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Arrays. CSC 171 FALL 2002 LECTURE 20. Arrays. Suppose we want to write a program that reads a set of test grades and prints them, marking the highest grade? 65.2 81.7 31.3 95.4 < - highest grade 76.1 58.6. Individual data items.

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Arrays l.jpg

Arrays

CSC 171 FALL 2002

LECTURE 20


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Arrays

  • Suppose we want to write a program that reads a set of test grades and prints them, marking the highest grade?

    65.2

    81.7

    31.3

    95.4 < - highest grade

    76.1

    58.6


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Individual data items

  • If we knew that there were always 150 students in the class, we could store the data as individual variables

    • score1,score2,score3, . . , score150

  • What would the data entry code look like?

  • What would the printing code look like?

  • How about finding the max?


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Arrays

  • An array is a collection of data items of the same type

  • Every element of the collection can be accessed separately.


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___________________ is a fixed length sequence of values of the same type.


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_An array__________ is a fixed length sequence of values of the same type.


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Constructing Arrays

double [] data = new double[10];


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double [] data = new double[10];

int x = data.length ;

//instance field on arrays, x = = 10


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Setting Array values

  • To get values into an array you need to specify which slot you want to use.

  • Specification is done with the [ ] operator

  • The [ ] operator follows the name of the array

  • The [ ] operator encloses and integer-valued expression called the index or subscript


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Setting array values

data[4] = 29.95;

0

1

2

3

4

5

6

7

8

9


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Using array values

  • Similar to setting

    int i = 4 ;

    System.out.println(“data[“+i+”] ==“+ data[i]);

    > data[4] ==29.95


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You access array elements with and integer position number, called the _______________, using the notation _______________.


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You access array elements with and integer position number, called the ___index_______, using the notation ____a[index]___________.


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Array data items

  • Suppose we want to write a program that reads a set of test grades and prints them, marking the highest grade?

  • What would the data entry code look like?

  • What would the printing code look like?

  • How about finding the max?


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Array issues

  • Does it work?

    double [] data = new double[10];

    data[10] = 5.4;


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Array issues

  • Does it work?

    double [] data = new double[10];

    data[10] = 5.4;

    When the program runs, an out-of-bounds subscript generates an exception and terminates the program – why?


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Position numbers of an array range from _________ to ____________.


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Position numbers of an array range from ___0_______ to ____a.length -1_________.


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Accessing a nonexistent position results is a ______________ error.


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Accessing a nonexistent position results is a _bounds_____ error.


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Use the ___________________field to find the number of elements in an array.


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Use the _____length______field to find the number of elements in an array.


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Array issues

  • Can we search for the top grade as follows

    double maxScore = data[0];

    for (int i = 1;I<=data.length;i++)

    If (data[i] > maxScore)

    maxScore = data[i];


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Array issues

  • An array of length n has index values from 0 to (n-1)

    double maxScore = data[0];

    for (int i = 1;I<data.length;i++)

    If (data[i] > maxScore)

    maxScore = data[i];


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Array issues

  • Does it work?

    public static void main(String[] args){ double[] data;

    If (data[0] > 4.0)

    System.out.println(“GT 4!”);

    }


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Array issues

  • Arrays must be allocated!

    public static void main(String[] args){ double[] data = new double[10];

    If (data[0] > 4.0)

    System.out.println(“GT 4!”);

    }


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Array issues

  • Arrays can be initialized!

    public static void main(String[] args){ double[] data = {2,3,4,5,6};

    If (data[0] > 4.0)

    System.out.println(“GT 4!”);

    }

    // note: new int[] {2,3,4,5,6} ; is also legal


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Copying Arrays

Is this ok?

double [] data = new double[10];

double[] testScores;

testScores = data;


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Copying Arrays

Is this ok?

double [] data = new double[10];

double[] testScores;

testScores = data;

How many arrays are there?


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Copying Array REFERENCES

double [] data = new double[10];

double[] testScores;

testScores = data;


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Copying Arrays

  • So, what if we want to make two “real” copies – what does the code look like?

  • Write a method so that

    int[] x = {3,4,5,6,7,8,9,10};

    int[] y = myCopy(x);


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Copying

public static int[] myCopy(int[] x){

int[] r_arry = new int[x.length];

for (int i = 0 ; i< x.length;i++)

r_arry[i] = x[i];

return r_arry;

}


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System.arrayCopy

//System.arraycopy(from,fromstart,to,toStart,count);

System.arraycopy(data,0,prices,0,data.length);


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Clone


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An array variable stores a ________________________. Copying the variable yields a second ____________________ to _________________ array.


