Introduction to Arrays in Java Corresponds with Chapter 6 of textbook.
-
Upload
rosanna-morrison -
Category
Documents
-
view
218 -
download
0
Transcript of Introduction to Arrays in Java Corresponds with Chapter 6 of textbook.
Introduction to Arrays in Introduction to Arrays in JavaJava
Corresponds with Chapter 6 Corresponds with Chapter 6 of textbookof textbook
What is an Array?What is an Array?Array is a data structure that represents a collection of the same types of data.
Java treats these arrays as objects. This means array variables are references, not value variable.
Individual data items in arrays are called elements.
Elements are stored consecutively (contiguously) in memory.
Each element of an array is indexed.Indexing begins at 0.
Introducing ArraysIntroducing ArraysArray is a data structure that represents a collection of the same types of data.
5.6
4.5
3.3
13.2
4
34.33
34
45.45
99.993
11123
double[] myList = new double[10];
myList reference myList[0]
myList[1]
myList[2]
myList[3]
myList[4]
myList[5]
myList[6]
myList[7]
myList[8]
myList[9]
Element value
Array reference variable
Array element at index 5
Declaring ArraysDeclaring Arrays datatype[] arrayname;datatype[] arrayname;
Example:Example:int[] myList;int[] myList;
datatype arrayname[];datatype arrayname[];
Example:Example:int myList[];int myList[];
Either syntax will work. The [] indicates that the variable being declared will be a reference to an array. Each element of the array will be a data item of the specified data type.
NOTE: declaring the array does not create it! No memory is allocated for individual array elements. This requires a separate creation step.
Creating ArraysCreating Arrays
arrayName = arrayName = newnew datatype[arraySize]; datatype[arraySize];
NOTE: the new keyword creates an object or array. The object or array is created in a location of memory called the heap. A reference (pointer) to the array is assigned (via =) to the variable.
Example:myList = new double[10];
Java reserved word for object
creation
Integer value indicates the number of elements
Declaring and CreatingDeclaring and Creatingin One Stepin One Step
datatype[] arrayname = newdatatype[] arrayname = new
datatype[arraySize];datatype[arraySize];
double[] myList = new double[10];double[] myList = new double[10];
datatype arrayname[] = newdatatype arrayname[] = new datatype[arraySize]; datatype[arraySize];
double myList[] = new double[10];double myList[] = new double[10];
NOTE: when creating an array, the value inside the square brackets [ ] indicates the size you want the array to be (i.e. the number of elements).
The Length of an ArrayThe Length of an Array
Once an array is created, its size is fixed. It Once an array is created, its size is fixed. It cannot be changed. You can find its size usingcannot be changed. You can find its size using
arrayRefVar.lengtharrayRefVar.length
For example,For example,
myList.length returns 10myList.length returns 10
Initializing ArraysInitializing Arrays
Using a loop:Using a loop:
for (int i = 0; i < myList.length; i++)for (int i = 0; i < myList.length; i++)
myList[i] = someValue;myList[i] = someValue;
Declaring, creating, initializing in one Declaring, creating, initializing in one step:step:
double[] myList = {1.9, 2.9, 3.4, 3.5};double[] myList = {1.9, 2.9, 3.4, 3.5};
NOTE: when using an array, the value inside the square brackets [ ] indicates the index that you are looking at (i.e. the specific element).
Declaring, creating, Declaring, creating, initializing Using the initializing Using the Shorthand NotationShorthand Notation
double[] myList = {1.9, 2.9, 3.4, 3.5};double[] myList = {1.9, 2.9, 3.4, 3.5};
This shorthand notation is equivalent This shorthand notation is equivalent to the following statements:to the following statements:double[] myList = new double[4];double[] myList = new double[4];
myList[0] = 1.9;myList[0] = 1.9;
myList[1] = 2.9;myList[1] = 2.9;
myList[2] = 3.4;myList[2] = 3.4;
myList[3] = 3.5; myList[3] = 3.5;
Listing 6.1 – TestArray Class
1) Declaring the reference variable
2) Creating the array
3) Assigning the array’s memory location to the reference variable
4) NOTE: TOTAL_NUMBERS is a constant with value 6
Note: new is an operator that creates an object (an array is an example of an object).
How Variables are Kept in How Variables are Kept in MemoryMemory
Two categories of variablesTwo categories of variables ValueValue – contains actual data (integer, float, double, – contains actual data (integer, float, double,
boolean, or other primitive data type)boolean, or other primitive data type) ReferenceReference – contains a reference (pointer) to an object – contains a reference (pointer) to an object
or array that is located on the heap (as a result of using or array that is located on the heap (as a result of using the the newnew operator). operator).
Two different locations of memory (more to come Two different locations of memory (more to come later)later) FrameFrame StackStack – contains local variables and parameters – contains local variables and parameters
of methodsof methods HeapHeap – contains – contains objectsobjects and and arraysarrays The The newnew keyword always creates an object or array and keyword always creates an object or array and
places it in the heap.places it in the heap.
HeapFrame Stack
main’sframe
args
1) Declaring the reference variable
numbers
2) Creating the array on the heap
0 1 2 3 4 5
3) Assigning the array’s memory location to the reference variable
Listing 6.1 – TestArray Class
In a loop, read user input and insert into the array.
Note use of length property to decide when to end loop
Assume the user enters the numbers: 3, 2, 7, 5, 7, 1
HeapFrame Stack
main’sframe
argsnumber
s 0 1 2 3 4 5
i 0
3 2
1
7
2
5
3
7
4
1
56
Looping through the array
Note the use of the loop counter to index into the array
length is a property of array objects, indicating the number of elements
Listing 6.1 – TestArray Class
This loop is a useful technique for finding the largest value in an array.
