csci 51 introduction to programming dr. joshua stough january 27, 2009
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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
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