gp4, martin lillholm 1 introductory programming (gp) spring 2006 lecture 4 we start at 13:00 slides...

1GP4, Martin Lillholm

Introductory Programming (GP)Spring 2006

Lecture 4

We start at 13:00 Slides are available from the course home pageFeel free to print them now

www.itu.dk/courses/GP/F2006

Martin Lillholm


Mandatory Assignment

• How did it go with last week’s mandatory assignment ?– Easy?– Hard?– Many didn’t hand in – why ?


Last Week

• Classes, objects (instances)

• Attributes (instance variables)

• Methods and constructors

• A first look a static methods and variables (class variables)

• Encapsulation

• Object/reference variables – variable of non-primitive type

• The String, Random, Math og Scanner classes

• Formatted output


The Scanner class again

Input several (e.g.) integers using one line of input:


Formatting Phone Numbers

Assignment from last week: L&L Programming Project 3.4

23 00023

DecimalFormat fmt = new DecimalFormat("00000");

System.out.println(fmt.format(23));


Java API Specification

– Appendix M

– http://java.sun.com/j2se/1.5.0/docs/api/index.html


What Have We Skipped so Far?

• Earlier:

– Enumerated types– Wrapper classes and auto boxing– Some GUI

• Today:

– Iterators section 5.6


Today

• More on flow of control

• Boolean Expressions

• Relational and logical operators

• if and switch statements

• Loop statement– for, do, while

• Block statements

• Graphics using loops and conditions – if time allows


How Does Today’s Chapter Fit In ?

• Program execution (normally) begins in the main method. Statements in the main method are executed sequentially – flow of control.

• Method calls causes flow of control to execute statements in the method (and possible nested method calls) and then return to the next statement in the main method.

• Conditional statements:

• Loops

statement1

statement2statement3statement4

statement5

statement1statement2if (condition) statement3 // truestatement4

e.g. 10 times

Both used in methods bodies incl. main


Boolean Expressions

• Boolean/logical expressions true or false

Has type boolean

• As opposed to arithmetic expressions 1, 45.78

Has type integer, double, ...

• Boolean expressions are normally formed using relational and logical operators.


Operator Meaning Example

< Less than x < 60

<= Less than or equal x <= 60

> Greater than x > 60

>= Greater than or equal x >= 60

== Equal to x == 60

!= Not equal to x != 60

Relational Operators

Argument type typically integer, double, char (primitive types)Resultant type is boolean and has value true or false

Has lower precedence than the arithmetic operators

<, <=, >, >= has higher precedence than == and !=


Boolean Expressions using Relational Operators

• Examples (x=2 and y=4):

Expression Value

false false

true true

true == false false

x != y true

x < 3 + y true

y < x + 3 true

(x + y > 3) == false false

false != x < 3 true

x == y == false true


Logical Operators

Operator Meaning Eksempel

! Not (unary) !(x == 60)

&& And (binary) 0<= x && x <= 60

|| Or (binary) x < 0 || x>=60

Both argument and resultant type is boolean

! has higher precedence than && which has higher precedence than ||

! has higher precedence than both the relational and arithmetic operators

&& and || has lower precedence than both the relational and arithmetic operators

If expr1 is false in expr1 && expr2 then expr2 isn’t evaluatedIf expr1 is true in expr1 || expr2 then expr2 isn’t evaluated


Logical Operators, Truth Tables

x !x

true false

false true

x y x && y x || y

false false false false

false true false true

true false false true

true true true true


• Examples (x=2 and y=4):

Boolean Expressions with Logical Operators

Udtryk Værdi

!false true

!true false

!true == false true

!(true == false) true

true && false false

false || true true

(x + y > 3) && x < y true

x + y == 3 || x < 4 true

x < y && (3*4 == 2*6-1*2+2) == !(3<x) true


The if Statement

• Used for affecting flow of control based on boolean expressions

• And is thus used to make choices based on data:– Is one number bigger than another? – Is a number within a certain interval or outside?– Are two Strings identical?

• Is typically used when we need to:– React to data - unknown at compile time– Entered numbers, strings etc.– Data stored in files– Random numbers– ...

condition

Sstatement(s)

true

false


The if statement – Syntax

if ( expr ) statement;

if is a reserved word

expr has to be boolean and thus evaluate to either true or false

If expr evaluates to true, statement is executedand otherwise skipped


The if Statement – Examples

int a=5; if (a > 10)

a = a – 5;System.out.println(”a is: ” + a);

if (a>0 && a <=10)System.out.println(”a is in the interval from 1 to 10”);

if (a<1 && a>10)System.out.println(”a isn’t in the interval from 1 to 10”);

if (a == 5)a = 3;

if (a != 5)a = 5;

if (a < 0 || a > 10000)System.out.println(”a is negative or very large”);


The if Statement – Example

Limitations ? Either or ... Several statements in the true branch


The if-else Statement

• Syntax

if (expr)statement1;

elsestatement2;

• Examples:

if (level > MAX)System.out.println(”Level Critical!”);

elseSystem.out.printlnt(”Level ok”);

if (a > b - .01 && a < b + .01)a = b;

elseSystem.out.println(”a is different from b”);

condition

statement1

true false

statement2


The if-else – Example


if-else – Example


Block Statements

if (a > b) // swap a and b tmp = a; a = b; b = tmp;else a = a + 1;

if (a > b) { // swap a and b tmp = a; a = b; b = tmp;}else a = a + 1;

• { ... } is used to combine statements in blocks as we do in classes, constructor, and methods.