Note the if statement nested in the for loop.
Note the use of the loop counter to index into the array.
HeapFrame Stack
main’sframe
argsnumber
s 0 1 2 3 4 5
i 1
Looping through the array
3 2 7 5 7 1
3max
3
2
7
3456
Listing 6.1 – TestArray Class
This use of a loop with an array is similar to a linear search. More on search routines later.
Listing 6.1 – TestArray Class
Note the concatenation of array elements to a string, again, repetitively in a loop.
Parallel ArraysParallel Arrays
Parallel arrays should have same length
Same index position same entity
Program output:
MMultidimensional Arrayultidimensional ArrayssA multidimensional array is a data structure that represents a collection of the same types of data.
Java treats these arrays as objects. This means array variables are references, not value variable.
Individual data items in arrays are called elements.
Elements are stored consecutively (contiguously) in memory.
Can vary in size from two to n.
Element [0][0]Element [0][1]
Element [1][0]
Element [3][1]
Element [2][1]
Element [1][1]
Element [3][0]
Element [2][0]
Declaring Multidimensional Declaring Multidimensional ArraysArrays
datatype[][] arrayname;datatype[][] arrayname;
Example:Example:int[][] myList;int[][] myList;
datatype arrayname[][];datatype arrayname[][];
Example:Example:int myList[][];int myList[][];
Either syntax will work. The [][] indicates that the variable being declared will be a reference to an array. Each element of the array will be a data item of the specified data type.
NOTE: declaring the array does not create it! No memory is allocated for individual array elements. This requires a separate creation step.
Creating Multidimensional Creating Multidimensional ArraysArrays
arrayName = arrayName = newnew datatype[rowSize][columnSize]; datatype[rowSize][columnSize];
NOTE: the new keyword creates an object or array. The object or array is created in a location of memory called the heap. A reference (pointer) to the array is assigned (via =) to the variable.
Example: myList = new double[10][4];
Java reserved word for object
creation
Integer value indicates the number of columns
Integer value indicates the
number of rows
Declaring and CreatingDeclaring and Creatingin One Stepin One Step
datatype[][] arrayname = newdatatype[][] arrayname = new
datatype[rowSize][columnsize];datatype[rowSize][columnsize];
double[][] myList = new double[10][3];double[][] myList = new double[10][3];
datatype arrayname[][] = newdatatype arrayname[][] = new datatype[rowSize][columnSize]; datatype[rowSize][columnSize];
double myList[][] = new double[10][3];double myList[][] = new double[10][3];
NOTE: when creating a multidimensional array, the value inside the square brackets [ ] [ ] indicates the number of rows and columns. The total size of the array is the rows multiplied by the columns (10 * 3 = 30 elements for the example above).
Initializing 2-Dimensional Initializing 2-Dimensional ArraysArrays
for (int r =0; r<row; r++) { for (int c = 0; c < column; c++){ myList[r][c]=someValue; } }
double[][] myList = {double[][] myList = { {1.9, 2.9},{1.9, 2.9}, {3.4, 3.5}{3.4, 3.5}};};
NOTE: when using an array, the values inside the square brackets [] [] indicate the indexes that you are looking at (i.e. the specific element).
A 2-D array is an array of arrays
Use a nested loop to access all elements of a 2-D array, loop counters represent row and column indexes to the array.
Obtaining Lengths of Obtaining Lengths of Multidimensional ArraysMultidimensional Arrays
for (int r=0; r<myList.length; r++){
for (int c = 0; c<myList[r].length; c++) {
print(myList[r][c]);
}
}
NOTE: The length determines the number of elementsmyList.length determines the number of rowsmyList[r].length determines the number of elements for a given row
Ragged ArraysRagged Arrays
When rows in a 2-d array can vary in When rows in a 2-d array can vary in sizesize
Int [] [] myList = {Int [] [] myList = {
{1, 2, 3},{1, 2, 3},
{1, 2},{1, 2},
{1}{1}
};};
Listing 6.12 – TotalScore Class
1) Declaring the reference variable
2) Creating the 3-d arrayd1 = student (7)d2 = exam (5)d3 = exam part (2)
3) Assigning the array’s memory location to the reference variable
How Variables are Kept in How Variables are Kept in MemoryMemory
Two categories of variablesTwo categories of variables ValueValue – contains actual data (integer, float, double, – contains actual data (integer, float, double,
boolean, or other primitive data type)boolean, or other primitive data type) ReferenceReference – contains a reference (pointer) to an object – contains a reference (pointer) to an object
or array that is located on the heap (as a result of using or array that is located on the heap (as a result of using the the newnew operator). operator).
Two different locations of memory (more to come Two different locations of memory (more to come later)later) FrameFrame StackStack – contains local variables and parameters – contains local variables and parameters
of methodsof methods HeapHeap – contains – contains objectsobjects and and arraysarrays The The newnew keyword always creates an object or array and keyword always creates an object or array and
places it in the heap.places it in the heap.
HeapFrame Stack
main’sframe
args
1) Declaring the reference variable
scores
3) Assigning the array’s memory location to the reference variable
20.5 22.5127.5 …
[0,0,0] [0,0,1]
[0,1,0] [0,1,1]
…
Note: 7 rows, 5 columns with 2 parts each for a total of 70 elements – only 4 are shown
Listing 6.12 – TotalScore Class
In a loop, total all of the exam parts together.
Display the total for each student.
Note:
3-D array loop nested 3-deep.
Program output:
Contents of memory at breakpoint
Note: a 2-dimensional array is implemented as an array of arrays. Each element of the first dimension is a reference to a single dimensional array.