This week in CS 5

• HW 9 (2 problems)

M/T sections

W/Th sections

due Sunday, 11/4 at midnight

due Monday, 11/5 at midnight

Recitation for HW9 -- Friday 11/2

A dizzying array of possibilities...

Reading: Week 9’s online notes

This week’s honorees are truly frightening...

John Conway

Carl Gauss

“Pass By Value”

public static void main(String[] args){ int age = 18; H.out.print(“After a semester at HMC, you’ll feel ”);

change(age);

H.out.println(age + “ years old.”);}

public static void change(int age){ age = 42;

return;}

PASSBY

VALUE

“Pass by value” means that data is copied when sent to a method

Passing Arrays by Value

public static void main(String[] args){ int[] age = new int[2]; age[0] = 18; age[1] = 19;

H.out.print(“After a semester at HMC, you’ll feel ”);

change(age);

H.out.println(age[0] + “ or ” + age[1] + “ years old.”);}

public static void change(int[] age){ age[0] = 42; age[1] = 42;

return;}

Watch out!

public static void main(String[] args){ int[] age = new int[2]; age[0] = 18; age[1] = 19;

H.out.print(“After a semester at HMC, you’ll feel ”);

change(age);

H.out.println(age[0] + “ years old.”);}

public static void change(int[] age){ age = new int[2]; age[0] = 42; age[1] = 42; return;}

Arrays and Methods

Engineers think their equations are an approximation to reality. Physicists think reality is an approximation to their equations. Mathematicians don't care. 

Simple Facts -> build structures from them

Simple Actions (computations) -> build structures from them

2d arrays

Arrays can be of ANY type -- even other arrays!

double[] arr;

double[][] arr;

arr = new double[nRows][nCols];

arr[0][2] = 10.0;

2d arrays: Input and Output

public static void main(String[] args){ double[][] arr = new int[3][4];

// getting the array from input for (int r=0 ; r<3 ; ++r) { for (int c=0 ; c<4 ; ++c) { bigarray[r][c] = H.in.nextDouble(); } }

Using 2d arrays

picture before & picture after -- fill in the code...

Problem 1

1 Display prices2 Compute average of prices3 Compute variance of prices4 Display index and value of lowest price5 Display index and value of highest price6 Your TTS investment strategy

9 Quit

Which choice would you like?

Menu

An array of different methods...

Initial Input Get the number of stock prices from the user

Then, get each stock price from the user into the array.

Create an array of the appropriate number of elements.

Problem 1

How do we do this ? Pseudocode...

Methods

What does this code do ?

public static void main(String[] args){

}

public static double sumArray(double[] arr){ double sum = 2.0; sum = sum + 40.0; return sum;}

Calling Methods

What happens back in main()?

Method notes

double averageArray(double[] arr)

double variance(double[] arr)

int indexOfSmallest(double[] arr)

int indexOfLargest(double[] arr)

Problem 2

0 1 2 3 4 5

0 1 2 3 4 5

0 1 2 3 4 5

A starting row of lights

2 is selected Each turn, a light is selected -- It and its neighbors switch states.

Goal: get all the lights off…

on on off off off on

Problem 2

What is a light ?

0 1 2 3 4 5

How should we represent these lights in Java ?

on on off off off on

Lights Out -- Printing

// draw the current set of lights

0 1 2 3 4 5

| |****|****|****| |****| | |****|****|****| |****| | |****|****|****| |****| | |****|****|****| |****|

0 1 2 3 4 5 6 7

lights should be separated with vertical bars

may display all light numbers up to 15

print light numbers close to the center of each light

“off” lights should be 4x4 blocks of spaces

“on” lights should be 4x4 blocks of stars

H.out.println(“Type the number of words: ”);int len = H.in.nextInt();

String[] quip; quip = new String[len];

for (int i=0 ; i<len ; ++i){ quip[i] = H.in.nextWord();}

for (int i=len-1 ; i>=0 ; --i){ H.out.print( quip[i] + “ ” );}

Arrays in pictures

int len

String[] quip

quip[0]quip[1]

quip[2] quip[4]quip[3]

5

fall

leaves

after

leaves

fall

i is 4 i is 3 i is 2 i is 1 i is 0

fall leaves after leaves fall

public static int numSyllables(String w){ int numSyls = 0; int len = w.length();

if ( isVowel(w.charAt(0)) ) // an initial vowel ? ++numSyls;

for (int i=1 ; i<w.length() ; ++i) { // vowel preceded by a consonant if ( isVowel(w.charAt(i))&& !isVowel(w.charAt(i-1)) ) ++numSyls; } // final ‘e’ preceded by a consonant if ( w.charAt(len-1) == 'e’ && len >= 2 && !isVowel(w.charAt(len-2)) ) --numSyls;

if (numSyls < 1) // every word has at least 1 syllable numSyls = 1;

return numSyls;}

Syllable counting

Take in a number of words and print them out in reverse order. (To be specific, suppose it’s 5 words.)

