integer value which is the greatest one.
Read
a, b, c
a>b
Read three integer values (a, b and c) from screen.
Y
a>c
Y The biggest
value is a.
N
b>c
Y
The biggest
value is b.
N
The biggest
value is c.
Show the user this biggest value.
1. Compare a and b and
find the big one.
2. Compare the big value
and c and decide the
biggest value.
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Pseudocode
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• Pseudocode
– Artificial, informal language that helps us develop algorithms
– Similar to everyday English
– Not actually executed on computers
– Helps us “think out” a program before writing it
• Easy to convert into a corresponding C program
• Consists only of executable statements
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The if Selection Structure
• Selection structure:
– Used to choose among alternative courses of action
– Pseudocode:
If student’s grade is greater than or equal
to 60
Print “Passed”
• If condition true
– Print statement executed and program goes on to next
statement
– If false, print statement is ignored and the program goes onto
the next statement
– Indenting makes programs easier to read
• C ignores whitespace characters
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The if Selection Structure
• Same statement in C:
if ( grade >= 60 )
printf( "Passed\n" );
– C code corresponds closely to the pseudocode
• Diamond symbol (decision
symbol)
– Indicates decision is to be made
– Contains an expression that can be true or false
– Test the condition, follow appropriate path
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The if Selection Structure
• if structure is a single-entry/single-exit
structure
true
grade >= 60
A decision can be
made on any
expression.
print “Passed”
zero - false
nonzero - true
false
Example:
3 - 4 is true
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The if/else Selection Structure
• if
– Only performs an action if the condition is true
• if/else
– Specifies an action to be performed both when the condition
is true and when it is false
• Psuedocode:
If student’s grade is greater than or equal
to 60
Print “Passed”
else
Print “Failed”
– Note spacing/indentation conventions
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The if/else Selection Structure
• C code:
if ( grade >= 60 )
printf( "Passed\n");
else
printf( "Failed\n");
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The if/else Selection Structure
• Flow chart of the if/else selection
structure
false
grade >= 60
true
print “Failed”
print “Passed”
• Nested if/else structures
– Test for multiple cases by placing if/else selection
structures inside if/else selection structures
– Once condition is met, rest of statements skipped
– Deep indentation usually not used in practice
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The if/else Selection Structure
– Pseudocode for a nested if/else structure
If student’s grade is greater than or equal
to 90
Print “A”
else
If student’s grade is greater than or
equal to 80
Print “B”
else
If student’s grade is greater than
or equal to 70
Print “C”
else
If student’s grade is greater
than or equal to 60
Print “D”
else
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The if/else Selection Structure
• Compound statement:
– Set of statements within a pair of braces
– Example:
if ( grade >= 60 )
printf( "Passed.\n" );
else {
printf( "Failed.\n" );
printf( "You must take this course
again.\n" );
}
– Without the braces, the statement
printf( "You must take this course
again.\n" );
would be executed automatically
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The if/else Selection Structure
• Block:
– Compound statements with declarations
• Syntax errors
– Caught by compiler
• Logic errors:
– Have their effect at execution time
– Non-fatal: program runs, but has incorrect
output
– Fatal: program exits prematurely
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The while Repetition Structure
• Repetition structure
– Programmer specifies an action to be repeated
while some condition remains true
– Psuedocode:
While there are more items on my
shopping list
Purchase next item and cross it off
my list
– while loop repeated until condition becomes
false
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The while Repetition Structure
• Example:
int product = 100;
while ( product <= 1000 )
product = 2 * product;
printf(“product= %d \n”, product);
product <= 1000
false
true
product = 2 * product
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Formulating Algorithms 44
(Counter-Controlled Repetition)
• Counter-controlled repetition
– Loop repeated until counter reaches a certain value
– Definite repetition: number of repetitions is known
– Example: A class of ten students took a quiz. The grades
(integers in the range 0 to 100) for this quiz are available
to you. Determine the class average on the quiz
– Pseudocode:
Set total to zero
Set grade counter to one
While grade counter is less than or equal to
ten
Input the next grade
Add the grade into the total
Add one to the grade counter
Set the class average to the total divided by
ten
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1 /*
Outline
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Class average program with
