Strings CSE 1310 – Introduction to Computers and Programming Alexandra Stefan University of Texas at Arlington

Summary String type, String objects Concatenation: + Escape sequences: \\, \n (vs %n), \t, \" Methods: length, charAt, indexOf, Discuss signature and method call int len = "hello".length(); // len is now 5 char c = "hello".charAt(1); // c is 'e' int idx = "hello".indexOf("ll"); // idx is now2 Null Strings Error: Index out of bound See line number with the error Do not use == to compare strings! Use compareTo next() vs nextLine() Substring Substring vs char

Problems Solving problems: User enters name, print initial: Alexandra -> A User enters name, correct it: alexandra stefan-> Alexandra Stefan Horoscope: Recognize if a date is under a specific sign (e.g. Leo) Recognize if a string has a date format. Discuss: assumptions, test cases

The String Type In the same way that int and double are designed to store numerical values, the String type is designed to store text. Text for strings must be enclosed in double quotes. Examples: String name = "George"; String phone_number = "310-123-987";

A Simple Program Using Strings import java.util.Scanner; public class example1 { public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Hi, my name is Java.\n"); System.out.printf("What is your first name? "); String first_name = in.next(); System.out.printf("What is your last name? "); String last_name = in.next(); System.out.printf("Hello %s %s, nice to meet you!\n", first_name, last_name); } Example Output: Hi, my name is Java. What is your first name? Mary What is your last name? Smith Hello Mary Smith, nice to meet you!

String Input from the User import java.util.Scanner; public class example1 { public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Hi, my name is Java.\n"); System.out.printf("What is your first name? "); String first_name = in.next(); System.out.printf("What is your last name? "); String last_name = in.next(); System.out.printf("Hello %s %s, nice to meet you!\n", first_name, last_name); } As you see above, to read a string from user input, you use the Scanner.next() method. Note: although the code calls in.next(), the name of the method is Scanner.next(), because in is just an arbitrary variable name.

Length of a String import java.util.Scanner; public class example1 { public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("Hi, my name is Java.\n"); System.out.printf("What is your name? "); String name = in.next(); int length = name.length(); System.out.printf("Your name has %d letters!\n", length); } Example Output: Hi, my name is Java. What is your name? Vassilis Your name has 8 letters! To obtain the length of a string, we use the String.length() method.

String Concatenation Using + import java.util.Scanner; public class example1 { public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("What is your first name? "); String first_name = in.next(); System.out.printf("What is your last name? "); String last_name = in.next(); String name = first_name + last_name; System.out.printf("Hello %s!\n", name); } Example Output: What is your first name? Mary What is your last name? Smith Hello MarySmith! string1 + string2 returns the result of putting those strings together. We call this string concatenation.

String Concatenation Using + import java.util.Scanner; public class example1 { public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("What is your first name? "); String first_name = in.next(); System.out.printf("What is your last name? "); String last_name = in.next(); String name = first_name + " " + last_name; System.out.printf("Hello %s!\n", name); } Example Output: What is your first name? Mary What is your last name? Smith Hello Mary Smith! When you concatenate strings, make sure that you put spaces where they are needed.

Escape Sequences If you want to put a " character in a string: use \" If you want to put a newline character in a string: use \n public class example1 { public static void main(String[] args) { String a = "He said \"Hello\""; String b = "C:\\users\\jane\\note.txt"; String c = "*\n**\n***"; System.out.println(a); System.out.println(b); System.out.println(c); } Output: He said "Hello" C:\users\jane\note.txt * ** ***

Escape Sequences In order to print Use: Code example Output " \" \ \\ System.out.println("He said \"Hello\""); He said "Hello" \ \\ System.out.println("C:\\users\\jane"); C:\users\jane % \% System.out.println("Exams are 75\%."); Exams are 75%. newline \n System.out.println("*\n**\n***"); * ** ***

Characters and Substrings The position of string characters are numbered starting from 0. To get the character at position p: use charAt(p); To get the substring from position s up to and not including position t, use substring(s, t) H e l o , w r d ! 1 2 3 4 5 6 7 8 9 10 11 12 Index public class example1 { public static void main(String[] args) { String str = "Hello, world!"; char first = str.charAt(0); char fifth = str.charAt(4); String sub = str.substring(2, 9); System.out.println(first); System.out.println(fifth); System.out.println(sub); } Output: H o llo, wo

Characters and Substrings IndexOutOfBoundsException Error Valid indexes = indexes corresponding to characters in the string. If you try to grab a character from an invalid index for the given string you get an IndexOutOfBoundsException, explained as: String index out of range: badIndex H e l o , w r d ! 1 2 3 4 5 6 7 8 9 10 11 12 Index String str = "Hello, world!"; char bad1 = str.charAt(-2); // String index out of range: -2 char bad2 = str.charAt(13); // String index out of range: 13 String subBad1 = s.substring(8,16); // String index out of range: 16 Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: -2 at java.lang.String.charAt(String.java:646) at TryOutErase.main(TryOutErase.java:5) Java Result: 1 5 is the number of line in your code that produced the error.

indexOf indexOf returns the index of the first occurrence of a substring in another string Returns: -1 if there is no occurrence and the index if found, H e l o , w r d ! 1 2 3 4 5 6 7 8 9 10 11 12 Index public class example1 { public static void main(String[] args) { String str1 = "Hello, world!"; int idx1 = str1.indexOf("or"); // found, starts at index 8 int idx2 = str1.indexOf("old"); // not found: -1 int idx3 = str1.indexOf("o"); // index of first o System.out.println("idx1 = " + idx1); System.out.println("idx2 = " + idx2); System.out.println("idx3 = " + idx3); } Output: idx1 = 8 idx2 = -1 idx3 = 4

Printing Characters with printf To print a value of type char with System.out.printf, you can use either %s or %c (they both work).

Example 1: Printing Name Initial Write a program that: Asks the user: What is your name? Gets the name from user input. Prints: Your initial is X where X is the first letter of the name that the user typed.

Example 1: Printing Name Initial import java.util.Scanner; public class example1 { public static void main(String[] args) { Scanner in = new Scanner(System.in); System.out.printf("What is your name? "); String name = in.next(); char initial = name.charAt(0); System.out.printf("Your initial is %s\n", initial); } Example Output: What is your name? Mary Your initial is M Example Output: What is your name? John Your initial is J

Practice: Capitalize name Write a program that: Asks the user: What is your full name? Gets the name from user input. “extracts” the first and last names Prints them afterward Asks the user: What is your full name? Fixes the name to make each part start with a capital letter E.g. user enters alex stefan and the program prints Alex Stefan Hint: find intermediary steps to go from the above problem to this one.

Extra material

String Concatenation Using += import java.util.Scanner; public class example1 { public static void main(String[] args) { Scanner in = new Scanner(System.in); String message = "Hello "; System.out.printf("What is your first name? "); String first_name = in.next(); message += first_name; System.out.printf("%s!\n", message); } Example Output: What is your first name? Mary Hello Mary! The following two lines do the EXACT SAME THING: variable_name += value; variable_name = variable_name + value;