programming methodology (1). temperature1 temperature2 temperature3 temperature4 temperature5...
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Programming Methodology (1)
temperature1temperature2
temperature3
temperature4
temperature5
temperature6 temperature7
System.out.println( temperature1 );
System.out.println( temperature2 );
System.out.println( temperature3 );
System.out.println( temperature4 );
System.out.println( temperature5 );
System.out.println( temperature6 );
System.out.println( temperature7 );
for ( int i = 1 ; ; i++ )
{
}
System.out.println( );
i <= 7
temperature1
temperature1temperature2
temperature3
temperature4
temperature5
temperature6 temperature7
Arrays
temperature
Arrays
Learning objectives
• create arrays;
• use loops to process arrays;
• use an enhanced for loop to process an array;
• use arrays as method inputs and outputs;
• develop routines for accessing and manipulating arrays.
What is an array?
An array is a data type that stores a collection of items!
These items are referred to as the elements of the array!
All elements must be of the same type BUT there is no restriction on which type this is!
1.75
75.1
12.5
1.75
31.5
‘T’
‘a’
‘s’
‘s’
‘W’
“Blue”
“Red”
“White”
“Black”
“Green”
double char String
Examples
9.2
“Java”
12
‘j’
true
mixed
Examples
How do you create an array?
Array creation is a two-step process:
STEP 1:Declare an array variable.
typeOfElement arrayName;[ ]
int score;[ ]
An array of exam scores:Exam scores are whole numbers
typeOfElement arrayName;[ ]
String name;[ ]
An array of student names:Names are Strings
typeOfElement arrayName;[ ]
temperature;double[ ]
An array of temperatures:temperatures are real numbers
The effect on computer memory of declaring an array …..
Java Instructions
double[ ] temperature ;
Computer Memory
temperature
?
This is called a reference variable
STEP 2:Allocating memory to store the array elements!
What information do we need to provide ?
a) The size of the array.
b) The type of each element.
Combine the two with a new operator.
Example: an array of 10 scores:
nameOfArray = new arrayType [ size ];
score = new int [ 10 ];
Example: an array of 20 student names:
nameOfArray = new arrayType [ size ];
name = new String [ 20 ];
nameOfArray = new arrayType [ size ];
Returning to the temperature array:
temperature = new double [ 7 ];
The effect on computer memory of sizing an array….
Java Instructions
double[ ] temperature ;
Computer Memory
temperature
?
item of type 'double'
item of type 'double'
item of type 'double'
item of type 'double'
item of type 'double'
item of type 'double'
item of type 'double' temperature = new double[7];
Combining steps 1 and 2
double[ ] temperature ;
= new double [7];temperature
= new double [7];
Initializing an array
double[ ] temperature = new double [7];{9, 11.5, 11, 8.5, 7, 9, 8.5} ;
Naming the array elements….
0 1 2 3 4 5 6
temperature
First item is
temperature[0]
Last item is
temperature[6]
Array variables can be used like any other variable of the given type in Java.
score[0] = 12;
Entering 12 into the first position of the score array:
name[2] = “Aaron”;
Entering “Aaron” into the third position of the name array:
temperature[6] = 21.5;
Entering 21.5 into the 7th position of the temperature array:
Entering values into an array from the keyboard..
double num;
num = sc.nextDouble();
Entering into a simple double variable
temperature [ ] = sc.nextDouble();
Entering into first array variable
0
double num;
num = sc.nextDouble();
Entering into a simple double variable
temperature [ ] = sc.nextDouble();
Entering into second array variable
1
double num;
num = sc.nextDouble();
temperature [ ] = sc.nextDouble();
Entering into last array variable
6
Entering into a simple double variable
More examples of accessing array elements…
0 1 2 3 4 5 6
temperature
Printing 6th element on the screen…
0 1 2 3 4 5 6
temperature
0 1 2 3 4 5 6
temperature
System.out.println( num );
0 1 2 3 4 5 6
temperature
System.out.println( temperature[5] );
0 1 2 3 4 5 6
temperature
Double the 5th element….
0 1 2 3 4 5 6
temperature
num = num * 2;
0 1 2 3 4 5 6
temperature
temperature[4] = temperature[4] * 2;
Check the temperature on the third day…..
0 1 2 3 4 5 6
temperature
if (num >= 18){
System.out.println("it was hot today");}
0 1 2 3 4 5 6
temperature
if ( temperature[2] >= 18){
System.out.println("it was hot today");}
0 1 2 3 4 5 6
temperature
What is wrong here?