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counter-controlled repetition */
1. Initialize Variables
4 #include <stdio.h>
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6 int main()
2. Execute Loop
7 {
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int counter, grade, total, average;
3. Output results
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/* initialization phase */
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total = 0;
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counter = 1;
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/* processing phase */
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while ( counter <= 10 ) {
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printf( "Enter grade: " );
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scanf( "%d", &grade );
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total = total + grade;
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counter = counter + 1;
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}
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/* termination phase */
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average = total / 10;
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printf( "Class average is %d\n", average );
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return 0;
/* program ended successfully */
27}
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Outline
Enter
Enter
Enter
Enter
Enter
Enter
Enter
Enter
Enter
Enter
Class
grade: 98
grade: 76
grade: 71
grade: 87
grade: 83
grade: 90
grade: 57
grade: 79
grade: 82
grade: 94
average is 81
Program Output
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Formulating Algorithms with TopDown, Stepwise Refinement
• Problem becomes:
Develop a class-averaging program that will
process an arbitrary number of grades each
time the program is run.
– Unknown number of students
– How will the program know to end?
• Use sentinel value
–
–
–
–
Also called signal value, dummy value, or flag value
Indicates “end of data entry.”
Loop ends when user inputs the sentinel value
Sentinel value chosen so it cannot be confused with a
regular input (such as -1 in this case)
Formulating Algorithms with 48
TopDown, Stepwise Refinement
• Top-down, stepwise refinement
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– Begin with a pseudocode representation of the top:
Determine the class average for the quiz
– Divide top into smaller tasks and list them in order:
Initialize variables
Input, sum and count the quiz grades
Calculate and print the class average
• Many programs have three phases:
– Initialization: initializes the program variables
– Processing: inputs data values and adjusts program
variables accordingly
– Termination: calculates and prints the final results
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Formulating Algorithms with TopDown, Stepwise Refinement
• Refine the initialization phase
from Initialize variables to:
Initialize total to zero
Initialize counter to zero
• Refine Input, sum and count the
quiz grades to
Input the first grade (possibly the
sentinel)
While the user has not as yet entered
the sentinel
Add this grade into the running
total
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Formulating Algorithms with TopDown, Stepwise Refinement
• Refine Calculate and print the
class average to
If the counter is not equal to zero
Set the average to the total
divided by the counter
Print the average
else
Print “No grades were entered”
1 /*
Outline
2
Class average program with
3
sentinel-controlled repetition */
4 #include <stdio.h>
5
6 int main()
7 {
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float average;
/* new data type */
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int counter, grade, total;
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/* initialization phase */
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total = 0;
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counter = 0;
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/* processing phase */
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printf( "Enter grade, -1 to end: " );
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scanf( "%d", &grade );
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while ( grade != -1 ) {
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total = total + grade;
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counter = counter + 1;
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printf( "Enter grade, -1 to end: " );
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scanf( "%d", &grade );
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}
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29
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33
34
35}
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/* termination phase */
if ( counter != 0 ) {
average = total / counter;
printf( "Class average is %.2f", average );
}
else
printf( "No grades were entered\n" );
return 0;
Outline
/* program ended successfully */
3. Calculate Average
Enter
Enter
Enter
Enter
Enter
Enter
Enter
Enter
Enter
Class
grade, -1 to end:
grade, -1 to end:
grade, -1 to end:
grade, -1 to end:
grade, -1 to end:
grade, -1 to end:
grade, -1 to end:
