COMP 110:Introduction to Programming

Tyler JohnsonFeb 4, 2009

MWF 11:00AM-12:15PMSitterson 014

COMP 110: Spring 20092

Announcements

Program 1 due tonight by midnight

Lab 2 due tomorrow by midnight

Lab 1 has been graded

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Questions?

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Today in COMP 110

Lab 1

In-Class Exercise

Floating-Point Numbers

Loops

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Floating-Point Numbers

Floating-point numbers (double, float) store real numbers in a computer

Numbers with some fractional component such as 8.5, 99.9, 3.14159

Floating-point numbers are approximate quantities and are not exact!

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Floating-Point Numbers

Consider the number 1/3 = 0.333333

Computers have finite storage and cannot store infinitely repeating decimals

In a computer, the number 1/3 is stored as 0.333333, with part of the fraction missing or truncated

Since 0.333333 is < 1/3, this representation is inexact

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Floating-Point Numbers

When using floating-point numbers, small errors will accumulate over time as operations are performed

== tests whether two numbers are exactly equal

You should never use == or != to compare floating-point numbers

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Floating-Point Comparison

To compare two floating-point numbers, check if the difference is within some tolerance

float a,b;final float EPSILON = 1e-6; //10^-6…if(Math.abs(a – b) < EPSILON) //Math.abs() is absolute

valueSystem.out.println(“a and b are the same!”);

elseSystem.out.println(“a and b are different!”);

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Loops

Programs often need to repeat some action

For example, when grading a test, the process of grading is repeated for each student

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Loops

A portion of a program that repeats some action is called a loop

The statements that are repeated are called the loop body

Each repetition of the loop body is called an iteration

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Loops

All loops must also have a stopping condition

Describes under what condition(s) iteration should cease

ExampleWhen the process of grading is completed for all students, the instructor will stop grading

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Loop Example

A loop that counts up to a certain number n

Evaluate

i <= n ?

Execute

Print i

i = i + 1;

Execute

End Program

Start

i = 1

false

true

Loop Body

Stopping Condition

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While Statements

The while statement can be used to construct a loop in Java

while(Boolean_Expression) {Statement_1Statement_2…Statement_N

}

As long as Boolean_Expression is true, the statements in the loop body are executed

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While Example

A program that counts up to a certain number

int number;

… //number is set to some value, e.g. user input

int count = 1;while(count <= number) {

System.out.print(count + ", ");count++;

}

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While Example

int number = 4;int count = 1;

while(count <= number) {System.out.print(count + ", ");count++;

}

Iteration

count Output

1 1 1,

2 2 1, 2,

3 3 1, 2, 3,

4 4 1, 2, 3, 4,

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Programming Demo

How can we extend MenuCalculator.java to allow the user to re-enter input when the input is invalid?

Step 1: Ask the user for inputStep 2: Check the inputStep 3: If the input is invalid, go to Step 1Step 4: Perform the selected operation

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Programming Demo

Evaluate

Input Valid?

Execute

Present Menu

Read Input

Execute

Perform User Selection

Start

true

false

Loop Body

Stopping Condition

Allowing the user to re-enter input

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Friday

Recitation

BringLaptops (fully charged)Textbook