CS 106A, Lecture 8 - Stanford University

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CS106A,Lecture8CharactersandStrings

suggestedreading:JavaCh.8.1-8.4

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Learning Goals•Beabletoconfidentlywriteandcallmethodsthatuseparametersandreturnvalues.

•Beabletogeneraterandomvaluesinyourprograms.

•Beabletouseandmanipulatechars.•Beabletowritestringalgorithmsthatoperateoneachcharacter.

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Plan For Today•Announcements•Recap–Parameters–Return

•RandomNumbers•TextProcessing–Characters–Strings

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Parameters

Parametersletyouprovideamethodsomeinformationwhenyouarecallingit.

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Return

Returnvaluesletyougivebacksomeinformationwhen

amethodisfinished.

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Methods = Toasters

parameter

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Methods = Toasters

parameter

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Methods = Toasters

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Methods = Toasters

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Methods = Toasters

return

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Example: readInt

int x = readInt(”Your guess? ");

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Example: readInt


We give readInt some information (the text to

print to the user)

We call readInt

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Example: readInt


When we include values in the parentheses of a method call, this means we are passing them as parameters to this

method.

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Example: readInt


When finished, readInt gives us information back (the user’s number) and we put it in x.

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Example: readInt


When we set a variable equal to a method, this tells Java to save the return value of the method in that variable.

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Parameters Example: drawBox

private void drawBox(int width, int height) {// use width and height variables// to draw a box

}

Tells Java this method needs two ints in order to

execute.

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}

Inside drawBox, refer to the first parameter value

as width…

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}

…and the second parameter value as height.

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drawBox

drawBox(10, 4);

We give drawBox some information (the size of

the box we want)

We call drawBox

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drawBox

int width = readInt("Width? ");int height = readInt("Height? ");...

drawBox(width, height);

We give drawBox some information (the size of

the box we want)

We call drawBox

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drawBox

int width = readInt("Width? "); 7int height = readInt("Height? "); 4...

drawBox(width, height);

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drawBox


drawBox(width, height);7 4

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drawBox


drawBox(7, 4);

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drawBox

7 4

Second parameter to

drawBox

First parameter to drawBox

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drawBox

private void drawBox(int width, int height) {

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drawBox


}

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drawBox

private void drawBox(int width, int height) {...println(width); // prints 7println(height); // prints 4...

}

7 4

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Parameter Names

Parameternamesdonotaffectprogrambehavior.

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Parameter Namespublic void run() {

int width = readInt("Width? "); 7int height = readInt("Height? "); 4drawBox(width, height);

}

private void drawBox(int width, int height) {...

}

run

width height

7 4drawBox

width height

7 4

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Parameter Namespublic void run() {

int width = readInt("Width? "); 7int height = readInt("Height? "); 4drawBox(width, height);

}

private void drawBox(int w, int h) {...

}

run

width height

7 4drawBox

w h

7 4

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Return Example: metersToCm

private double metersToCm(double meters) {...

}

When this method finishes, it will return a double.

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private double metersToCm(double meters) {double centimeters = meters * 100;return centimeters;

}

Returns the value of this expression (centimeters).

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public void run() {double cm = metersToCm(10);...

}

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public void run() {double cm = metersToCm(10);...

}

Setting a variable equal to a method means we save the method’s return

value in that variable.

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Return Example: metersToCmpublic void run() {

double meters = readDouble("# meters? ”);...

double cm = metersToCm(meters);println(cm + " centimeters.");

}


}

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}


}

7

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}


}

7

7

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}


} 700

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7

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}

700

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Return Values and Expressionspublic void run() {

double meters = readDouble("# meters? ”);println(metersToCm(meters) + " cm.");

}


}

7

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Return Values and Expressionspublic void run() {

double meters = readDouble("# meters? ”);println(metersToCm(meters) + " cm.");

}


}

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You can use a method’s return value directly in an expression.

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Buggy Example!public void run() {


metersToCm(meters); // Does nothing!...

