CSCI 51 Introduction to Programming Dr. Joshua Stough January 27, 2009.

CSCI 51Introduction to Programming

Dr. Joshua StoughJanuary 27, 2009

Announcements

•Problem Set 1 due Monday.•Building access request

PS 1

• It may well serve you better to type the code yourself, even if it’s just copying code from another program.– That way, you make mistakes that

you know how to fix, and hopefully fewer mistakes later.

Last Time in CSCI 51

• Parts of a Computer• Binary Numbers• Algorithms

• PS 1 assigned– due Monday, Feb 2

Review, Hardware vs. Software

A computer is made up of hardware and software

Hardware Software• CPU

- ex: 1.8 GHz Core 2 Duo• input/output

- keyboard- 15.4” wide screen LCD- network card

• main memory- ex: 2 GB RAM

• secondary memory- ex: 160 GB hard drive

• operating systems- Windows XP- Mac OS X

• applications- games- Microsoft Word- Mozilla Firefox

ReviewHardware Organization

motherboard

CPU

memory

hard drive

ReviewMain Memory (with 100 cells)

Each memory cell has a numeric address, which uniquely identifies it

QuestionsBinary Numbers

• What’s the maximum value a 6-bit number can represent?

• What’s the decimal representation of 111010?

• What’s the binary representation of 35?

63

58 = 32+16+8+2

100011

KB 210 = 1024

MB 220 (over 1 million)

GB 230 (over 1 billion)

TB 240 (over 1 trillion)

Unit Symbol Number of Bytes

kilobyte

megabyte

gigabyte

terabyte

Storage Capacity

• Every memory device has a storage capacity, indicating the number of bytes (8 bits) it can hold

• Various units:

Questions

1. - computer components including the CPU, main memory, I/O devices, and secondary storage

2. - the brain of the computer, containing the PC, IR, ALU, and ACC

3. - computer instructions to solve a problem

4. The digits 0 and 1 are calledor the shortened term

From Java to Machine Language• Computers understand only 0 and 1 (machine

language)• Compiler translates source code into machine

code

• Java compiler translates source code (file ending in .java) into bytecode (file ending in .class)– bytecode is portable (not machine-specific)

• Java interpreter reads and executes bytecode– different Java interpreters for different types of CPUs

and operating systems (OS)• Intel/Windows, Motorola/Mac OS X, Intel/Linux

Problem Solving• The purpose of writing a program is to have the

computer solve a problem

• But:– “Computers do not solve problems; they

implement solutions.” – Laurent Gasser

• The general steps in problem solving are:– understand the problem– dissect the problem into manageable pieces– design a solution– consider alternatives to the solution and refine it– implement the solution– test the solution and fix any problems that exist

Algorithm

• Sequence of instructions used to carry out a task or solve a problem

• May be written in either English or pseudocode– outline of a program that could be

translated into actual code

• May need refinement as you work

Always write/think out your algorithm before you begin programming

Problem-Analysis-Coding-Execution

most important stepwithoutcomputer

withcomputer

Algorithm Design Example

Problem: Convert change in cents to number of half-dollars, quarters, dimes, nickels, and pennies to be returned.

Example: – given 646 cents– number of half-dollars: divide 646 by 50

• quotient is 12 (number of half-dollars)• remainder is 46 (change left over)

– number of quarters: divide 46 by 25• quotient is 1 (number of quarters)• remainder is 21 (change left over)

– number of dimes, nickels, pennies– result: 12 half-dollars, 1 quarter, 2 dimes, 0

nickels, 1 penny

Resulting Algorithm

1. Get the change in cents2. Find the number of half-dollars3. Calculate the remaining change4. Find the number of quarters5. Calculate the remaining change6. Find the number of dimes7. Calculate the remaining change8. Find the number of nickels9. Calculate the remaining change10.The remaining change is the number of

pennies.

