Transcript Lecture slides for week 3

Primitive data
Week 3
Lecture outcomes
• Primitive data
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integer
double
string
char
Float
Long
boolean
Declaration
Initialisation
Assignments
Arithmetic operators
Boolean operators
Example
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123 (int)
1.5 (double)
“HelloWorld” (String)
`H’ (Char)
….
Data Types
• Constants
• Variables
What is a Constant?
• 456—a literal numerical constant
– System.out.println(456); // Java
– Console.writeline(456); // Visual C#
• “A Literal String Constant”
– System.out.println(“My First Java”); // Java
– Console.writeline(“My First C#”); // Visual C#
What is a variable?
• It is a named computer location in memory
that holds values that might vary
• Must that location have an address?
– YES
• What has addresses? Bits, bytes, words,
what?
– Bytes
• Can a variable be more than one byte long?
– YES
Data type Declarations
• Specify the type of data and the length of the
data item in bytes
• int, short, long
• float, double
• boolean
• char
Data Types -- Integer
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Int – the default declaration – 4-byte integer
Byte—1-byte integer
Short—2-byte integer
Long—8-byte integer
Floating Point
• Float—a 4-byte floating point number
• Double—an 8-byte floating point number
There are eight primitive data types
• Boolean, byte, char, int, double, float, long,
short
• In bytes, how long is the short data type? The
int data type, the long data type?
• In bytes, how long is the float data type? The
double data type?
• How long is the char data type?
Primitives sizes and Ranges
PRIMITIVE
SIZE IN BITS
RANGE
int
32bits (4 bytes)
-231 to 231 -1
-2,147,483,648 to +2,147,483,647
long
64bits -- 8 bytes
-263 to 263 - 1
float
32bits- -4 bytes
-+(1.40129846432481707e-45 to
3.40282346638528860e+38}
double
64
+-(4.94065645841246544e-324 to
1.79769313486231570e+308)
char
16bits
One character
string
16bits per char
Not applicable
bool (boolean in Java)
8bits--1-byte
True or false
Examples
Type
Set of values
Sample literal vlues
int
interges
99 (-12) 214748647
double
Floating-point numbers
3.14 (-1.5) 6.0021 1023
boolean
Boolean values
True or false
char
characters
‘a’ ‘1’ ‘£’ ‘%’ ‘\n’
String
Sequence of characters
“AC” ”Hello” ” 1.5”
Variable declaration
declaration
Variable name
Variable type
Int x;
x
integer
double d;
d
double
char c;
c
character
String s;
s
string
Float f;
f
float
The assignment operator =
declaration
Variable name
Int x;
Declare the variable x as an integer
x = 36;
Sets x to constant 36 at execution time
Sets x = to the constant 36 at compile time
int x = 36;
Initializes x to 36 at the time memory is set aside for it
String y;
Declare the variable x as an integer
y = “Hellow”;
Sets y to constant “Hello” at execution time
Sets y = to the constant “Hello” at compile time
String y = “Hello”;
Initializes x to “Hello” at the time memory is set aside for it
Initialisation
• If no value is assigned prior to use, then the compiler
will give an error
• Java sets primitive variables to zero or false in the
case of a boolean variable
• All object references are initially set to null
• An array of anything is an object
– Set to null on declaration
– Elements to zero false or null on creation
Declaration Examples
int index = 1.2;
// compiler error
boolean retOk = 1;
// compiler error
double fiveFourths = 5 / 4; // no error!
float ratio = 5.8f;
// correct
double fiveFourths = 5.0 / 4.0;
// correct
• 1.2f is a float value accurate to 7 decimal places.
• 1.2 is a double value accurate to 15 decimal places.
