Download Lecture 3 Last lecture Java`s primitive number types Numeric type

Last lecture
Lecture 3
Looked at refinement – converting specification to code
Seminars
Added input statements
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Seminar 2s begin today at 5pm
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integer variables (int) were used to store values
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These will take place in the rooms indicated on the timetable
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we looked at declaration and assignment
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You should complete problem sheet 2
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studied the capacity (range) of these variables
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Hand in your solutions beforehand if you want feedback
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other number types including byte, short and long
Added a calculation part
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Post the solutions in the CS118 letterbox at the back of 006
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Make sure they include your name and your seminar tutors name
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that needed some comparison operators (==, !=, <)
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that repeated code using the Java while construct
Added an output part
Coursework 1
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Start reading the material and looking at the exercises
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More info at the end of this lecture
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CS118: Programming for Computer Scientists
need to strengthen the precondition (e.g. a and b > 0)
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Numeric type conversion
Java’s primitive number types
We have seen four primitive number types
using Java library functions (e.g. System.out.print)
Discussed the importance of pre- and post-conditions
Primitive
datatypes
5&6
Java is strict with the types of different variables
int i;
We need two more to represent floating point numbers
Type
float
Size
4 bytes
double
8 bytes
Range
-3.4e38 to 3.4e38
6 to 7 significant
digits of accuracy
-1.7e308 to 1.7e308
14 to 15 significant
digits of accuracy
Numeric operators include:
i = 10.3 * x; illegal - i is less accurate type
However, Java automatically converts some types
x = i + 10; legal - x is more accurate type
This is called Automatic Type Conversion
2/4=0
2.0 / 4.0 = 0.5
4%2=0
+, -, *, /, %
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Type conversion allows smaller formats to be converted to
larger formats (with increased level of accuracy)
byte
short
int
long
float
double
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Type casting
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Introducing features of Java
Java provides the programmer with a means of converting a
variable of one type to another
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double x = 3.1;
When introducing new language features, we will consider
the Syntax
this is called Type Casting
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rules governing what the layout in Java looks like
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i.e. the grammatical arrangement of the language
Difference between Type Casting and Type Conversion:
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type conversion (automatic - Java driven)
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type casting (manual - programmer driven)
the Semantics
Type casting allows larger formats to be converted to smaller
formats (with possible loss of accuracy)
byte
short
int
long
float
double
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rules governing what the Java means
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i.e. the relationship between the symbols and what they represent
Compilation & execution
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the Java compiler is good at spotting syntax errors
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you will only discover semantic errors by running the code
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Type casting (again)
Type casting cont…
Example
Type casting shortcuts
double d = 10.125;
float f = (float) 10.1;
int i = 0;
float f = 10.1f;
i = (int) d;
// equivalent statements
Long (trailing l or L); Float (trailing f or F); Double
(trailing d or D) etc.
results in i = 10
Syntax
Type casting can of course create errors :-(
(targetType) variableName/Value
int i;
Semantics
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// ...some assignment and calculation involving i
The variable or value (to the right of the cast) has its type narrowed to
the given target type; there may be a loss in accuracy.
Default Java type for fractional numbers is double; for whole
numbers it is int
CS118: Programming for Computer Scientists
This will work if -128 <= i <= 127, otherwise you may get a runtime
error! Or even worse … it may fail quietly!
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Variable assignment
Variable declaration
Syntax:
dataType variableName;
Example:
int x; // x is declared as an integer
variable = value;
variable = expression;
Example:
x = 24;
// assign a value to x
Semantics: Store the value or the result of the expression in
the variable location named variable
Use of variables distinguishes imperative programming
languages (e.g. C, Java, Pascal, VB etc.) from declarative
languages (e.g. purely functional languages like Haskell)
Expressions that denote locations (e.g. x) are known as lvalues; expressions that denote values (e.g. 5) are known as
r-values. Interestingly x is both an l-value and an r-value
Languages that use variables are known as state-based
Operations on variables are known as side effects
s1
s1
The declaration of a variable x
creates a program state
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Syntax:
x = (x * 34) / y;
Semantics: Create a variable container of type dataType
whose name is variableName
x
byte b = (byte) i;
x=0
x = x + 1;
s2
x=1
print(x);
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Variable declaration and assignment
Dog’s dinner
Syntax:
dataType variable = value;
dataType variable = expression;
Because Java is based on the C
programming language, a number of
programming short cuts exist
Example:
int x = 24;
// declare and assign value to x
Semantics: Create a variable container of type dataType
whose name is variable, then store the value or the result of
the expression in that variable
int x;
x = x + 1;
System.out.println(x);
i = i + 2; same as
i += 2;
i = i - 1;
i -= 1; same as
same as
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i- -;
While these might seem like a good idea they
are the cause of common mistakes
And they do not improve the readability of
your program: what values do x and y have?
int x = 5;
int y = ++x;
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x=1
i =+ 2;
This is useful as declaration on its own does not
guarantee an initial value
int x;
s3
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x = 7;
y=x++;
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Primitive
datatype #7
Boolean data type
Boolean data types cont...