System.out.println( temperature[ 7 ] );
The index must be valid (0 -6)
otherwise you will get an error at run-time.
ArrayIndexOutOfBoundsException
Using a variable as the array index…
System.out.println( temperature[ ] ); i0123456
for ( ; ; )
{
}
int i = 0 i++ i < 7i <= 7
This will cause your program to crash!What will happen here?
7
ArrayIndexOutOfBoundsException
System.out.println( temperature[ ] ); i0123456
for ( ; ; )
{
}
int i = 0 i++ i < 7 i < temperature.length; i++)
for ( ; ; )
{
} temperature[ ] = sc.nextDouble(); i01
76
int i = 0 i++ i < temperature.length; i++)
System.out.println ("enter max temperature for day " + ); (i+1)12
Putting it all together…..
public static void main(String[ ] args){ Scanner sc = new Scanner(System.in); double[ ] temperature = new double[7]; for (int i = 0; i < temperature.length; i++) { // code to enter temperatures } for (int i = 0; i < temperature.length; i++) {
// code to display temperatures }}
Re-writing this program using methods enterTemps and displayTemps …..
public static void main(String[ ] args){ Scanner sc = new Scanner(System.in); double[ ] temperature = new double[7]; for (int i = 0; i < temperature.length; i++) { // code to enter temperatures } for (int i = 0; i < temperature.length; i++) {
// code to display temperatures }}
Put this loop into enterTemps method
Put this loop into displayTemps method
public static void main(String[ ] args){ Scanner sc = new Scanner(System.in); double[ ] temperature = new double[7];
}
private static void enterTemps ( ){
// loop to enter into temperature array here}private static void displayTemps ( ){
// loop to display temperature array here}
enterTemps( );displayTemps( );temperature
temperature
temperatureIn
temperatureIn
double[ ]
double[ ]
// loop to enter into temperatureIn array here
// loop to display temperatureIn array here
Passing arrays as parameters…
private static void enterTemps(double[ ] temperatureIn){ Scanner sc = new Scanner(System.in); for (int i = 0; i < temperatureIn.length; i++) { System.out.println ("enter max temperature for day " + (i+1)); temperatureIn[i] = sc.nextDouble(); }}
public static void main (String [ ] args){
double[ ] temperature = new double[7];enterTemps( temperature );displayTemps( temperature );
}
temperature
temperatureIn
The effect on computer memory of passing an array as a parameter………
Java Instructions
main (String[ ] args){ // create temperature array
}
Computer Memory
temperature
[ 6 ]
[ 5 ]
[ 4 ]
[ 3 ]
[ 2 ]
[ 1 ]
[ 0 ]enterTemps( temperature );
enterTemps (double[] temperatureIn)
{
// update temperatureIn
}
temperatureIn
Returning an array from a method……..
private static void enterTemps(double[ ] temperatureIn){ Scanner sc = new Scanner(System.in); for (int i = 0; i < temperatureIn.length; i++) { System.out.println ("enter max temperature for day " + (i+1)); temperatureIn[i] = sc.nextDouble(); }}
private static ? enterTemps( ){ Scanner sc = new Scanner(System.in); for (int i = 0; i < temperatureIn.length; i++) { System.out.println ("enter max temperature for day " + (i+1)); temperatureIn[i] = sc.nextDouble(); }}
private static double[ ] enterTemps( ){ Scanner sc = new Scanner(System.in); for (int i = 0; i < temperatureIn.length; i++) { System.out.println ("enter max temperature for day " + (i+1)); temperatureIn[i] = sc.nextDouble(); }}
private static double[ ] enterTemps( ){ Scanner sc = new Scanner(System.in); for (int i = 0; i < temperatureIn.length; i++) { System.out.println ("enter max temperature for day " + (i+1)); temperatureIn[i] = sc.nextDouble(); }}
double[ ] temperatureOut = new double[7];
private static double[ ] enterTemps( ){ Scanner sc = new Scanner(System.in); for (int i = 0; i < temperatureOut.length; i++) { System.out.println ("enter max temperature for day " + (i+1)); temperatureOut[i] = sc.nextDouble(); }}
double[ ] temperatureOut = new double[7];
private static double[ ] enterTemps( ){ Scanner sc = new Scanner(System.in); for (int i = 0; i < temperatureOut.length; i++) { System.out.println ("enter max temperature for day " + (i+1)); temperatureOut[i] = sc.nextDouble(); } return temperatureOut;}
double[ ] temperatureOut = new double[7];
Modifying the main method..
public static void main (String [ ] args){
double[ ] temperature
displayTemps( temperature );}
= new double[7];;enterTemps( temperature );enterTemps( );temperature = enterTemps( );
The enhanced 'for' loop….
for ( ){
}
int i = 0; i < temperatureIn.length ; i++
System.out.println( );
temperatureIn[ i ]
temperatureIn item : double
item
“For every item in the temperatureIn array”
Some useful array methods…
public class SomeUsefulArrayMethods{
public static void main (String[] args){ Scanner sc = new Scanner(System.in);
int[ ] someArray; System.out.println("How many elements to store?"); int size = sc.nextInt(); someArray = new int[size];
// call methods here }
// methods to process an array here}
Allows user to size the array.