Declaration (Cont)
int a, b, c ;
b =1;
a=b;
c =a;
System.out.print(“c= “ + c);
• What is the value of a, b & c
Example
Int1.java
// uninitialised data
// this program will declare and print a number
Public class int3
{
public static void main(String[] arguments)
{
int weight;
System.out.println("your weight is " + weight);
}
}
//end of program
Example
Int2.java
// this program will declare and print a number
class int2
{
public static void main(String[] arguments)
{
int weight = 68;
System.out.println("your weight is " + weight);
}
}
//end of program
Example
Int5.java
// uninitialised data
// this program will declare and print a number
class int5
{
public static void main(String[] arguments)
{
int weight;
weight = 65 ;
//65 = weight ;
System.out.println("your weight is " + weight);
}
}
//end of program
Example
String2.java
// this program will declare and print a string
class string2
{
public static void main(String[] arguments)
{
String name = "Lahcen";
String x = "my name is ";
System.out.println( x + name ); //print string x and then string name
}
}
//end of program
Basic Mathematical Operators
• * / % + - are the mathematical operators
• * / % have a higher precedence than + or double myVal = a + b % d – c * d / b;
• Is the same as:
double myVal = (a + (b % d)) –
((c * d) / b);
Basic arithmetic Operators
Operator
Meaning
E=3 xample
+
Addition
2+3 = 5
*
Multiplication
2*3=6
-
subtraction
3-2=1
/
division
4/2=2
%
mod
5 % 2 = 1, 6 % 2= 0
Precedence Rules
1. Evaluate all sub-expressions in parentheses
2. Evaluate nested parentheses from the inside out
3. In the absence of parentheses or within
parentheses
a. Evaluate *, /, or % before + or –
b. Evaluate sequences of *, /, and % operators from left to
right
c. Evaluate sequences of + and – operators from left to
right
Example
SumStr.java
public class SumStr
{
public static void main(String args[])
{
System.out.print(args[0] + args[1] );
Java Argument 2 7
}
}
}
}
27
parse a string to integer.
SumInt.java
public class SumInt
{
public static void main(String args[])
{
System.out.print( Integer.parseInt(args[0] )+ Integer.parseInt(args[1] ) );
Java SumInt 2 7
}
}
}
}
9
Basic boolean Operators
Operator
Meaning
E=3 xample
==
equal
(4-2)==(8-6)
!=
Not equal
3 !=2 is true but
4!=(6-2) is false
>
Greater than
(3>2) is true
>=
Greater or equal
(3>=2) is true
<
Less than
(3<2) is false
<=
Less or equal
(3<=4) is true
Statements & Blocks
• A simple statement is a command terminated by a
semi-colon:
name = “Fred”;
• A block is a compound statement enclosed in curly
brackets:
{
name1 = “Fred”;
name2 = “Bill”;
}
• Blocks may contain other blocks
Flow of Control
• Java executes one statement after the other in the
order they are written
• Many Java statements are flow control statements:
Alternation: if, if else, switch
Looping:
for, while, do while
Escapes:
break, continue, return
If – The Conditional Statement
• The if statement evaluates an expression and if that
evaluation is true then the specified action is taken
if ( x < 10 ) x = 10;
• If the value of x is less than 10, make x equal to 10
• It could have been written:
if ( x < 10 )
x = 10;
• Or, alternatively:
if ( x < 10 ) { x = 10; }
If… else
• The if … else statement evaluates an expression and performs
one action if that evaluation is true or a different action if it is
false.
if (x != oldx) {
System.out.print(“x was changed”);
}
else {
System.out.print(“x is unchanged”);
}
Nested if … else
if ( myVal > 100 ) {
if ( remainderOn == true) {
myVal = mVal % 100;
}
else {
myVal = myVal / 100.0;
}
}
else
{
System.out.print(“myVal is in range”);
}
else if
• Useful for choosing between alternatives:
if ( n == 1 )
{
// execute code block #1
}
else if ( j == 2 )
{
// execute code block #2
}
else {
// if all previous tests have failed,
execute code block #3
}
A Warning…
WRONG!
if( i == j )
if ( j == k )
System.out.print(
“i equals k”);
else
System.out.print(
“i is not equal
to j”);
CORRECT!
if( i == j ) {
if ( j == k )
System.out.print(
“i equals k”);
}
else
System.out.print(“i
is not equal to j”);
// Correct!
The switch Statement
switch ( n ) {
case 1:
// execute code block #1
break;
case 2:
// execute code block #2
break;
default:
// if all previous tests fail then
//execute code block #4
break;
}
Summary
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Different data type
Declarations
Arithmetic operators
Parse string to integer.
Boolean operators
Assignments
If statement
Switch statement.
Home work
• Practice with all the exercise on the website
• Read chapter 4 on the study guide:
– Redo all the examples
– Do all the exercises.