1. Lecture 10
Strings
CSE 1322
4/26/2018

2. String class
A string is a sequence of characters stored in a certain address in memory.
Once created, it cannot be changed. It is immutable because the string class has no mutators methods.
It does not need to be explicitly instantiated (new is not needed)
Once created, the content does not change directly!
If we try to change the value, it will be saved to a new location in dynamic memory and the variable will point to it.
4/26/2018

3. Main Ideas String are objects, so they have a ton of methods!
Length – the number of characters in the string
Substring – returns only part of a string
Append – creates a new string made from other strings
Upper/Lowercase
IndexOf – returns the location of a character(s)
Most often, you can access them similar to arrays
String programming appears a lot in job interviews
4/26/2018

4. Note In Java, Strings start with a capital S
In C#, strings start with a lowercase s
For many of these examples, we don’t care and will use generic:
S/string
4/26/2018

5. Other note
Strings usually have a hidden character '\0' that denotes the end of the string
It’s called a “null terminator” to create a “null terminated string”
This takes up one character of memory
Job interview question: how much memory needed to store the word “cat”? Answer: 4 characters (usually 8 bytes)
4/26/2018

6. S/strings Strings are objects of the S/string class Examples:
A constant or literal: e.g. "Hello"
A variables: S/string message = "Hello, World!";
String length: int n = message.length();
Empty string: ""
4/26/2018

7. String Constant Pool
Java tries to be efficient and store (unchanging) strings in a “pool”
Adds as much to the pool as possible
String s1 = new String (“A”);
This adds a new string in memory
AND to the string pool
Memory - Heap s1
String Object “A”
The “pool”
String Object “A”
4/26/2018

8. String Constant Pool
Java tries to be efficient and store (unchanging) strings in a “pool”
Adds as much to the pool as possible
String s1 = new String (“A”);
String s2 = “A”;
Memory - Heap s1
String Object “A”
The “pool”
String Object “A” s2
4/26/2018

9. String Constant Pool
Java tries to be efficient and store (unchanging) strings in a “pool”
Adds as much to the pool as possible
String s1 = new String (“A”);
String s2 = “A”;
String s3 = “A”;
So only two objects in memory
Memory - Heap s1
String Object “A”
The “pool”
String Object “A” s3 s2
Note: this is somewhat esoteric, but Java employers like it
4/26/2018

10. Concatenation Use the + operator:
S/string name = "Dave"; S/string message = "Hello, " + name; // message is “Hello, Dave”
If one of the arguments of the + operator is a string, the other is converted to a string
S/string a = "Agent"; int n = 7; String bond = a + n; // bond has the value Agent7
4/26/2018

11. Concatenation in Print Statements
Useful to reduce the number of Print instructions
PRINT ("The total is "); PRINT (total);
versus
PRINT ("The total is " + total);
4/26/2018

12. Converting between Strings and Numbers
Convert to number:
int n = Integer.parseInt(str); double x = Double.parseDouble(str);
Convert to string (easy!):
String str = "" + n; // OR str = Integer.toString(n);
4/26/2018

13. Converting between Strings and Numbers
Convert to number:
int n = int.ParseInt(str); double x = double.Parse(str);
Convert to string (easy!):
string str = "" + n; // OR int many = 5; str = many.ToString(n);
4/26/2018

14. Substrings Java First position is at 0, last is 5H e l o , W r d ! 1 2 3 4 5 6 7 8 9 10 11 12
Supply start and “past the end” position (be careful!)
First position is at 0, last is 5
S/string greeting = "Hello, World!"; S/string sub = greeting.substring(0, 5);
// sub is "Hello"
4/26/2018

15. Substrings Java First position is at 7, last is 12H e l o , W r d ! 1 2 3 4 5 6 7 8 9 10 11 12
Supply start and “past the end” position (be careful!)
First position is at 7, last is 12
S/string greeting = "Hello, World!"; S/string sub = greeting.substring(7, 12);
// sub is "World"
4/26/2018

16. Length Property Strings usually have either:
A getter (C#) like public int Length { get; }
A method (Java) like length();
These return the number of characters in the string.
The Length property returns the number of Char objects in this instance, not the number of Unicode characters.
Example:
string h= “hello”;
int len = h.Length;
// len has a value of 5
4/26/2018

17. Challenge string s ="Agent";
What is the effect of the assignment s = s + s.Length;
string river ="Mississippi";
What is the value of river.Substring(1, 2)?
What is the value of river.Substring(2, river.Length-3)?
What is the value of river.Substring(1)?
4/26/2018

18. Answers
Agent5 i
ssiss
ississippi
4/26/2018

19. Additional methods ToUpper/toUpperCase and ToLower/toLowerCase
indexOf()
charAt (Java) and array access (C#) of individual characters
Replace/replace (Java)
Trim/trim – a *wonderful* way to get rid of leading/trailing whitespace!
4/26/2018

20. To upper and lower case
public string ToLower() Return Value: A string in lowercase. public string ToUpper() Return Value: A string in uppercase. Example: string myString = “good luck”; myString = myString.ToUpper(); //myString now has the value “GOOD LUCK”
4/26/2018

21. To upper and lower case
public string toLowerCase() Return Value: A string in lowercase. public string toUpperCase() Return Value: A string in uppercase. Example: String myString = “good luck”; myString = myString.toUpperCase(); //myString now has the value “GOOD LUCK”
4/26/2018

22. IndexOf methods public int IndexOf( char value )
The zero-based index position of value if that character
is found, or -1 if it is not.
public int IndexOf( string value)
The zero-based index position of value if that string is
found, or -1 if it is not.
string myString= “hello world”;
int e_index=myString.IndexOf(‘e’); // e_index= 1
int or_index= myString.IndexOf(“or”); //or_index=7
4/26/2018

23. indexOf methods public int indexOf( char value )
The zero-based index position of value if that character
is found, or -1 if it is not.
public int indexOf( string value)
The zero-based index position of value if that string is
found, or -1 if it is not.
String myString= “hello world”;
int e_index=myString.indexOf(‘e’); // e_index= 1
int or_index= myString.indexOf(“or”); //or_index=7
4/26/2018

24. Accessing Individual Characters
Java - use charAt(int x);
String bob = "Bob";
System.out.println (bob.charAt(2)); // Prints b
C# - use array indices
string bob = "Bob";
Console.WriteLine (bob[2]); // Prints b
4/26/2018

25. Trimming Extra Whitespace (super common)
Java - use trim();
String bob = " \t   Bob    ";
System.out.println (bob.trim()); // Prints “Bob” – no spaces
C# - use array indices
    string bob = " \t   Bob    ";
    Console.WriteLine (bob.Trim()); // Prints “Bob”
4/26/2018

26. Final Notes A null string is not the same thing as an empty string:
S/string s1 = null;
S/string s2 = “”;
s1 is “dead” and points to null
s2 points to something “alive” that contains no text
Often, you can convert a string to/from a character array (char[]). Quite useful for manipulation!
4/26/2018

27. In-class Problems Write the code to:
See if the word “Bubba” appears in a string
Count how many vowels are in a string
Reverse a string
Determine if a string is a palindrome
Check to see if the first letter of each word is capitalized
Replaces all tabs '\t’ with newlines '\n’
Adds one to each letter (e.g. ‘a’ becomes ‘b’, ‘b’ becomes ‘c’) so that “CSE 1321” would become “DTF 2432”
Print all possible substrings of a given string (that’s a fun one!)
4/26/2018

28. Frequently Used String Methods
string-function.php
4/26/2018