What is computer science? WELCOME TO CSE 142! computers? science? programming? late lonely nights in front of the computer? ALGORITHMIC THINKING host: benson limketkai al·go·rithm: a step-by-step procedure for solving a problem or accomplishing some end especially by a computer University of Washington, Summer 2007 How does that relate to programming? 1 2 Just like Legos… 3 How to do well in this course Keep up with the assignments The course material is cumulative From a former student: “Procrastination will eventually come around to bite you in the ass!” If you don’t understand something, ask questions (especially “WHY?”). “There’s no such thing as a dumb question.” 6 1 Your first Java program! Basic Java programs Java is a programming language . public class Hello { public static void main(String[] args) { System.out.println("Hello, world!"); } Readings: 1.1 – 1.3 } What does this code output (print to the user) when you run (execute) it? 7 8 Running a program Program execution Before you run a program, you must compile it. The output is printed to the console . compiler: Translates a computer program Some editors pop up the console as another written in one language (i.e., Java) to another window. language (i.e., byte code) compile execute source code byte code output (Hello.java ) (Hello.class ) 9 10 Another Java program Writing your own Java programs public class Hello2 { public class <name> { public static void main(String[] args) { public static void main(String[] args) { System.out.println("Hello, world!"); <statement>; System.out.println(); <statement>; System.out.println("This program produces"); … System.out.println("four lines of output"); <statement>; } } } } Every executable Java program consists of a class that contains a method called main that contains the statements (commands) to be executed 11 12 2 Syntax Syntax Errors syntax: The set of legal structures and syntax error: A problem in the structure of a commands that can be used. program. 1 public class Hello { Examples: 2 pooblic static void main(String[] args) { 3 System.owt.println("Hello, world!") Every basic statement ends with a semi-colon. 4 } The contents of a class occur between curly 5 } compiler output: braces. 2 errors found: File: Hello.java [line: 2] Error: Hello.java:2: <identifier> expected File: Hello.java [line: 3] Error: Hello.java:3: ';' expected 13 14 Finding syntax errors First lesson in computer science Error messages do not always help us Computers are stupid. understand what is wrong: Computers can’t read minds. File: Hello.java [line: 2] Computers don’t make mistakes. Error: Hello.java:2: <identifier> expected If the computer is not doing what you want, poo blic static void main(String[] args) { it’s because YOU made a mistake. Why can’t the computer just say “ You misspelled ‘public’ ”? 15 16 More on syntax errors System.out.println Java is case-sensitive System.out.println : A statement to print a line Hello and hello are not the same of output to the console. pronounced “ print-linn ” 1 Public class Hello { 2 public static void main(String[] args) { Two ways to use System.out.println : 3 System.out.println("Hello, world!"); System.out.println("<message> "); 4 } Prints the given message as a line of text to the console. 5 } compiler output: 1 error found: System.out.println(); File: Hello.java [line: 1] Error: Hello.java:1: class, interface, or enum Prints a blank line to the console. expected 17 18 3 Strings Details about strings string: A sequence of text characters. A string may not span across multiple lines. "This is not Start and end with quotation mark characters a legal string." Examples: A string may not contain a “ character. "hello" The ‘ character is okay. "This is a string" "This is not a "legal" string either." "This, too, is a string. It can be very long!" "This is 'okay' though." This begs the question… 19 20 Escape sequences Questions A string can represent certain special characters by preceding them What is the output of each of the following println statements? with a backslash \ (this is called an escape sequence ). \t tab character System.out.println("\ta\tb\tc"); \n newline character \" quotation mark character System.out.println("\\\\"); System.out.println("'"); Example: System.out.println("\"\"\""); System.out.println("Hello!\nHow are \"you\"?"); System.out.println("C:\nin\the downward spiral"); Output: Hello! Write a println statement to produce the following line of How are "you"? output: This begs another question… / \ // \\ /// \\\ 21 22 Questions What println statements will generate the following output? This program prints a Procedural decomposition quote from the Gettysburg Address. "Four score and seven years ago, using static methods our 'fore fathers' brought forth on this continent a new nation." What println statements will generate the following output? A "quoted" String is Readings: 1.4 – 1.5 'much' better if you learn the rules of "escape sequences." Also, "" represents an empty String. Don't forget to use \" instead of " ! '' is not the same as " 23 24 4 Algorithms The “Bake sugar cookies” algorithm Recall: An algorithm is a list of steps for Mix the dry ingredients. solving a problem. Cream the butter and sugar. Beat in the eggs. Stir in the dry ingredients. Set the oven for the appropriate temperature. What is the algorithm to bake sugar cookies? Set the timer. Place the cookies into the oven. Allow the cookies to bake. Mix the ingredients for the frosting. Spread frosting and sprinkles onto the cookies. 