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Gentle Introduction to Programming
Assaf ZaritskySummer 2009
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Administration
• September 6-17• Odd week days 9:00 – 13:30001דן דויד •
• No grading• Web: http://www.cs.tau.ac.il/summer2009/
Check what’s new!• My email: [email protected]
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Course Description
This course will provide a gentle introduction to programming
using Scala for highly motivated students with little or no prior experience in programming
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Objective
Bridge the gap for students without prior programming knowledge
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Course Description
Lectures will be interactive featuring in-class exercises with lots of support
You are expected to work hard!
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Course PlanSession Material
1 Basic concepts in CS and programming, basic Scala
2 Basic Scala (cont.), Functions
3 Recursion
4 Arrays, sorting algorithms
5 Lists
6 Object oriented programming
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Course Plan – More Admin.
Faculty guests lectures
Today: pretty intense, from Tuesday – (maybe) more relaxed
Lab sessions – today!14:30-15:3015:30-16:30
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Questions?
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Agenda for Today
• Administration• General introduction• The Scala programming language:
• Variables and Types• Operators• Branching and Repetition
• Compiler, Interpreter• Working environment• Home work
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Basic Terms
• Computer Science• Computer• Hardware / Software• Algorithm• Input / Output• Pseudo-Code• Programming Language• Computer Program
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Computer Science
ענף מדעי העוסק בלימוד הבסיס התיאורטי למידע ולחישוביות, והחלתם על מערכות
מחשב
מדעי המחשב אינם עוסקים במחשב יותר משאסטרונומיה עוסקת בטלסקופ", דייקסטרה"
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Computer
מכונה המעבדת נתונים על פי רצף פקודות נתון מראש
מחשב = חומרה + תכנה
מעבד אמצעי פלט
אמצעי קלט
זכרון
עכבר, מקלדת, )(דיסק קשיח
מדפסת, מסך, )(דיסק קשיח
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Algorithm
סדרת פעולות סופית לביצוע משימה מסויימת
Algorithm
Input Output
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לביצוע משימה סופיתסופיתסדרת פעולות אלגוריתם: מסוימת
תרשים זרימה
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Pseudo Code
תיאור מצומצם ולא רשמי לאלגוריתם. מיועד לקריאה של בני אדם
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Example
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Programming Language
Machine-readable artificial language designed to express computations that
can be performed by a computer
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Machine Code (Language)
• Computers understand only machine language• Basically looks like a sequence of 1’s and 0’s• Very inconvenient to work with and non intuitive
• All other computer languages were created for human convenience• The computer does not understand C/C#/Java/Scala• Must be “translated” into machine language
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There are Many Programming Languages
Over 500 different computer languages are listed by Wikipedia
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Language Selection
• Goal• Runtime vs. Development time• Operating systems• Platforms
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Computer Program
מימוש של אלגוריתם באמצעות שפת תכנות
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Computer Program(more technically)
• A sequence of processor instructions designed to achieve a specific purpose
• The instructions are executed sequentially. No instruction is executed before the previous has been completed
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Agenda for Today
• Administration• General introduction• The Scala programming language:
• Variables and Types• Operators• Branching and Repetition
• Compiler, Interpreter• Working environment• Home work
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Scala History
• Created by Martin Odersky at EPFL• Released Jan 2004• Twitter lately switched large portions of their
code to Scala (and intend to convert the rest)• Over 4000 downloads per month
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Why Scala?
• Semester A: Scheme• Semester B: Java• Scala language has some features similar to
Scheme and some to Java• Scala is cool!
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My First Scala Program: Hello World!
Hello.scala
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Memory
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Memory (Cont.)
• The computer memory is composed of a long list of bits
• Bits are grouped into bytes and words
• Every byte is numbered sequentially
• This number is called an address
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Variables and Types
): יחידת מידע המאכסנת ערך variable (משתנה•במהלך ריצת התכנית
ניתן ל"גשת" למשתנה (לשנות או לקבל את ערכו)•ניתן לבצע פעולות חשבוניות (ואחרות) בעזרת •
משתניםשקובע אילו סוגי )type (טיפוסלכל משתנה מוגדר •
ערכים המשתנה יכול להכיל (דוגמאות: מספר שלם, מספר ממשי, תו)
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Define Variables • Use var to declare variables:
var x : Int = 3
var y : Int = 5
var z : Int = x + y
var s1 : String = “Hello World”
define variable
variable name
type valueassignment operator
Variable x “is of type” Int and is assigned the value 3
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Why do We Need Variables?