}

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Buggy Example!public void run() {


metersToCm(meters); // Does nothing!...

}

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700

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private int max(int num1, int num2) {if(num1 >= num2) {

return num1;}return num2; // here only if num1 < num2

}

Return Stops Method Execution

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Returning Booleans

private boolean isEven(int number) {return number % 2 == 0;

}

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Returning Booleans


}

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Returning Booleans


}

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true

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Returning Booleans


}

// Examplepublic void run() {

if (isEven(2)) {...

}}

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Returning Booleans

private boolean isDivisibleBy(int a, int b) {return a % b == 0;

}

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Returning Booleans


}

24 9

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Returning Booleans


}

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false

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Returning Booleans


}

// Examplepublic void run() {

if (isDivisibleBy(4, 2)) {...

}}

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Factorial Code Walkthrough

Seenextslides

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public void run() {for(int i = 0; i < MAX_NUM; i++) {

println(i + "! = " + factorial(i));}

}i

4

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}0i

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}0i

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}0i

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}0i

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}0

i

private int factorial(int n) {int result = 1;for (int i = 1; i <= n; i++) {

result *= i;}return result;

}

result0n i

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}0

i



}

1result0n i

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}0

i



}

1result0n 1i

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}0

i



}

1result0n 1i

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}0

i



}

1result0n 1i

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}0i

1

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}0i

1

0! = 1

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}1i

0! = 1

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}1i

0! = 1

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}1i

0! = 1

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}1i

0! = 1

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}0

i



}

result1n i

0! = 1

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}0

i



}

1result1n i

0! = 1

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}0

i



}

1result1n 1i

0! = 1

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}0

i



}

1result1n 1i

0! = 1

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}0

i



}

1result1n 1i

0! = 1

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}0

i



}

1result1n 2i

0! = 1

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}0

i



}

1result1n 2i

0! = 1

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}0

i



}

1result1n 2i

0! = 1

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}1i

1

0! = 1

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}1i

1

0! = 11! = 1

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}2i

0! = 11! = 1

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}2i

0! = 11! = 1

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}2i

0! = 11! = 1

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}2i

0! = 11! = 1

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}2i

2

0! = 11! = 1

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}2i

2

0! = 11! = 12! = 2

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}3i

0! = 11! = 12! = 2

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}3i

0! = 11! = 12! = 2

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}3i

0! = 11! = 12! = 2

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}3i

0! = 11! = 12! = 2

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}3i

6

0! = 11! = 12! = 2

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}3i

6

0! = 11! = 12! = 23! = 6

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}4i

0! = 11! = 12! = 23! = 6

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}4i

0! = 11! = 12! = 23! = 6

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RandomGenerator• import acm.util.*;

// random number from 0-9 inclusiveint digit = RandomGenerator.getInstance().nextInt(0, 9);println(digit);

// prints "hello!" between 3-6 timesint times = RandomGenerator.getInstance().nextInt(3, 6);for (int i = 0; i < times; i++) {

println("hello!");}

Method DescriptionRandomGenerator.getInstance().nextInt(min, max) arandomintegerinthe

givenrange,inclusive

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RandomGeneratorThe RandomGenerator class defines the following methods:

int nextInt(int low, int high)Returns a random int between low and high, inclusive.

int nextInt(int n)Returns a random int between 0 and n-1.

double nextDouble(double low, double high)Returns a random double d in the range low ≤ d < high.

double nextDouble()Returns a random double d in the range 0 ≤ d < 1.

boolean nextBoolean()Returns a random boolean value, which is true 50 percent of the time.

boolean nextBoolean(double p)Returns a random boolean, which is true with probability p, where 0 ≤ p ≤ 1.

Color nextColor()Returns a random color.

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Extra: Dice exercise• WriteaconsoleprogramRollTwoDice thatrepeatedlyrollstwo6-sideddiceuntiltheyarriveatagivendesiredsum.