Array summation..
SET total TO zero
LOOP FROM first element TO last element
BEGIN
END
SET total TO total + current element
RETURN total
private static sum ( int[ ] arrayIn ) int
{
}
int total = 0;LOOP FROM first element TO last element
BEGIN
END
SET total TO total + current element
RETURN total
private static sum ( int[ ] arrayIn ) int
{
}
int total = 0;for ( int i = 0 ; i < arrayIn.length ; i++ ) BEGIN
END
SET total TO total + current element
RETURN total
private static sum ( int[ ] arrayIn ) int
{
}
int total = 0;for ( int i = 0 ; i < arrayIn.length ; i++ ) {
}
SET total TO total + current element
RETURN total
private static sum ( int[ ] arrayIn ) int
{
}
int total = 0;for ( int i = 0 ; i < arrayIn.length ; i++ ) {
}
total = total + arrayIn [ i ] ;
RETURN total
private static sum ( int[ ] arrayIn ) int
{
}
int total = 0;for ( int i = 0 ; i < arrayIn.length ; i++ ) {
}
total = total + arrayIn [ i ] ;
return total;
private static sum ( int[ ] arrayIn ) int
{
}
int total = 0;for ( int currentElement: arrayIn ) {
}
total = total + arrayIn [ i ] ;
return total;
private static sum ( int[ ] arrayIn ) int
{
}
int total = 0;for ( int currentElement: arrayIn ) {
}
total = total + currentElement ;
return total;
private static sum ( int[ ] arrayIn ) int
{
}
Consider the following explicit creation of an array:
a) What would be the value of someArray.length ?
int[ ] someArray = {2, 5, 1, 9, 11};
Consider the following explicit creation of an array:
int[ ] someArray = {2, 5, 1, 9, 11};
5
2
1
9
someArray
11
a) What would be the value of someArray.length ?
Consider the following explicit creation of an array:
a) What would be the value of someArray.length ?
int[ ] someArray = {2, 5, 1, 9, 11};
5
2
1
9
someArray
11
5
Consider the following explicit creation of an array:
b)What is the value of someArray[2]?
int[ ] someArray = {2, 5, 1, 9, 11};
5
2
1
9
someArray
11
Consider the following explicit creation of an array:
b)What is the value of someArray[2]?
int[ ] someArray = {2, 5, 1, 9, 11};
5
2
1
9
someArray
11
[0]
[1]
[2]
[3]
[4]
Consider the following explicit creation of an array:
b)What is the value of someArray[2]?
int[ ] someArray = {2, 5, 1, 9, 11};
5
2
1
9
someArray
11
[0]
[1]
[2]
[3]
[4]
Consider the following explicit creation of an array:
c)What would happen if you tried to access someArray[6]?
int[ ] someArray = {2, 5, 1, 9, 11};
5
2
1
9
someArray
11
[0]
[1]
[2]
[3]
[4]
Consider the following explicit creation of an array:
c)What would happen if you tried to access someArray[6]?
int[ ] someArray = {2, 5, 1, 9, 11};
5
2
1
9
someArray
11
[0]
[1]
[2]
[3]
[4]
Consider the following explicit creation of an array:
c)What would happen if you tried to access someArray[6]?
int[ ] someArray = {2, 5, 1, 9, 11};
5
2
1
9
someArray
11
[0]
[1]
[2]
[3]
[4]ArrayIndexOutOfBoundsException
5
2
1
9
someArray
11
[0]
[1]
[2]
[3]
[4]
d)Write a standard for loop that will double the value of every item in someArray.