A puzzle...

Take in a number of words and print them out in reverse order. (To be specific, suppose it’s 5 words.)

A puzzle...

String s1 = H.in.nextWord();String s2 = H.in.nextWord();String s3 = H.in.nextWord();String s4 = H.in.nextWord();String s5 = H.in.nextWord();

H.out.print( s5 + “ ” );H.out.print( s4 + “ ” );H.out.print( s3 + “ ” );H.out.print( s2 + “ ” );H.out.print( s1 + “\n” );

Not a very flexible solution...

Arrays - lists of data items

String[] quip;

quip = new String[5];

for (int i=0 ; i<5 ; ++i){ quip[i] = H.in.nextWord();}

declares a String array named quip

declares five Strings named quip[0]…quip[4]

loop through the array

index

Arrays in code

H.out.println(“Type the number of words: ”);int len = H.in.nextInt();

String[] quip; // create an empty arrayquip = new String[len]; // create array elements

for (int i=0 ; i<len ; ++i){ quip[i] = H.in.nextWord(); // input each element}

// now print them out in reverse order…

Sentence palindromes

bores are people that say that people are bores

fall leaves after leaves fall

you can cage a swallow, can’t you, but you can’t swallow a cage, can you?

First Ladies rule the state and state the rule, “Ladies First!”

Strings

Be careful not to go out of bounds!

Element typesdouble, int, String,boolean, char, (any type) … char

The ith element

Example Declaration

Length

arr[i] s.charAt(i)

double[] arr;arr = new double[100];

String s;

java.lang.StringIndexOutOfBoundsException: -1

java.lang.ArrayIndexOutOfBoundsException: -1

arr.length s.length()

Warning

Range from 0 up to length-1

vsArrays

T. T. Securities

Input stock prices for a number of days in a row, and then analyze that data… .

1 Display prices2 Compute average of prices3 Compute standard deviation of prices4 Display index and value of lowest price5 Display index and value of highest price6 Your TTS investment strategy

9 Quit

Which choice would you like?

Menu:

Arrays and Methods

public static double sumArray(double[] arr)

Using sumArray

public static void main(String[] args){ // prompt for and input nStocks double[] stocks = new double[nStocks]; // input and assign each stocks[i] double stockSum = sumArray(stocks); H.out.println(“The sum is ” + stockSum);}

public static double sumArray(double[] arr){ // see previous page … return sum;}

double[] stocks

90.0 10.0 60.0 42.0 75.0 70.0

double[] arr

Using sumArray

public static void main(String[] args){ // prompt for and input nStocks double[] stocks = new double[nStocks]; // input and assign each stocks[i] double stockSum = sumArray(stocks); H.out.println(“The sum is ” + stockSum);}

public static double sumArray(double[] arr){ // see previous page … return sum;}

double[] stocks

90.0 10.0 60.0 42.0 75.0 70.0

double[] arr

2 references referring to the same list of data

Array Searching

public static double findMax(double[] arr)

T. T. Securities

Find the most profitable strategy for buying and selling the stock among the prices in the array...

Day 0 Price is 90.0Day 1 Price is 10.0Day 2 Price is 60.0Day 3 Price is 42.0Day 4 Price is 75.0Day 5 Price is 70.0

Lights Out !

0 1 2 3 4 5

0 1 2 3 4 5

0 1 2 3 4 5

A starting row of lights

2 is selected Each turn, a light is selected -- It and its neighbors switch states.

Goal: get all the lights off…

on on off off off on

Lights Out !

Features of the game:

// allow the user to set the // number of lights from 3 to 15

// start each light randomly on or off

// draw the current set of lights

// allow the user to select a light// only allow valid lights !

// update the set of lights and repeat

Lights Out !

// draw the current set of lights

0 1 2 3 4 5

| |****|****|****| |****| | |****|****|****| |****| | |****|****|****| |****| | |****|****|****| |****|

0 1 2 3 4 5 6 7

lights should be separated with vertical bars

may display all light numbers up to 15

print light numbers close to the center of each light

“off” lights should be 4x4 blocks of spaces

“on” lights should be 4x4 blocks of stars

Lights Out !

// allow the user to select a light

// only allow valid lights !