Desired sum? 93 and 4 = 72 and 1 = 35 and 5 = 106 and 2 = 86 and 5 = 114 and 5 = 9

• TrysolvingthisonyourownonCodeStepByStep!

RollTwoDice

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Text Processing

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Goal

After learning text processing, we will be able to write our own encryption program!

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CharAchar isavariabletypethatrepresentsasinglecharacteror“glyph”.

char letterA = 'A';char plus = '+';char zero = '0';char space = ' ';char newLine = '\n';char tab = '\t';char singleQuote = '\'';char backSlash = '\\';

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CharUnderthehood,Javarepresentseachchar asaninteger (its“ASCIIvalue”).

•Uppercaselettersaresequentiallynumbered•Lowercaselettersaresequentiallynumbered•Digitsaresequentiallynumbered

char uppercaseA = 'A'; // Actually 65char lowercaseA = 'a'; // Actually 97char zeroDigit = '0'; // Actually 48

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Char Math!WecantakeadvantageofJavarepresentingeachchar asaninteger (its“ASCIIvalue”):

boolean areEqual = 'A' == 'A'; // trueboolean earlierLetter = 'f' < 'c'; // falsechar uppercaseB = 'A' + 1;int diff = 'c' - 'a'; // 2int numLettersInAlphabet = 'z' – 'a' + 1;// orint numLettersInAlphabet = 'Z' – 'A' + 1;

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Char Math!WecantakeadvantageofJavarepresentingeachchar asaninteger (its“ASCIIvalue”):

// prints out every characterfor (char ch = 'a'; ch <= 'z'; ch++) {

print(ch);}

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Char Math!Noteveryintegermapstoacharacter.Sowhenyouhaveanexpressionwithints andchars,Javapicksint asthemostexpressivetype.

'A' + 1 // evaluates to 66 (int)'c' + (2*5) – 1 // evaluates to 108

Wecanmakeitacharbyputtingitinacharvariable.char uppercaseB = 'A' + 1;// orchar uppercaseB = 66;

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Side Note: Type-castingIfwewanttoforceJavatotreatanexpressionasaparticulartype,wecanalsocastit tothattype.

'A' + 1 // evaluates to 66 (int)(char)('A' + 1) // evaluates to 'B' (char)

1 / 2 // evaluates to 0 (int)(double)1 / 2 // evaluates to 0.5 (double)1 / (double)2 // evaluates to 0.5 (double)

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Character MethodsTherearesomehelpfulbuilt-inJavamethodstomanipulatechars.

char lowercaseA = 'a';char uppercaseA = Character.toUpperCase(lowercaseA);

char plus = '+';if (Character.isLetter(plus)) {

...}

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Character MethodsMethod Description

Character.isDigit(ch) trueifch is'0' through'9'

Character.isLetter(ch) trueifch is'a' through'z' or'A' through'Z'

Character.isLetterOrDigit(ch) trueifch is'a' through'z','A' through'Z' or '0' through'9'

Character.isLowerCase(ch) trueifch is'a' through'z'

Character.isUpperCase(ch) trueifch is'A' through'Z'

Character.toLowerCase(ch) returnslowercaseequivalentofaletter

Character.toUpperCase(ch) returnsuppercaseequivalentofaletter

Character.isWhitespace(ch) trueifch isaspace,tab,newline,etc.

Remember: these returnthe new char, they cannot modify an existing char!

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Character MethodsRemembertoalwayssavethereturnvalueofCharactermethods!

char lowercaseA = 'a';Character.toUpperCase(lowercaseA); // Does nothing!println(lowercaseA); // prints ’a’!

char uppercaseA = Character.toUpperCase(lowercaseA); // OK

println(uppercaseA); // prints ’A’!

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StringsAString isavariabletyperepresentingasequenceofcharacters.

String text = "Hi parents!";

– Eachcharacterisassignedanindex,goingfrom0tolength-1– Thereisachar ateachindex

index 0 1 2 3 4 5 6 7 8 9 10

character 'H' 'i' ' ' 'p' 'a' 'r' 'e' 'n' 't' 's' '!'