• Computer programs manipulate data• Data is given as input or calculated throughout
the program• To be later accessed, variables must be
remembered• Thus, variables are stored in the memory• Variable name memory address
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Types
טיפוס של משתנה קובע:• אילו ערכים יכול המשתנה להכיל• מהן הפעולות שניתן לבצע על המשתנה•
שדורש (יחידה בסיסית של זכרון המחשב)מספר הבתים •אחסון של משתנה בזכרון תלוי בטיפוסו
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Some Basic Types
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Why Do We Need Different Types?
• Saving memory
• Execution speed
• Makes compiler “life” easier
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Define Variables • Int:
var x : Int = 5
• Double:var pi : Double = 3.14
• Boolean:var term : Boolean = true
• Char:var c : Char = ‘A’
• String:var st : String = “I want a break!”
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Arithmetic Operators
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Example 1 var a : Int = 3var b: Int = 5var c : Int = a + bprintln(c)c = c * 2println(c)var first : Int = (a + b) * 2var second : Int = a + b * 2println(first)println(second)
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Example 2 // Demonstrate strings additionvar s1 : String = “He”var s2 : String = “llo”var s3 : String = s1 + s2println(s3)var s4 : String = s3 + “ World ”var c : Char = ‘!’println(s4 + 2009 + c)
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Example 3 // Reverse a 3-digits numberprintln(“Enter a 3-digit number”)var num : Int = Console.readInt() // read a number from the user
var ones : Int = num % 10var tens : Int = (num % 100) / 10var hundreds : Int = num / 100
var reverseNum : Int = (ones * 100) + (tens * 10) + hundreds println(“the reverse number is “ + reverseNum)
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Rational Operators
Operator
Name Description
x < y Less than true if x is less than y, otherwise false.
x > y Greater than true if x is greater than y, otherwise false.
x <= y Less than or equal to
true if x is less than or equal to y, otherwise false.
x >= y Greater than or equal to
true if x is greater than or equal to y, otherwise false.
x == y Equal true if x equals y, otherwise false.
x != y Not Equal true if x is not equal to y, otherwise false.
Compares two numbers and returns a Boolean
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Example // Demonstrate rational operatorsprintln(“Enter the first number”)var x : Int = Console.readInt()println(“Enter the second number”)var y : Int = Console.readInt()
println(“x < y is “ + (x < y))println(“x > y is “ + (x > y))println(“x <= y is “ + (x <= y))println(“x >= y is “ + (x >= y))println(“x == y is “ + (x == y))println(“x != y is “ + (x != y))
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Logical Operators Operates on two Booleans and returns a Boolean
Operator
Name Description
x && y And True if both x and y are true, otherwise false.
x || y OrTrue if at least one of x or y are true, otherwise false.
! X Not True if x is false, otherwise false.
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&&, ||, !=
0 1
0 0 0
1 0 1
0 1
0 0 1
1 1 1
0 1
1 0
&&
||!
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Example
// Demonstrate logical operators println(“Enter the first number”)var x : Int = Console.readInt()println(“Enter the second number”)var y : Int = Console.readInt()
println(“(x < 10) && (y < 10) is “ + ((x < 10) && (y < 10)))println(“(x < 10) || (y < 10) is “ + ((x < 10) || (y < 10)))var state : Boolean = ((x < 10) || (y < 10) )println(“state is “ + state)
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All Examples at BasicExamples.scala
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Flow Control
• Usually a program is executed line after line in order
• It is reasonable to expect different execution orders on different inputs• Computer games• Illegal input
• Control structures• if-else• for loop• while loop
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Conditional Statement: if
• Used to execute conditionally a statement (or block of code)
• Syntax: if (expression) statement
• If expression is true, statement is executed
• statement can be replaced by a block of statements, enclosed in curly braces
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if-else Statement
if (expression) statement1
else statement2
• if expression is true, statement1 is executed. • if expression is false, statement2 is executed • both statements can be (and very often are) replaced
by blocks of statements (“compound statements”)
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Example
// Demonstrate logical operators println(“Enter the first number”)var a : Int = Console.readInt()println(“Enter the second number”)var b : Int = Console.readInt()println(“Enter the third number”)var c : Int = Console.readInt()
if ((a + b <= c) || (a + c <= b) || (b + c <= a))println(“There is no triangle with these sides”)
elseprintln(“There is a triangle with these sides”)
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Example
if (price > 100)
if (price < 200)
println(“reasonable price”)
if (price > 100 && price < 200)
println(“reasonable price”)
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Loops
• Used to repeat the same instructions until a stop criterion is met
• for, while loops:• while (<condition>)
<statement>• for (<range and update>)
<statement>
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Example - while
// factorialprintln(“Enter a non-negative number”)var n : Int = Console.readInt()
var fact : Int = 1var i : Int = 1
while (i <= n) {fact = fact * ii = i + 1
}println(n + “! = “ + fact)
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Example - for
// factorialprintln(“Enter a non-negative number”)var n : Int = Console.readInt()
var fact : Int = 1
for (i <- 1 to n) {fact = fact * i
}println(“n! = “ + fact)
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Examples at Factorial.scala
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Example - Prime
// Find whether a number is a primeprintln(“Enter a non-negative number”)var n : Int = Console.readInt()
var j : Int = 2var stop : Boolean = falsewhile ((j <= n/2) && !stop) {
stop = (n%j == 0) j = j + 1
}if (!stop) {
println(n)} Prime.scala
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When to use for/while?
