Thisdocumentiscopyright(C)StanfordComputerScienceandMartyStepp,licensedunderCreativeCommonsAttribution2.5License.Allrightsreserved.BasedonslidescreatedbyKeithSchwarz,MehranSahami,EricRoberts,StuartReges,andothers.

CS106A,Lecture9Problem-SolvingwithStrings

suggestedreading:JavaCh.8.5

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Learning Goals•Beabletowritestringalgorithmsthatoperateoneachcharacter.

•Beabletobuildupnewstringsfromexistingstringsusingbuilt-inStringmethods.

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Plan For Today•Recap:CharactersandStrings•LoopingoverStrings•Practice:ReversingaString•Practice:Palindromes•Practice:CaesarCipher

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Plan For Today•Recap:CharactersandStrings•LoopingoverStrings•Practice:ReversingaString•Practice:Palindromes•Practice:CaesarCipher

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Text Processing

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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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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 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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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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Remember:charsandlength-1stringsaredifferent!

char ch = 'A' DIFFERENT FROM String str = "A"

Strings vs. Chars

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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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char c1 = 'a';char c2 = 'b';

// How do we concatenate these characters?

String str = c1 + c2; // ERROR: this is an int!

String str = "" + c1 + c2; // ✔

From Chars to Strings

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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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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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SubstringsAsubstring isasubsetofastring.

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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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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Plan For Today•Recap:CharactersandStrings•LoopingoverStrings•Practice:ReversingaString•Practice:Palindromes•Practice:CaesarCipher

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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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Looping Over StringsAcommonStringprogrammingpatternisloopingoverastringandoperatingoneachcharacter.

// 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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Looping Over StringsAcommonStringprogrammingpatternisloopingoverastringandoperatingoneachcharacter.

// 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 = newStr + Character.toUpperCase(ch);

}println(newStr); // HELLO!

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Looping Over StringsAcommonStringprogrammingpatternisloopingoverastringandoperatingoneachcharacter.

// 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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Building Up New StringsAnothercommonStringprogrammingpatternisbuildingupanewstringbyaddingcharacterstoitovertime.

// Creates a new String in all capsString str = "";for (int i = 0; i < 5; i++) {

str += i;}println(str); // 01234

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Plan For Today•Recap:CharactersandStrings•LoopingoverStrings•Practice:ReversingaString•Practice:Palindromes•Practice:CaesarCipher

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Exercise: Reversing a StringLet’swriteamethodcalledreverseString thattakesoneStringparameter,andreturnsanewStringwiththecharactersintheoppositeorder.

reverseString("Hello!") -> "!olleH"

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H e l l o !

Reversing a String

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H e l l o !

!

Reversing a String

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H e l l o !

!

Reversing a String

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H e l l o !

! o

Reversing a String

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H e l l o !

! o

Reversing a String

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H e l l o !

! o l

Reversing a String

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H e l l o !

! o l

Reversing a String

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H e l l o !

! o l l

Reversing a String

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H e l l o !

! o l l

Reversing a String

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H e l l o !

! o l l e

Reversing a String

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H e l l o !

! o l l e

Reversing a String

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H e l l o !

! o l l e H

Reversing a String

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H e l l o !

! o l l e H

Reversing a String

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Reversing a StringString str = "Hello!";String newStr = "";for (??? ; ??? ; ???) {

...}

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Reversing a StringString str = "Hello!";String newStr = "";for (int i = str.length() - 1; ??? ; ???) {

...}

H e l l o !

!

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Reversing a StringString str = "Hello!";String newStr = "";for (int i = str.length() - 1; i >= 0; ???) {

...}

! o l l e H

H e l l o !

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Reversing a StringString str = "Hello!";String newStr = "";for (int i = str.length() - 1; i >= 0; i--) {

...}

! o l l e H

H e l l o !

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Reversing a StringString str = "Hello!";String newStr = "";for (int i = str.length() - 1; i >= 0; i--) {

newStr += str.charAt(i);}

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H e l l o !

Reversing a String

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H e l l o !

H

Reversing a String

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H e l l o !

He

Reversing a String

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H e l l o !

Hel

Reversing a String

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H e l l o !

Hell

Reversing a String

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H e l l o !

Hello

Reversing a String

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H e l l o !

Hello!

Reversing a String

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Reversing a Stringpublic void run() {

println("This program reverses a string.");String str = readLine("Enter a string: ");String rev = reverseString(str);println(str + " spelled backwards is " + rev);

}

ReverseString

str

STRESSED

This program reverses a string.

STRESSED spelled backwards is DESSERTSSTRESSEDEnter a string:

rev

DESSERTS

private String reverseString(String str) {String result = "";for ( int i = 0; i < str.length(); i++ ) {

result = str.charAt(i) + result;}return result;

}

istrresult

STRESSED 012345678STSRTSERTSSERTSSSERTSESSERTSDESSERTS

Using portions of slides by Eric Roberts

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Plan For Today•Recap:CharactersandStrings•LoopingoverStrings•Practice:ReversingaString•Practice:Palindromes•Practice:CaesarCipher

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Exercise: PalindromesLet’swriteamethodcalledisPalindrome thattakesoneStringparameter,andreturnswhetherornotthatStringisapalindrome(thesameforwardsandbackwards).

isPalindrome("racecar") -> trueisPalindrome("hi there") -> falseisPalindrome("kayak") -> true

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Plan For Today•Recap:CharactersandStrings•LoopingoverStrings•Practice:ReversingaString•Practice:Palindromes•Practice:CaesarCipher

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A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

D E F G H I J K L M N O P Q R S T U V W X Y Z A B C

• Rotatealphabetbyn letters(n =3inbelow)– n iscalledthekey

• Wrap-aroundattheend

• Substitutelettersbasedonthismapping

original

encrypt

Exercise: Caesar Cipher

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A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

D E F G H I J K L M N O P Q R S T U V W X Y Z A B C

• Rotatealphabetbyacertainkey,withwrapping

original

encrypt

Exercise: Caesar Cipher

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Recap•Recap:CharactersandStrings•LoopingoverStrings•Practice:ReversingaString•Practice:Palindromes•Practice:CaesarCipher

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