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From Algorithms to Programs
• Algorithms are essentially abstract things
that can have
Lecture 13
– Linguistic realizations (in pseudocode or
programming languages)
– Hardware realizations (in digital circuitry)
Introduction to
High-Level Programming
(S&G, §§7.1
–7.6)
§§7.1–
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What Is a Programming
Language?
What Is a Program?
• Usually, one or more algorithms written in a
programming language that can be
translated to run on a real machine
• We sometimes call programs software
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• A bit like pseudocode, but with very strict
syntax rules
• Examples: Basic, Pascal, C++, Java
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The Program Development
Process (Data Flow)
From Algorithms to Hardware
Algorithm
Algorithm
Editor
Translate (by a human being)
Program
Program in machine’s language
A real computer
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Program in programming language
Compiler
Translate (by another program)
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Input
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A real computer
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Output
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The Program Development
Process (Control Flow)
Java
• Was developed at Sun Microsystems in the
early 1990s
• A general-purpose language but
Edit
Syntax errors
Compile
Input
Run
– Programs could run in small devices
(embedded systems)
– Internet-ready and Web-ready
– Standard support for multimedia applications
Output
Runtime errors
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Compiling and Running
a Java Program
Portability
• A Java compiler translates a Java program
to an equivalent program in byte code
• A Java virtual machine (JVM) is a program
that interprets a Java byte code program to
execute it
• A Java program translates to a standard byte
code
• A Java byte code can run on any JVM
• A JVM, and thus a Java program, can run
User inputs
Java code
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Compiler byte code
JVM
Program outputs
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– In a PDA
– In a desktop computer
– In a Web browser
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Example Java Statements
// This is a comment.
// Comments are not executable statements
// but exist solely for the benefit of human readers
Pseudocode:
Set the value of <identifier> to <arithmetic expr>
Java:
<identifier> = <arithmetic expr> ;
Examples:
i = i + 1;
force = mass * acceleration;
average = sum / n;
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Statements in Java: Assignment
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int width = 20;
// Declare an integer variable and set its
// value to 20
int height = 35;
int area;
// Declare an integer variable; its
// default value is 0
area = height * width;
// An assignment statement
// Pseudocode: set area to height * width
All variables must be declared before they are used in statements.
Each variable has a data type, such as int, double or String.
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Algorithms that use arrays, such
as Sequential Search
Basic Data Types
Type Name
Example Values
int
34, 0, 10000000
double
3.14,
0.563333333
String
"Java rules!"
char
'a', '9', '%'
boolean
true, false
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Given values for Name, N1, …, N 10 and for T1, …, T10
Set the value of i to 1 and set the value of Found to NO
Repeat until either Found = YES or i > 10
In pseudo code, we use
If Name is equal to Ni, then
subscripts to create a list …
Print Ti
Set the value of Found to YES
Else
… and to reference
Increment i
particular elements of it
If (Found = NO) then
Print “Sorry, but the name’s not in the directory”
Stop
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Statements in Java: Input
• Here are examples of creating arrays:
of 10 integers
a list of 2000
real numbers
a list of 40 strings
• Here are examples of referencing elements:
firstyear = years[0];
//
//
lastarea = area[1999]; //
//
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Translating to Java
int[ ] years = new int[10]; // a list
double[ ] area = new double[2000]; //
//
String[ ] name = new String[40];
//
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warning: array elements are
numbered starting at zero !
the last element in the
list of area’s
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Pseudocode:
Ask user to enter a value of <identifier>
Java:
<identifier> = Console.readInt(<prompt>);
<identifier> = Console.readDouble(<prompt>);
<identifier> = Console.readChar(<prompt>);
Examples:
int speed;
speed = Console.readInt ("Please enter" +
" the speed you traveled");
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Statements in Java: Output
Example Problem
Pseudocode:
Output the value of <identifier>
Write a program that computes the area of a circle.
Java w/ examples:
System.out.println("thanks for the data");
System.out.println("The time for your trip is " + time);
System.out.println("A trip of " + distance + " miles "
+ "at " + speed + " mph " + "requires " + time + " hours");
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From Problem to Algorithm
Pseudocode for Program
The algorithm
• uses area = πr2
• accepts as input a decimal point number
representing the radius of the circle
• computes and displays the area of the
corresponding circle
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input radius
area = π × radius × radius
output area
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Basic Computation
Complete Program
public class CircleArea
{
double radius, area;
radius = Console.readInt ("Radius?");
area = Math.PI * radius * radius;
System.out.println ("Area = " + area);
public static void main (String[] args)
{
radius = Console.readInt ("Radius?");
area = Math.PI * radius * radius;
System.out.println ("Area = " + area);
}
From pseudocode
From pseudocode
}
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Digression: What Does It All
Mean?
Java Methods
// A method is like a little program that performs a complex task
The language machine regulates and adjusts in advance the
mode of our possible usage of language through mechanical
energies and functions. The language machine is — and above
all, is still becoming — one way in which modern technology
controls the mode and the world of language as such.
Meanwhile, the impression is still maintained that man is the
master of the language machine. But the truth of the matter
might well be that the language machine takes language into its
management and thus masters the essence of the human being.