• Some applications are more natural to for, and others to while
• for is more suited when something is performed a predefined number of times, or when iterating over a list of objects
• while is more suited if the number of iterations is not known in advance (e.g., asking for legal input from a user)
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Infinite Loops
• What are they?
• Beware of them
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Questions
• How do I write…?
• Is it legal to…?
• What happens if I write…?
Try it yourself!
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Agenda for Today
• Administration• General introduction• The Scala programming language:
• Variables and Types• Operators• Branching and Repetition
• Compiler, Interpreter• Working environment• Home work
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Compiler
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Interpreter
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Interpreter
• The Read/Evaluate/Print Loop• Read an expression• Compute its value• Print the result• Repeat the above
• The Environment
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Compiler = Clerk"ראש קטן"
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Runtime
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Hello World ;-)
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Agenda for Today
• Administration• General introduction• The Scala programming language:
• Variables and Types• Operators• Branching and Repetition
• Compiler, Interpreter• Working environment• Home work
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Working Environment
• Interpreter vs. scripts vs. Eclipse
• Home vs. labs
VS.
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Setting Environment @ Home • Download Java (www.java.com/getjava/ ) • Download Scala (http://www.scala-lang.org/downloads )
• Download Eclipse (http://www.eclipse.org/downloads/ )• Set Scala plugin to Eclipse (http://www.scala-lang.org/node/94 )
• Start programming
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Download Scala http://www.scala-lang.org/downloads
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Download Scala
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Use the Interpreter Type Scala from the command prompt (linux) or using “run” (windows)
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Download Eclipse
http://www.eclipse.org/downloads/
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Set Scala Plugin to Eclipse
http://www.scala-lang.org/node/94
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Set Scala Plugin to Eclipse (new)
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Set Scala Plugin to Eclipse (new)
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Open Eclipse – Set Workspace
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Open Eclipse
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Define New Project
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If you don’t find the Scala new project…
File New Project
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New Object
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Write Some Code
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Run
output
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Agenda for Today
• Administration• General introduction• The Scala programming language:
• Variables and Types• Operators• Branching and Repetition
• Compiler, Interpreter• Working environment• Home work
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Exercise 0
• Write your first “Hello World!” program in Scala:• Use the interpreter• In Eclipse
• Make sure it compiles and executes properly
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Fibonacci
• Fibonacci series0, 1, 1, 2, 3, 5, 8, 13, 21, 34• Definition:
• fib(0) = 1• fib(1) = 1• fib(n) = fib(n-1) + fib(n-2)
en.wikipedia.org/wiki/Fibonacci_number
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סלט פיבונאצ'י
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Exercise 1
• Write a program that receives from the user an integer n > 0 and prints to the screen the n th Fibonacci number
a. Use a “for” loop
b. Use a “while” loop
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Exercise 2
• Write a program that receives from the user two integers 0 < n1 < n2 and prints to the screen all prime numbers between n1 and n2
• Divide the problem to smaller sub-problems:• How would you traverse over all the numbers
between n1 and n2?• How would you be sure that a given number is a
prime?
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Want More Exercises?
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Exercise – Integer Division
• Input:• Two integers – A and B
• Output:• How many times A contains B (it is the result of
the integer division A/B)
• Do not use the operators ‘/’, ‘*’
• Solution:
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Exercise – Power of Two
• Input: integer A
• Output: is there an integer N such that A == 2^N?
• Solution:
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Write a program that prints an upside-down half triangle of *.
The height of the pyramid is the input.
*****
*****
****
*
Exercise – Triangle Printout
