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CS 106A, Lecture 1 Introduction to Programming with Karel the Robot reading: Karel the Robot Learns Java, Chapters 1-2

This document is copyright (C) Stanford Computer Science and Marty Stepp, licensed under Creative Commons Attribution 2.5 License. All rights reserved. Based on slides created by Keith Schwarz, Mehran Sahami, Eric Roberts, Stuart Reges, and others.

What is programming? • program: A set of instructions to be carried out by a computer. • program execution: The act of carrying out the instructions contained in a program. • programming language: A systematic set of rules used to describe computations in a format that is editable by humans.

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Programming languages • procedural languages: programs are a series of commands – Pascal (1970): designed for education – C (1972):

low-level operating systems and devices

• functional programming: functions map inputs to outputs – Lisp (1958) / Scheme (1975), ML (1973), Haskell (1990)

• object-oriented languages: programs use interacting "objects" – Smalltalk (1980): first major object-oriented language – C++ (1985):

"object-oriented" improvements to C

• successful in industry; used to build OSes such as Windows

– Java (1995):

designed for embedded systems, web apps

• Runs on many platforms (Windows, Mac, Linux, cell phones...) • The language taught in this course and our textbook

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Karel the Robot • Karel the Robot is a simple programming environment for a simulated robot that lives in a 2-D world. – created in 1970s by Rich Pattis at Stanford – named for Karel Čapek ("Chopek") who was the author of a Czech play called R.U.R. (Rossum's Universal Robots) in 1921 – the word "robot" comes from this play (actually Karel's brother Josef)

• Karel was extremely influential on CS education and has been translated to many programming languages. – We will use Karel to learn basic programming concepts and Java commands. 4

Karel's World 3 2 1

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Streets (rows) 3 2 1

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Each row is called a street.

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Avenues (columns) 3 2 1

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Each column is called an avenue. 7

Corners (locations) 3 2 1

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The intersection of a street and an avenue is a corner.

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Walls 3 2 1

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Karel cannot move through walls.

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Beepers 3 2 1

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Beepers mark locations in Karel's world. Karel can pick them up and put them down.

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Karel commands: move Karel Commands 3 2 1

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move

– Every time move executes, Karel moves forward one square in the direction he is facing. 11

Commands: turnLeft Karel Commands 3 2 1

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turnLeft move

– Every time turnLeft executes, Karel rotates 90° counter-clockwise. – There is no turnRight command. (Why not?) 12

Commands: pickBeeper Karel Commands 3 2 1

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pickBeeper

– When pickBeeper executes, if there is a beeper under Karel's current corner position, it causes Karel to "pick up" that beeper. • Karel can hold multiple beepers at a time in his "beeper bag". 13

Commands: putBeeper Karel Commands 3 2 1

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putBeeper turnLeft move

– When putBeeper executes, if Karel is holding a beeper, it causes Karel to set a beeper down in his current location. • pickBeeper and putBeeper are used to move beepers around. 14

A Karel program import stanford.karel.*;

You must give your program a name. It will be stored in the file Name.java.

public class Name extends Karel { public void run() { command(); You write a series of commands to issue to Karel here. command(); ... Each command is followed by a pair of command(); parentheses and a semicolon. } }

– In syntax templates like these, parts in bold italic should be replaced by your own code; other parts must be written as-is. 15

Example program import stanford.karel.*; public class MyFirstKarel extends Karel { public void run() { move(); move(); turnLeft(); move(); pickBeeper(); turnLeft(); move(); putBeeper(); } } 16

Program structure class: a program public class Name extends Karel { public void run() { statement; statement; method: a named group of statements ... statement; } statement: a command to be executed }

• Every executable Java/Karel program consists of a class, – that contains a method named run, • that contains the statements (commands) to be executed. 17

Compiling/running 1.

Write it. –

code or source code: The set of instructions in a program.

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Compile it. − compile: Translate a program from one language to another. (Eclipse does this automatically when you save your file.) – byte code: The Java compiler converts your code into a format named byte code that runs on many computer types.

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Run (execute) it. – output: The behavior shown to the by a program. output

byte code

source code

compile

run

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Names and identifiers • You must give your program (class) a name. public class KarelIsGreat extends Karel { – Naming convention: capitalize each word (e.g. MyClassName) – Your program's file must match exactly (MyClassName.java) • includes capitalization (Java is "case-sensitive")

• identifier: A name given to an item in your program. – must start with a letter or _ or $ – subsequent characters can be any of those or a number • legal: • illegal:

_myName me+u

TheCure 49ers

ANSWER_IS_42 side-swipe

$bling$ Ph.D's 19

Keywords • keyword: An identifier that you cannot use because it already has a reserved meaning in Java. – A list of all Java keywords: abstract boolean break byte case catch char class const continue

default do double else extends final finally float for goto

if implements import instanceof int interface long native new package

private protected public return short static strictfp super switch synchronized

this throw throws transient try void volatile while

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Syntax • syntax: The set of legal structures and commands that can be used in a particular language. – Every basic Java statement ends with a semicolon ; – The contents of a class or method occur between { and }

• syntax error (compiler error): A problem in the structure of a program that causes the compiler to fail. – – – –

Missing semicolon Too many or too few { } braces Illegal identifier for class name Class and file names do not match ... 21

Syntax errors 1 2 3 4 5

public class Hello extends Karel { pooblic void run() { turnleft()_ } }

• Compiler output: Hello.java:2: expected pooblic void run() { ^ Hello.java:3: The method turnleft() is undefined turnleft() ^ Hello.java:4: ';' expected } ^ 3 errors – The compiler shows the line number where it found the error.

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Exercise: MeetKarel

MeetKarel

• Write a program named MeetKarel with the following behavior: – Karel begins at (1, 1) facing east. There is a beeper at (1, 2). – Deliver the beeper to (2, 4). – Park Karel at (2, 5) facing east.

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Exercise solution import stanford.karel.*; public class MeetKarel extends Karel { public void run() { move(); pickBeeper(); move(); turnLeft(); move(); turnLeft(); turnLeft(); turnLeft(); move(); putBeeper(); move(); } } 24

Comments • comment: A note written in source code by the programmer to describe or clarify the code. – Comments are not executed when your program runs.

• Syntax: // comment text, on one line or, /* comment text; may span multiple lines */

• Examples: // This is a one-line comment. /* This is a very long multi-line comment. */ 25

Solution w/ comments /* This is a simple Karel program designed to show off some * of the basic features of Karel the Robot. * It moves Karel from (1,1) to (2,5), and also transports * a beeper from (1,2) to (2,4) along the way. */ import stanford.karel.*; public class MeetKarel extends Karel { public void run() { move(); pickBeeper(); // grab beeper at move(); turnLeft(); move(); // right turn = 3 turnLeft(); turnLeft(); turnLeft(); move(); putBeeper(); // drop beeper at move(); // park Karel at } }

(1,2)

left turns

(2,4) (2,5) 26

More practice

ThreeBeepers

• Write a program named ThreeBeepers : – – – –

Karel begins at (1, 1) facing east. Beepers are at (2, 3), (6, 2), and (7, 2). Pick up the other two beepers and place them all at (6, 2). Leave Karel parked at (6, 2) on the three beepers.

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