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String str = "Hello, world!";String empty = "";println(str);

// Read in text from the userString name = readLine("What is your name? ");

// String concatenation (using “+”)String message = 2 + " cool " + 2 + " handle";int x = 2;println("x has the value " + x);

Creating Strings

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String str = "Hello, world!";

// Lengthint strLength = str.length(); // 13

// Access individual characterschar firstLetter = str.charAt(0);char lastLetter = str.charAt(strLength – 1);char badTimes = str.charAt(strLength); // ERROR

Common String Operations

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SubstringsAsubstring isasubsetofastring.

String str = "Hello, world!";String hello = str.substring(0, 5);

0 1 2 3 4 5 6 7 8 9 10 11 12'H' 'e' 'l' 'l' 'o' ',' ' ' 'w' 'o' 'r' 'l' 'd' '!'

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String str = "Hello, world!";String worldExclm = str.substring(7, 13);

0 1 2 3 4 5 6 7 8 9 10 11 12'H' 'e' 'l' 'l' 'o' ',' ' ' 'w' 'o' 'r' 'l' 'd' '!'

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String str = "Hello, world!";String worldExclm = str.substring(7); // to end

0 1 2 3 4 5 6 7 8 9 10 11 12'H' 'e' 'l' 'l' 'o' ',' ' ' 'w' 'o' 'r' 'l' 'd' '!'

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String MethodsMethodname Description

s.length() numberofcharactersinthisstring

s.charAt(index) charatthegivenindex

s.indexOf(str) indexwherethestartofthegivenstringappearsinthisstring(-1ifnotfound)

s.substring(index1, index2)ors.substring(index1)

thecharactersinthisstringfromindex1 (inclusive)toindex2 (exclusive);ifindex2 isomitted,goesuntilend

s.toLowerCase() anewstringwithalllowercaseletterss.toUpperCase() anewstringwithalluppercaseletters

• Thesemethodsarecalledusingdotnotation:

String className = "CS 106A yay!";println(className.length()); // 12

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Strings are ImmutableOnceyoucreateaString,itscontentscannotbechanged.

// Cannot change individual chars in the stringString typo = "Hello, warld!”;

TochangeaString,youmustcreateanew Stringcontainingthevalueyouwant(e.g.usingStringmethods).

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Strings are ImmutableString className = "cs 106a";className.toUpperCase(); // does nothing!

className = className.toUpperCase(); // ✔println(className); // CS 106A

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Comparing StringsString greeting = "Hello!”;if (greeting == "Hello!") { // Doesn’t work!

...}

// Instead:if (greeting.equals("Hello!")) {

...}

Always use .equals instead of == and !=

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Comparing StringsMethod Description

s1.equals(s2) whethertwostringscontainthesamecharacters

s1.equalsIgnoreCase(s2) whethertwostringscontainthesamecharacters,ignoringuppervs.lowercase

s1.startsWith(s2) whethers1 containss2’scharactersatstart

s1.endsWith(s2) whethers1 containss2’scharactersatend

s1.contains(s2) whethers2 isfoundwithins1

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Looping Over StringsAcommonStringprogrammingpatternisloopingoverastringandoperatingoneachcharacter.

String str = "Hello!";for (int i = 0; i < str.length(); i++) {

char ch = str.charAt(i);// Do something with ch here

}

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// Prints out each letter on a separate lineString str = "Hello!";for (int i = 0; i < str.length(); i++) {

char ch = str.charAt(i);println(ch);

}

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// Creates a new String in all capsString str = "Hello!";String newStr = "";for (int i = 0; i < str.length(); i++) {

char ch = str.charAt(i);newStr += Character.toUpperCase(ch);

}println(newStr); // HELLO!

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Recap•Recap–Parameters–Return


Nexttime:problem-solvingwithStrings

CS 106A, Lecture 8 - Stanford University

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