ReviewBinary Numbers

• N bits to represent 2N values• N bits represent values 0 to 2N-1• Example: 5 bits

– 32 unique values (0-31)– 00000 = 0– 11111 = 31

24 23 22 21 20

16 + 8 + 4 + 2 + 1

ReviewProblem Solving• Steps:

– understand the problem• how would you solve the problem (as a human,

without a computer)

– dissect the problem into manageable pieces– design a solution– consider alternatives to the solution and

refine it– implement the solution– test the solution and fix any problems that

exist

Review Questions

1. What is the maximum decimal value that a 4-bit binary number can represent?

2. What is the binary number for 17?

3. Before the CPU can execute instructions, they must first be loaded from

4. What is the first and most important step in writing a program?

15

10001

main memory

analysis, or understanding the problem

Today in CSCI 51

• Basic elements of a Java program – special symbols– identifiers– data types– operators– expressions– Strings

• Textbook Ref: Ch 2 and 3

Reading Check-Up

1. The rules of a language determine which instructions are valid.

2. True or False? Hello! is an example of a legal Java identifier.

3. If an operator has an integer and a floating-point operand, the result of the operation is a number.

4. The expression (int) (9.2) evaluates to

syntax

False

floating-point

9

Introduction

• Computer program: a sequence of statements to accomplish a task

• Programming: process of planning and creating a program

• Programming language: a set of rules, symbols, and special words

Sample Java Program

public class Hello {public static void main (String[] args){

System.out.println ("Hi There!");}

}

Upon execution, this program displaysHi There!

Programming Languages• Programming languages have rules of

grammar just as English does

• syntax rules - which statements are legal and which are not

• semantic rules - determine the meaning of the instructions

• token - smallest individual unit of a program– special symbols– word symbols– identifiers

Special Symbols

+ - * /

. ; ? ,

<= != == >=

Word Symbolsaka reserved words, or keywords• int• float• double• char

• void• public• static• throws • return

• reserved words are always all lowercase• each word symbol is considered to be a single symbol• cannot be used for anything other than their intendedpurpose in a program

Identifiers

• Names of things (variables, constants, methods) in your programs

• Can be composed of any combination of letters, digits, underscore (_), and dollar sign ($)

• Cannot begin with a digit• May be any length• Java is case-sensitive

– Total, total, and TOTAL are different identifiers

Illegal Identifiers

Questions

Classify the following as legal or illegal identifiers:

1. My First Program2. my1stProgram3. 1stProgram4. $money5. an_identifier6. Jane'sProgram

illegal

legal

illegal

legal

legal

illegal

Primitive Data TypesWhat’s A Data Type?• A set of values and the operations that

can be performed on those values

• Primitive data are fundamental values such as numbers and characters

• Operations are performed on primitive types using built-in operators

Primitive Data Types

• 8 primitive data types in Java– 4 represent integers

•byte, short, int, long– 2 represent floating point numbers

•float, double– 1 represents characters

•char– 1 represents boolean values

•boolean

Primitive Data TypesNumeric Types

• The difference between the various numeric primitive types is their size, and therefore the values they can store:

Typebyteshortintlong

floatdouble

Storage8 bits16 bits32 bits64 bits

32 bits64 bits

Min Value-128-32,768-2,147,483,648< -9 x 1018

+/- 3.4 x 1038 with 7 significant digits+/- 1.7 x 10308 with 15 significant digits

Max Value12732,7672,147,483,647> 9 x 1018

Integers

• Examples: -6728, -67, 0, 78, 36782

• Positive integers do not have a '+' sign in front of them (but they can)

• No commas are used in an integer– commas in Java are used to separate

items in a list

Primitive Data TypesCharacters

• A char stores a single character from the Unicode character set– an ordered list of characters, and each

character corresponds to a unique number– uses 16 bits per character, allowing for

65,536 unique characters

• Character literals are delimited by single quotes:'a' 'X' '7' ' ' '$' ',' '\n'

newline character(we'll discuss later)

Primitive Data TypesBooleans

• Only two valid values– true or false– uses 1 bit for storage

• Represent any situation that has 2 states– on - off– true - false

• true and false are reserved words

• Expression - a combination of one or more operands and their operators

• Arithmetic expressions compute numeric results and make use of the arithmetic operators:

• If either or both operands associated with an arithmetic operator are floating point, the result is a floating point

Addition +Subtraction -Multiplication *Division /Remainder %

Arithmetic Expressions

• If both operands to the division operator (/) are integers, the result is an integer (the fractional part is discarded)

• The remainder, or modulus, operator (%) returns the remainder after dividing the second operand into the first (only works with integer types)

14 / 3 equals?