5
2
1
9
someArray
11
[0]
[1]
[2]
[3]
[4]
d)Write a standard for loop that will double the value of every item in someArray.
for ( ? ; ? ; ? )
{
// code to double a value here
}
5
2
1
9
someArray
11
[0]
[1]
[2]
[3]
[4]
d)Write a standard for loop that will double the value of every item in someArray.
for (int i = 0; ? ; ? )
{
// code to double a value here
}
5
2
1
9
someArray
11
[0]
[1]
[2]
[3]
[4]
d)Write a standard for loop that will double the value of every item in someArray.
for (int i = 0; ? ; i++)
{
// code to double a value here
}
5
2
1
9
someArray
11
[0]
[1]
[2]
[3]
[4]
d)Write a standard for loop that will double the value of every item in someArray.
for (int i = 0; i <= 4 ; i++)
{
// code to double a value here
}
5
2
1
9
someArray
11
[0]
[1]
[2]
[3]
[4]
d)Write a standard for loop that will double the value of every item in someArray.
for (int i = 0; i < 5 ; i++)
{
// code to double a value here
}
5
2
1
9
someArray
11
[0]
[1]
[2]
[3]
[4]
d)Write a standard for loop that will double the value of every item in someArray.
for (int i = 0; i < someArray.length ; i++)
{
// code to double a value here
}
5
2
1
9
someArray
11
[0]
[1]
[2]
[3]
[4]
d)Write a standard for loop that will double the value of every item in someArray.
for (int i = 0; i < someArray.length ; i++)
{
someArray[i] = someArray[i] * 2;
}
10
4
2
18
someArray
22
[0]
[1]
[2]
[3]
[4]
d)Write a standard for loop that will double the value of every item in someArray.
for (int i = 0; i < someArray.length ; i++)
{
someArray[i] = someArray[i] * 2;
}
10
4
2
18
someArray
22
[0]
[1]
[2]
[3]
[4]
e)Should you use an enhanced for loop here?
for (int i = 0; i < someArray.length ; i++)
{
someArray[i] = someArray[i] * 2;
}
10
4
2
18
someArray
22
[0]
[1]
[2]
[3]
[4]
e)Should you use an enhanced for loop here?
for ( int item: someArray )
{
someArray[i] = someArray[i] * 2;
}
10
4
2
18
someArray
22
[0]
[1]
[2]
[3]
[4]
e)Should you use an enhanced for loop here?
for ( int item: someArray )
{
item = item * 2;
}
10
4
2
18
someArray
22
[0]
[1]
[2]
[3]
[4]
e)Should you use an enhanced for loop here?
for ( int item: someArray )
{
item = item * 2;
}
Enhanced for loops should not modify the original array!
f) Place the original for loop in a method, increaseMarks.
for (int i = 0; i < someArray.length ; i++)
{
someArray[i] = someArray[i] * 2;
}
public static void main (String [ ] args)
{
int [ ] someArray = {2, 5, 1, 9, 11};
}
private static ? increaseMarks( ? )
{
}
f) Place the original for loop in a method, increaseMarks.
for (int i = 0; i < someArray.length ; i++)
{
someArray[i] = someArray[i] * 2;
}
public static void main (String [ ] args)
{
int [ ] someArray = {2, 5, 1, 9, 11};
// call method
}
private static ? increaseMarks( int[ ] someArray )
{
}
f) Place the original for loop in a method, increaseMarks.
for (int i = 0; i < someArray.length ; i++)
{
someArray[i] = someArray[i] * 2;
}
public static void main (String [ ] args)
{
int [ ] someArray = {2, 5, 1, 9, 11};
// call method
}
private static void increaseMarks( int[ ] someArray )
{
}
f) Place the original for loop in a method, increaseMarks.
for (int i = 0; i < someArray.length ; i++)
{
someArray[i] = someArray[i] * 2;
}
public static void main (String [ ] args)
{
int [ ] someArray = {2, 5, 1, 9, 11};
// call method
}
private static void increaseMarks( int[ ] someArray )
{
}
f) Place the original for loop in a method, increaseMarks.
for (int i = 0; i < someArray.length ; i++)
{
someArray[i] = someArray[i] * 2;
}
public static void main (String [ ] args)
{
int [ ] someArray = {2, 5, 1, 9, 11};
increaseMarks( );
}
private static void increaseMarks( int[ ] someArray )
{
}
f) Place the original for loop in a method, increaseMarks.
for (int i = 0; i < someArray.length ; i++)
{
someArray[i] = someArray[i] * 2;
}
public static void main (String [ ] args)
{
int [ ] someArray = {2, 5, 1, 9, 11};
increaseMarks( someArray);
}
Practical Task…
Room 1 Room 2 Room 3 Room 4 Room 5
30 25 50 30 40
a) Declare and initialise an array, rooms, to hold the following capacities:
c) Add an instruction in main to call the displayRooms method.
b) Write a method, displayRooms, that accepts the array of room capacities and displays the capacity of each room.
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