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double d = Math.sqrt(25);
// d is 5.0
int i = Math.abs(-4);
// i is 4
i = Math.pow(2, 10);
// i is 1024
System.out.println("Hello there!");
// Displays output in
// the terminal window
The data in the parentheses are called parameters.
— Martin Heidegger
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Methods receive information in parameters and can return
information to the rest of the program.
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Structure of Simple Program:
The Main Method
public class <name of program>
{
public static void main (String[] args)
{
<declarations>
<statements>
} //end of main method
}
//end of program
Java Control Structures
Conditionals
(if statements)
A Java method describes data and an algorithm as a chunk of
program code.
The main method is run when the program starts up.
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Syntax of Compound Statements
Syntax of Selection Statements
{
}
<statement 1>
<statement 2>
…
<statement n>
A compound statement is a sequence of zero or more
statements.
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if (<Boolean expression>)
<statement>
// One-way decision
if (<Boolean expression>)
<statement>
else
<statement>
// Two-way decision
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Semantics of
Selection Statements
if (<Boolean expression>)
<statement>
true
?
false
Java Control Structures
statement
if (<Boolean expression>)
<statement>
else
<statement>
?
true
statement
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Iteration/Loops
(while statements and for statements)
false
statement
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Example: Summation
Example: Summation
Write a program that computes the sum of the
numbers between 1 and 10.
Write a program that computes the sum of the
numbers between 1 and 10.
Set counter to 1
Set sum to 0
While counter ≤ 10 do
Set sum to sum + counter
Increment counter
Print sum
int counter = 1;
int sum = 0;
while (counter <= 10)
{
sum = sum + counter;
counter = counter + 1;
}
System.out.println (sum);
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Count-controlled Loops
// General form
<initialize a counter variable>
while (<test counter for termination condition>){
<do something>
<change the value of counter>
}
while (<Boolean expression>)
<statement>
…
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Syntax and Semantics of
while Statements
while (<Boolean expression>)
{
<statement 1>
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true
?
false
statement
<statement n>
}
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Count-controlled Loops
Count-controlled Loops
// General form
<initialize a counter variable>
while (<test counter for termination condition>){
<do something>
<change the value of counter>
}
// General form
<initialize a counter variable>
while (<test counter for termination condition>){
<do something>
<change the value of counter>
}
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// Count up
<initialize a counter variable>
while (<counter is less than a limit value>){
<do something>
<increase the value of counter>
}
// Count up
<initialize a counter variable>
while (<counter is less than a limit value>){
<do something>
<increase the value of counter>
}
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int counter = 1;
while (counter <= 10){
<do something>
counter = counter + 1;
}
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Count-controlled Loops
Count-controlled Loops
// General form
<initialize a counter variable>
while (<test counter for termination condition>){
<do something>
<change the value of counter>
}
// General form
<initialize a counter variable>
while (<test counter for termination condition>){
<do something>
<change the value of counter>
}
// Count down
<initialize a counter variable>
while (<counter is greater than a limit value>){
<do something>
<decrease the value of counter>
}
// Count down
<initialize a counter variable>
while (<counter is greater than a limit value>){
<do something>
<decrease the value of counter>
}
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Increment and Decrement
Increment and Decrement
int counter = 1;
while (counter <= 10){
<do something>
counter = counter + 1;
}
int counter = 1;
while (counter <= 10){
<do something>
counter++;
}
int counter = 10;
while (counter > 0){
<do something>
counter = counter - 1;
}
int counter = 10;
while (counter > 0){
<do something>
counter--;
}
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Designing Correct Loops
Off-by-One Error
int counter = 1;
while (counter <= 10){
<do something>
counter++;
}
– Plan how many iterations, then set the counter
and the limit accordingly
int counter = 1;
while (counter < 10){
<do something>
counter++;
}
• Check the logic of the termination condition
• Update the loop control variable properly
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• Initialize all variables properly
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int counter = 10;
while (counter > 0){
<do something>
counter = counter - 1;
}
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40
// Executes 10 passes
// Executes 9 passes
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The for Loop
Infinite Loop
int counter = 1;
while (counter <= 10){
<do something>
counter = counter + 2;
}
int counter = 1;
while (counter <= 10){
<do something>
counter++;
}
// Executes 5 passes
for (int counter = 1; counter <= 10; counter++)
<do something>
int counter = 1;
while (counter != 10){
<do something>
counter = counter + 2;
}
// Runs forever
In general, avoid using != in loop termination conditions.
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The for Loop
Syntax and Semantics of
the for Loop
int counter = 10;
while (counter > 0){
<do something>
counter--;
}
for (<initializer>; <termination>; <update>)
<statement>
for (int counter = 10; counter > 0; counter--)
<do something>
false
initializer
termination
true
statement
update
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The Life Cycle of Programs:
Waterfall Model
Maintenance
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Analysis
-----------Verify
• Programs might have a lifetime of 5-20
years
• Requirements might change, and minor or
major changes would be necessary
Design
-----------Verify
Implementation
------------------Test
Integration
------------Test
Maintenance
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The Relative Costs of the
Phases
The Cost of Correcting an Error
Cost of
Correcting
a Fault
implementation
maintenance
analysis
integration
design
Software Development Phase
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