Comes from Boolean Algebra (true, false)
Boolean operators (have boolean operands)
boolean lightsOn = true;
Comparison operators (return boolean values)
Operator
Name
Example
Answer
<
less than
1<2
true
<=
less than or equal
1 <= 2
true
>
greater than
1>2
false
>=
greater than or equal
1 >= 2
false
==
equal
1 == 2
false
!=
not equal
1 != 2
true
Operator
Name
Example
!
not (negation)
!true
Answer
&&
and (conjunction) true && true
||
or (disjunction)
true || false
true
^
exclusive or
false ^ false
false
false
true
(exclusion)
Note that ! is a unary operator, it only requires one operand
Note these are binary operators, i.e. they take two operands
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Strict and lazy operators
Character data type
Java (and C) have strict (e.g. &) and lazy (e.g. &&) versions of
the Boolean operators
a
b
a&b
a
F
F
F
F
b
F
F
T
F
F
F
T
F
F
T
F
F
T
T
T
T
T
T
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Primitive
datatype #8
The char data type is used to represent a single
character. A character is enclosed in single ‘ ’s
a && b
char letter = ‘a’;
char number = ‘4’;
For multiple characters, or a string, we use the String
class. A string is enclosed in “ ”s
String message = “this is a string”;
Need to be careful which operator you use
String is not one of Java’s 8 basic data types - strings
are objects not primitive values
If x=0, for example, then what happens here?
(x == 1) && ((100/x) != 2)
(x > -2) | (x++ < 2)
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Operator precedence
RobotRobot-maze
Determines the order in which expressions are
evaluated
Two pieces of coursework
3 + 4 * 4 > 5 * (4 + 3) - 1++
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Wednesday 26 October (week 5)
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Friday 2 December (week 10)
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Expressions in parentheses evaluated first
Precedence
Operator
Highest
++, -*, /, %
+, <, <=, >, =>
==, !=
&&
||
Lowest
Both are based on a robot-maze problem
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You have a robot, a target and a maze
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The aim is to control the robot so that it reaches the target
You are required to design the robot-controller code
=, +=, -=, *=, /=, %=
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18 exercises for coursework 1
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8 exercises for coursework 2
Parentheses tend to improve readability
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RobotRobot-maze environment
Controlling the robot
Where do you want the robot to move?
Getting started
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Select maze
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Select robot
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New Maze
1. Draw maze
2. Draw robot
3. Enact move
Robot
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Direction
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Start position
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Speed
Maze
Controller
Target
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1. Detect walls etc
2. Point robot
3. Signal move
Moveable
Maze
I want the robot to move left…
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Sizeable
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Editable
This control loop is carried out once for each move
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Robot controller code
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Dumbo controller code
import uk.ac.warwick.dcs.maze.logic.IRobot;
Stored in a small Java file
Library methods
public class DumboController
{
public void controlRobot(IRobot robot) {
int randno;
int direction;
The robot interacts with the maze through a set of
library methods
Control method
// Select a random number (0..3 inclusive)
robot.look(direction)
randno = (int) Math.round(Math.random()*3);
robot.face(direction)
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Where direction can be AHEAD, BEHIND, LEFT and RIGHT
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robot.look returns PASSAGE, WALL or BEENBEFORE
The library method robot.advance() instructs the robot to
move in the specified direction
// Convert this to a direction
Selects a random
number (0..3)
if (randno == 0)
direction = IRobot.LEFT;
else if (randno == 1)
direction = IRobot.RIGHT;
else if (randno == 2)
direction = IRobot.BEHIND;
else
direction = IRobot.AHEAD;
Convert to a
direction
robot.face(direction);
robot.advance();
// face
// move
Face robot in that
direction and move
}
}
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Coursework 1
What should you do next?
Your job is to modify this controller code
Read The Guide and lecture notes
Part 1
Source a book
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Stop the robot crashing into walls
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Print out log information
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Smarten the way in which directions are chosen
Download the robot-maze
Read up on control statements
Part 2
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Design a homing robot
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Sense the target, aim robot towards target
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if else
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while
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do-while
Get started!
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