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An array variable stores a ______reference__________. Copying the variable yields a second ______reference______________ to ____the same_____________ array.


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Use the ________________ method to copy the elements of an array.


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Use the ____clone______ method to copy the elements of an array.


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Use the _____________________________ method to copy elements from one array to another.


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Use the _System.arraycopy____________________ method to copy elements from one array to another.


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If you run out of space in an array you need to _______________ a larger array and copy the elements into it.


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If you run out of space in an array you need to __allocate_____ a larger array and copy the elements into it.


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Getting a Bigger Array


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More Arrays

Suppose we want to write a program that reads a set of test product names, prices, and quality scores prints them, marking the best value? (score/prices)

Digital 500X, $3499.00, score 73

ZEOS Pentium-III/500, $2545.00, score 70

Micro Express MF, $2195.00, score 72 < - best value

Polywell Poly 450IP, $2099.00, score 30


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Parallel Arrays

  • One solution, can you think of a better one?


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Arrays of Objects

  • Easier to deal with - arrays hold references


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Avoid ___________________________ arrays by changing them into arrays of objects.


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Avoid ____________parallel_______ arrays by changing them into arrays of objects.


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Multidimensional Arrays

  • Arrays of arrays

    • Arrays are objects

    • Arrays hold references to objects

    • Ergo, arrays can hold arrays


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Tables are 2D arrays

// how easy to modify code ????

int[][] mtable = new int[5][5];

for(i=0;i<5;i++)

  • for(int j=0;j<5;j++)

    mtable[i][j] = (i+1)*(j+1);

int size = 5;

int[][] mtable = new int[size][size];

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

for(int j=0;j<size;j++)

mtable[i][j] = (i+1)*(j+1);


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Arrays of Arrays

int [][] powers = new int[10][10];


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Arrays of Arrays

int [][] pow2 = {{1,2,3},{4,5,6},{7,8,9}};


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Allocating arrays of arrays

int size = 10;

int[][] mtable = new int[size][];

for(i=0;i<mtable.length;i++) {

mtable[i]= new int[size];

for(int j=0;j<mtable[i].length;j++)

mtable[i][j] = (i+1)*(j+1);

}


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Alternately

  • The following is legal.

  • What is the structure?

    int[][] b = new int[5][];

    for (int i=0;i<b.length;i++)

    b[i] = new int[i+1];


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Alternately

int[][] b = new int[5][];

for (int i=0;i<b.length;i++)

b[i] = new int[i+1];

This is known as a triangular array

Is b[3][1] a legal reference or b[1][3]?


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Two dimensional arrays form a ___________________________ arrangement. You access elements with an index pair using the notation ______________.


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Two dimensional arrays form a _tabular, two dimensional_______ arrangement. You access elements with an index pair using the notation _____a[i][j]_________.


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Passing Arrays

  • In JAVA

    • Primitives are passed by value

      • A copy of the variable is made & used

      • Modifications made do not affect calling value

      • public void myAdd(int x) { x++;}

    • Objects are passed by reference

      • Since the reference is passed it is possible to change the value in the calling method

      • Public void myAdd (int[] x) {x[0]++;}


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Pass by value

public class passArry {

public static void main(String args[]){

int x1 = 3;

System.out.println("x1 == "+x1);

myAdd(x1);

System.out.println("x1 == "+x1);

}

public static void myAdd(int x) {x++;}

}


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Pass by reference

public class passArry {

public static void main(String args[]){

int[] x2 = {4,5,6};

System.out.println("x2[0] == "+x2[0]);

myAdd(x2);

System.out.println("x2[0] == "+x2[0]);

}

public static void myAdd(int[] x) {x[0]++;}

}


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Local reference

class Student {

int age;

// …..

public void setAge(int age){

age = age;

}

}


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Local reference

class Student {

int age;

// …..

public void setAge(int age){

this.age = age;

}

}


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Exercise

  • Write a method that takes an integer array and prints the values


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Exercise

  • Write a method that takes an integer array and prints the values

    public static int myPrint(int[] x){

    for (int i = 0 ; i<x.length;i++)

    System.out.println(x + “[“+i+”] == “+ x[i]);

    }


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Exercise

  • Write a method that takes an integer array, and two integer indices and swaps the value


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Exercise

Write a method that takes an integer array, and two integer indices and swaps the value

public static void mySwap(int[] x, int i, int j){

int temp = x[i];

x[i] = x[j];

x[j] = temp;

}


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Interpretive Dance

Some people are “visual” learners

Some people are “aural” learners

Some people are “symbolic” learners

Some people are “body” learners

Every kindergardener learns how to sort.