grade, -1 to end:
grade, -1 to end:
average is 82.50
75
94
97
88
70
64
83
89
-1
3.1 Print Results
Program Output
10 Nested control structures
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• Problem
– A college has a list of test results (1 = pass, 2 = fail) for
10 students
– Write a program that analyzes the results
• If more than 8 students pass, print "Raise Tuition"
• Notice that
– The program must process 10 test results
• Counter-controlled loop will be used
– Two counters can be used
• One for number of passes, one for number of fails
– Each test result is a number—either a 1 or a 2
• If the number is not a 1, we assume that it is a 2
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10 Nested control structures
• Top level outline
Analyze exam results and decide if tuition
should be raised
• First Refinement
Initialize variables
Input the ten quiz grades and count passes
and failures
Print a summary of the exam results and
decide if tuition should be raised
• Refine Initialize variables to
Initialize passes to zero
Initialize failures to zero
Initialize student counter to zero
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10 Nested control structures
• Refine Input the ten quiz grades and count
passes and failures to
While student counter is less than or equal to ten
Input the next exam result
If the student passed
Add one to passes
else
Add one to failures
Add one to student counter
• Refine Print a summary of the exam results
and decide if tuition should be raised to
Print the number of passes
Print the number of failures
If more than eight students passed
Print “Raise tuition”
1/*
2
Analysis of examination results */
3#include <stdio.h>
4
5int main()
6{
7
/* initializing variables in declarations */
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int passes = 0, failures = 0, student = 1, result;
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/* process 10 students; counter-controlled loop */
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while ( student <= 10 ) {
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printf( "Enter result ( 1=pass,2=fail ): " );
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scanf( "%d", &result );
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if ( result == 1 )
/* if/else nested in
16
passes = passes + 1;
while */
17
else
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failures = failures + 1;
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student = student + 1;
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}
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printf( "Passed %d\n", passes );
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printf( "Failed %d\n", failures );
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if ( passes > 8 )
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printf( "Raise tuition\n" );
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return 0;
/* successful termination */
30 }
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Outline
1. Initialize
variables
2. Input data
and count
passes/failures
3. Print results
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Outline
Enter Result
Enter Result
Enter Result
Enter Result
Enter Result
Enter Result
Enter Result
Enter Result
Enter Result
Enter Result
Passed 6
Failed 4
(1=pass,2=fail):
(1=pass,2=fail):
(1=pass,2=fail):
(1=pass,2=fail):
(1=pass,2=fail):
(1=pass,2=fail):
(1=pass,2=fail):
(1=pass,2=fail):
(1=pass,2=fail):
(1=pass,2=fail):
1
2
2
1
1
1
2
1
1
2
Program Output
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Assignment Operators
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• Assignment operators abbreviate
assignment expressions
c = c + 3;
can be abbreviated as c += 3; using the
addition assignment operator
• Statements of the form
variable = variable operator expression;
can be rewritten as
variable operator= expression;
• Examples of other assignment
operators:
d
e
f
g
-=
*=
/=
%=
4
5
3
9
(d
(e
(f
(g
=
=
=
=
d
e
f
g
*
/
%
4)
5)
3)
9)
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12
Increment and Decrement Operators
• Increment operator (++)
– Can be used instead of c+=1
• Decrement operator (--)
– Can be used instead of c-=1
• Preincrement
– Operator is used before the variable (++c or --c)
– Variable is changed before the expression it is in is evaluated
• Postincrement
– Operator is used after the variable (c++ or c--)
– Expression executes before the variable is changed
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12
Increment and Decrement Operators
• If c equals 5, then
printf( "%d", ++c );
– Prints 6
printf( "%d", c++ );
– Prints 5
– In either case, c now has the value of 6
• When variable not in an expression
– Preincrementing and postincrementing have the same effect
++c;
printf( “%d”, c );
– Has the same effect as
c++;
printf( “%d”, c );
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Exercise - 1
• Write pseudocode and draw a flowchart that
calculate the average of the 10 numbers.
• Input = ?
• Output = ?Read 10 integer numbers from the screen.
• Logic = ? Show the user the avarage of these 10 integer numbers.
1. Find the cumulative value of the numbers and
2. divide this number with the count of numbers.
Draw flow chart and write pseudocode.