• A block statement can always substitute any single statement.

• Indentation doesn’t mean anything – logically speaking – but makes the program a lot more readable


Block Statements - Example


Nested if Statements – Example


Nested if Statements – A Pitfall …

if (b < 0)

if (a < b)

System.out.println(”Både a og b er negative”);

else

System.out.println(”b er negativ”);

if (b < 0) {

if (a < b)

System.out.println(”Både a og b er negative”);

}

else

System.out.println(”b er negativ”);

else always “belongs” to closest unmatched if – bar {}’s


Comparing Data

• Some primitive types (integer, long, short, byte) are compared as we seen so far.

• Decimal type (float, double) are still approximations and always be compared within and certain tolerance and not using e.g ==

final double TOLERANCE = 0.00001;

if (Math.abs(f1-f2) < TOLERANCE)

System.out.println(”f1 and f2 are approx. equal”);

• Characters char is compared using their Unicode:

’0’ .. ’9’ < ’A’ .. ’Z’ < ’a’ ... ’ z’‘A’ != ‘a’


Comparing Data (String)

• Think of strings as a sequence of characters (from left to right). This gives an ordering (lexicographic ordering).“garden” before “gardens” and “Martin” before “anders”

• Vi cannot use e.g. == and <= directly – see next slide

equals and compareTo methods I the String class

if (name1.equals(name2)) System.out.println (”The names are the same”);else System.out.println (”The names are not the same”);

int result = name1.compareTo(name2);if (result < 0) System.out.println(name1 + ” comes before ” + name2);else if (result == 0) System.out.println (”The name are the same”); else System.out.println (name1 + ” follows ” + name2);


Comparing Data (Objects)

• We’ll return to object comparison in general.

• But == applied to objects compares references.


The switch Statement

• Alternative construction to select one or more of several statements

• Based on an integral expression one or more options from a given list is selected – possibly the default option is none matches.

• Program execution continues a the option that matches the expression option

switch (option){ case 'A': aCount++; break; case 'B': bCount++; break; case 'C': cCount++; break;

default: System.out.println(”No match”);

}


The switch Statement

• The purpose af break

• The purpose of default

• Implicit blocks

• The expression must be an integral type:(int, char or (enumerated type))

• The cases must be literals

• Equivalent to a series of if statements but sometimes more elegant


switch – Example


while Loops

• Repeats a statement or block 0 or more times

while (expr) statement;

• If expr evaluates to true, statement is executed

• expr is evaluated again and if true, statement is executed again …

• Continues until expr evaluates to false

int count = 1;while (count <= 5) { Sytem.out.println (count); count++;}

Boolean expression

statement

true false

condigtion


while loops –Example


while loops – Example


Infinite Loops

int count = 1;

while (count <= 25) {

System.out.println (count);

count = count - 1;

}

while (true) {

System.out.println (”This will take a while ...”);

}

int j = 0;

while (j < 10) {

j = j + 0;

System.out.println(j);

}

double num = 1.0;

while (num != 0.0)

num = num – 0.1;


Nested Loops

int count1, count2;

count1 = 1;

while (count1 <= 10) {

count2 = 1;

while (count2 <= 50) {

System.out.println(”Here again”);

count2++;

}

count1++;

}


Nested Loops


do loops

• Very similar to while loops except that the condition is evaluated after the body – thus do loops always run at least once

do

statement;

while (expr)

int count = 0;

do {

count++;

System.out.println(count);

} while (count < 5);

true

condition

statement

false

Boolean expression


do loops – Example


for loops

for ( initialisation ; condition ; increment ) statement;

InitialisationExcuted once before

the loop body

statement is executed untilcondition evaluates to false

“increment” executed after the loop body at the end of each iteration


for loops

statement

true

conditionevaluated

false

increment

initialization


for loops - Examples

int i;

for (i=0; i<10; i++)

System.out.println(”i:” + i);

for (int j=1; j<10; j++)

System.out.println(”j:” + j);

for (i=9; i >= 0; i--) {

k = i + 2;

System.out.println(”k:” + k);

}

– L&L Multiples.java (side 248)

– L&L Stars.java (side 250)


All Loops Constructs are Equivalent

• while

• do

• for


Advise on Variable Declaration and Initialisation

• Declare variables as local as possible

• Declare variables close to their first use

• Declare loop variables close to or in the loop construct (only possible for for loops)


Graphics using Loops

– Bullseye.java og BullseyePanel.java i BlueJ(L&L page 251-254)

– Boxes.java(L&L page 255-257)

gp4, martin lillholm 1 introductory programming (gp) spring 2006 lecture 4 we start at 13:00 slides...

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