8 / 12 equals?

4

0

14 % 3 equals?

8 % 12 equals?

2

8

Division and Remainder

Unary vs. Binary Operators• Unary operators

– has only one operand– example: - (negative, not subtraction)-5

• Binary operators– has two operands– example: - (subtraction)5 - 3

Operator Precedence

• Determines the order in which operators are evaluated:1. multiplication, division, and remainder 2. addition, subtraction, and string

concatenation3. arithmetic operators with the same

precedence are evaluated from left to right

• Parentheses can be used to force the evaluation order (just like in math)

Operator Precedence

• What is the order of evaluation in the following expressions?

a + b + c + d + e1 432

a + b * c - d / e3 241

a / (b + c) - d % e2 341

a / (b * (c + (d - e)))4 123

Integral Expressions

• All operands are integers• Result is an integer

• Examples:2 + 3 * 53 + x – y / 7x + 2 * (y – z) + 18

Floating-point Expressions• All operands are floating-point numbers• Result is a floating-point

• Examples:12.8 * 17.5 – 34.50x * 10.5 + y - 16.2

7.0 / 3.5

Mixed Expressions

• Operands of different types• Examples:

2 + 3.56 / 4 + 3.9

• Integer operands yield an integer result• Floating-point operands yield a floating-

point result• If both types of operands are present, the

result is a floating-point number– implicit type coercion

• Precedence rules are followed

2.0 + 3.5

1 + 3.9 1.0 + 3.9

Type Conversion (Casting)• Used to avoid implicit type

coercion• Syntax(dataTypeName) expression

• Expression evaluated first, then type converted to dataTypeName

• Examples:(int) (7.9 + 6.7) = 14(int) (7.9) + (int)(6.7) = 13

QuestionsEvaluate These Expressions

1. (5 + 4) % 6

2. (5 + 6) % 3.5

3. (double) (13) / 2

4. (double) (13 / 2)

9 % 63

11 % 3.5not possible

(double) (6)6.0

13.0 / 26.5

The class String• String

– sequence of zero or more characters– enclosed in double quotation marks– null or empty strings have no characters– numeric strings consist of integers or decimal

numbers– length is the number of characters in a string

• The class String is used to manipulate strings• Examples:

– "Hello World"– "1234"– "45.67"– ""

Strings

• Every character has a position in the string (starting with 0)"Hello World"

• The length of the string is the number of characters in it– what's the length of "Hello World"?

0123456789...

11(count the space)

Parsing Numeric Strings• In Java, input from the user comes

in the form of a string– we need to know how to get the

number values out of the string

• Numeric String– a string with only integers or decimal

numbers– "6723", "-823", "345.76"

Parsing Numeric Strings• We will use Scanner and nextInt, next,

nextDouble, etc.• String to intInteger.parseInt(strExpression)Integer.parseInt("6723") 6723

• String to floatFloat.parseFloat(strExpression)Float.parseFloat("345.76") 345.76

• String to doubleDouble.parseDouble(strExpression)Double.parseDouble("1234.56") 1234.56

Summary

• Identifiers– can be composed of any combination of

letters, digits, underscore (_), and dollar sign ($)

– cannot begin with a digit– Java is case-sensitive

• Data Types– main integer type: int– main floating-point type: double– others: char, boolean

• Arithmetic Operators:

• If one of the operands is floating-point, the result is a floating-point

• Can only use % with two integer operands• Casting

– (int) (54.9) - truncates the floating-point number– (double) (23) - adds a .0 to the integer

addition +subtraction -multiplication *division /remainder (mod) %

Summary

Next Time in CSCI 51

• Wednesday– input, variables,

increment/decrement, Strings and '+' operator

– Remember, you’re responsible for Ch. 2,3