25 26 Structured algorithm How do we bake a double batch? 1. Make the cookie batter. Observation : Structured Unstructured: Structured: Mix the dry ingredients. algorithms are easier to Mix the dry ingredients. 1. Make the cookie batter. Cream the butter and sugar. Cream the butter and sugar. 2a. Bake the first batch of cookies. Beat in the eggs. understand. Beat in the eggs. 2b. Bake the second batch of Stir in the dry ingredients. cookies. Stir in the dry ingredients. 3. Add frosting and sprinkles. 2. Bake cookies. Set the oven … Set the oven for the appropriate Set the timer. temperature. Place the cookies into the oven. Observation : Structured Set the timer. Allow the cookies to bake. Place the cookies into the oven. algorithms eliminate Set the oven … Allow the cookies to bake. redundancy. Set the timer. 3. Add frosting and sprinkles. Place the cookies into the oven. Mix the ingredients for the frosting. Allow the cookies to bake. Spread frosting and sprinkles onto the cookies. Mix the ingredients for the frosting. Spread frosting and sprinkles onto the cookies. 27 28 Redundancy in programs How do we structure the program? public class FraggleRock { public static void main(String[] args) { static method : A group of statements given System.out.println("Dance your cares away,"); a name. System.out.println("Worry's for another day."); System.out.println("Let the music play,"); System.out.println("Down at Fraggle Rock."); System.out.println(); To use a static method: System.out.println("Dance your cares away,"); System.out.println("Worry's for another day."); 1. declare it (write down the recipe) System.out.println("Let the music play,"); System.out.println("Down at Fraggle Rock."); Write a group of statements and give it a name. } 2. call it (cook using the recipe) } Tell our program to execute the method. 29 30 5 Declaring a static method Calling a static method The syntax for declaring a static method (writing The syntax for calling a static method down the recipe): (cooking using the recipe): public class <class name> { ... <method name> (); public static void <method name> () { <statement>; <statement>; … <statement>; } } 31 32 Static method example Worry’s for another day! Declaring a static method public class FraggleRock { public static void printAffirmation() { public static void main(String[] args) { System.out.println("I am good enough!"); singChorus(); System.out.println("I am smart enough!"); System.out.println(); System.out.println("People like me!"); singChorus(); } } Calling a static method (possibly multiple times) printAffirmation(); public static void singChorus() { printAffirmation(); System.out.println("Dance your cares away,"); Output System.out.println("Worry's for another day."); I am good enough! System.out.println("Let the music play,"); I am smart enough! System.out.println("Down at Fraggle Rock."); People like me! } I am good enough! } I am smart enough! People like me! 33 34 Methods calling methods Control flow of methods One static method can call another: Output: public class MethodsExample { When a method is called, the execution public static void main(String[] args) { This is message1. message1(); This is message2. "jumps" into that method message2(); This is message1. Done with message2. executes all of the method’s statements System.out.println("Done with main."); Done with main. } "jumps" back to the statement after the method call public static void message1() { System.out.println("This is message1."); } public static void message2() { System.out.println("This is message2."); message1(); System.out.println("Done with message2."); } } 35 36 6 Output: Control flow of methods This is message1. Summary: To use or not to use… This is message2. This is message1. Done with message2. public class MethodsExample { Done with main. public static void main(String[] args) { Yes message1(); Statements that are related to each other ( structure ). Statements that are repeated ( redundancy ). public static void message1() { message2(); System.out.println("This is message1."); } No public static void message2() { ... System.out.println("This is message2"); Individual statements occurring only once and not related } message1(); to other statements } System.out.println("Done with message2."); Unrelated or weakly-related statements } Consider splitting the method into two smaller methods. Blank lines public static void message1() { System.out.println("This is message1."); Blank println statements can go in the main method.