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Sorting

Write a method that takes an array and sorts via the kindergarden method.


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Bubblesort

Write a method that takes an array and sorts via the kindergarden method.

public static void bubbleSort(int[] x){

for (int j=0;j<x.length;j++)

for (int i = 0 ; i<x.length-1;i++)

if (x[i] > x[i+1]) mySwap(x,i,i+1);

}

// how many comparisions do we make?


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Counting comparisons

If the length of the array is “n”

The outer loop executes “n” time

Each time the outer loop executes, the inner loop executes “(n-1)” time

So, we get n(n-1) comparisions


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Exercise: SEARCH

  • Write a method that takes an integer array and returns the index of the maximum value


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Exercise

  • Write a method that takes an integer array and returns the index of the maximum value

    public static int myMax(int[] x){

    int rvalue = 0;

    for (int i = 0 ; i<x.length;i++)

    if (x[i] > x[rvalue]) rvalue = i;

    return rvalue;

    }


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Exercise

  • Overload the max finder to take an array and an index – the method now returns the index of the maximum value >= the index passed in


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Exercise

  • Overload the max finder to take an array and an index – the method now returns the index of the maximum value >= the index passed in

    public static int myMax(int[] x, int j){

    int rvalue = j;

    for (int i = j ; i<x.length;i++)

    if (x[i] > x[rvalue]) rvalue = i;

    return rvalue;

    }


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Sorting

  • Write a method that takes an integer array

  • Loop through all the positions in the array, one after the other

  • At each (current) position, find the max from that position to the rest of the array.

  • Swap the value with the current position

  • This is termed selection sort


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Selection Sort

Does it work?

public void selSort(int x){

for (int i = 0 ; i<x.length;i++){

int temp = myMax(x,i);

mySwap(x,i,temp);

}

}

Can you prove it works?

What is the loop invariant?


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{1,9,2,8,3,7,4,6,5}

| ^

{9,1,2,8,3,7,4,6,5}

| ^

{9,8,2,1,3,7,4,6,5}

| ^

{9,8,7,1,3,2,4,6,5}

| ^

{9,8,7,6,3,2,4,1,5}

| ^

{9,8,7,6,5,2,4,1,3}

| ^

{9,8,7,6,5,4,2,1,3}

| ^

{9,8,7,6,5,4,3,1,2}

| ^

Swap max “^” with first “|”


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Run time analysis

  • How long does it take SectionSort to run on an array of a given length?


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Count the loops

  • n is the length of the array

  • The outer (i) loop executes from 0 to n-2

    • n-1 times

  • The on each cycle of the outer loop i has a certain (different) value

  • On each cycle of the outer loop, the inner loop executes from I + 1 to n –1

    • n – i – 1 times


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So, we get

N-2

S (n-i-1)

i=0


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Can we prove ?

N-2

S (n-i-1) = =

i=0

n(n-1)/2


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Array Lists

  • Consider Purse class

  • Purse doesn't remember individual Coin objects, just the total

  • Don't know how many coins--can't have variables coin1...coin10

  • Use ArrayList to store variable number of objectsArrayList coins = new ArrayList();coins.add(new Coin(0.1, "dime"));. . .

  • size method yields number of elements


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An ______________________ is a sequence of objects.


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An _array list______ is a sequence of objects.


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Retrieving Array List Elements

  • Use get method

  • Index starts at 0

  • Must cast to correct type

  • Coin c = coins.get(0); // gets first coin

  • Bounds error if index is out of range

  • Most common bounds error:int n = coins.size();c = (Coin)coins.get(n); // ERROR // legal index values are 0...n-1


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To store a primitive type in an array list you must use a ____________ class.


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To store a primitive type in an array list you must use a __wrapper__ class.


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When retrieving an element from an array list, you need to ________________ the return value of the ________________ method to the element class.


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When retrieving an element from an array list, you need to __cast____ the return value of the _______get__ method to the element class.


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Position number of an array list range from _________ to _______________. Accessing a nonexistent position results in a _________ error.


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Position number of an array list range from _____0____ to ________size() - 1_______. Accessing a nonexistent position results in a ___counds______ error.


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