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Control Flow Summary

Control flow. Sequence of statements that are

actually executed in a program. Conditionals and loops: enables us to

choreograph the control flow. Manipulate small amounts of data.

Straight-lineprograms

All statements areexecuted in the order given.

ConditionalsCertain statements are

executed depending on thevalues of certain variables.

ifif-else

LoopsCertain statements are

executed repeatedly untilcertain conditions are met.

whilefor

do-while

Control Flow Description Examples

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Manipulating More Data

Goal. 10 variables of the same type.

double a0, a1, a2, a3, a4, a5, a6, a7, a8, a9; a0 = 0.0;a1 = 0.0;a2 = 0.0;a3 = 0.0;a4 = 0.0;a5 = 0.0;a6 = 0.0;a7 = 0.0;a9 = 0.0;a9 = 0.0;

double x = a4 + a8;

1.4 Arrays

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Arrays

Arrays allow you to store and manipulate huge quantities of data.Program code remains concise.

Array. Indexed sequence of values of the same type.

Examples. 52 playing cards in a deck. 14 thousand undergrads at UVa. 1 million characters in a book. 10 million audio samples in an MP3 file. 4 billion nucleotides in a DNA strand. 1 trillion webpages crawled by Google!

horton0

gurumurthi1

humphrey2

knuth3

billg4

rms5

mst3k6

alvin7

index value

Ele

ments

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Arrays in Java

Java has special language support for arrays. To make an array: declare, create, and initialize it. To access element i of array named a, use a[i]. Array indices start at 0.

int N = 10; double[] a; // declare the arraya = new double[N]; // create the arrayfor (int i = 0; i < N; i++) // initialize the array a[i] = 0.0; // all to 0.0

public class HelloWorld { public static void main(String[] args) { System.out.println("Hello, World"); }}

HelloWorld.java

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Arrays in Java

Java has special language support for arrays. To make an array: declare, create, and initialize it. To access element i of array named a, use a[i]. Array indices start at 0.

Compact alternative. Declare, create, and initialize in one statement. Default initialization: all numbers automatically set to zero.

int N = 10; double[] a; // declare the arraya = new double[N]; // create the arrayfor (int i = 0; i < N; i++) // initialize the array a[i] = 0.0; // all to 0.0

int N = 10;double[] a = new double[N]; // declare, create, init

Initializing and Printing Arrays

• Alternative way of initializing an array:double[] a = {0.1, 0.2, 0.3};

• Printing an array:• INCORRECTSystem.out.print(a); //random looking junk• CORRECTSystem.out.print(a[3]); //print element a[3]• CORRECTfor(int i=0;i < N;i++) System.out.println(a[i]); //print the entire array

WHY DOES ARRAY MANIPULATION INVOLVE MORE WORK?

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Basic Computer Hardware

CentralProcessing

Unit(CPU)

Executes the machine-language instructions of the compiled program

Memory

Provides storage for program data

Stores the program and other large data as files

Disk

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Role of Memory

Provides storage for the data used by a program

Variables – One storage location per variable

Size of the location depends on the datatype of the variable

Arrays A sequence of variables of the same

type Need to store all the “variables” of the

array in memory as a single logical unit

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Memory

Memory is split into equal sized units known as “addresses”. (Address

Space)

Arrays occupy consecutive addresses in this linear space:

int[] a = {5, 6, 10};

Memory Allocation: Java allocates addresses for the array when you call new Key Point: Name of the array indirectly references its starting address.

int[] b = new int[3]; b=a;

//b references the SAME data in memory. BEWARE!

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0 1 2 . . . N-2 N-1

0 1 2 3 4 5 N-2 N-1 a[0] a[1] a[2] 5 6 10

Addresses

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Vector Dot Product

Dot product. Given two vectors x and y of length n, their dot

product is the sum of the products of their corresponding components.

1 2

1 2

1 1 2 2

1

[ , ,..., ]

[ , ,.., ]

. ...

n

n

n

i i n n

i

x x x

y y y

x y x y x y x y

x

y

x y

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Vector Dot Product

double[] x = { 0.3, 0.6, 0.1 }; double[] y = { 0.5, 0.1, 0.4 }; double sum = 0.0; for (int i = 0; i < 3; i++) { sum += x[i]*y[i]; // same as sum = sum + x[i]*y[i];}

1 2

1 2

1 1 2 2

1

[ , ,..., ]

[ , ,.., ]

. ...

n

n

n

i i n n

i

x x x

y y y

x y x y x y x y

x

y

x y

+=-=/=*=

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Array Processing Code

NOT i <= N

What DoesThis Code Do?

Dr. Java Demo of Array Reversal

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An array’s length is fixed

Each array keeps track of how many items it contains In length field. Example:

int[] list = { 3, -1, 0, 7, 5 } list.length == 5 // is true

Note use of “.” notation. More on this later, but for now think of it as “something inside of” the thing on the left

Important: an array’s length cannot change (grow, shrink)– Must know how many items will be stored when it’s created!

int target = Integer.parseInt(args[0]);int[] list = { -3, -1, 0, 3, 5, 9 };

// Find index of first occurrence of target in array int i = 0;while ( i < list.length && list[i] != target ) i = i + 1;if ( i != list.length ) System.out.println("Found at index " + i);

Auto-increment and arrays

Auto-increment and auto-decrement operators used with arrays. ++i means “add one to i’s value and use the updated value” i++ means “use i’s value now and then add one to the old

value” Can use on line by itself as a single statement

– In which case they do exactly the same thing! Differences between the auto-increment operators are

important when used in expressions: y = 0; x = y++; y = 0; z = ++y;

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int target = Integer.parseInt(args[0]);int[] list = { -3, -1, 0, 3, 5, 9 }; int i = 0;while ( i < list.length && list[i++] != target ) ; // empty loop body! if ( i != list.length ) System.out.println("Found at index " + (i-1) );

Y=0; x = y; y = y+1;Y=0; y = y+1; z = y;

Which Version of the Code is Better?

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int target = Integer.parseInt(args[0]);int[] list = { -3, -1, 0, 3, 5, 9 };

// Find index of first occurrence of target in array int i = 0;while ( i < list.length && list[i] != target ) i = i + 1;if ( i != list.length ) System.out.println("Found at index " + i);

int target = Integer.parseInt(args[0]);int[] list = { -3, -1, 0, 3, 5, 9 }; int i = 0;while ( i < list.length && list[i++] != target ) ; // empty loop body! if ( i != list.length ) System.out.println("Found at index " + (i-1) );