Strings

In these notes you will learn:

• What string literals are.
• How to use special escape characters in a string.
• How to test if two strings are the same.
• How to combine strings with concatenation.
• How to represent characters with the char type.

Introduction

In Processing, a string is a sequence of 0 or more characters. For example, these are all strings:

"mouse"
"Once upon a time ..."
"604-555-3536"
"X"
"1\ntwo\nthree"
"Her name is \"Mary\""
"\\n is a return character"
""

More precisely, these are string literals because they begin and end with a " (double-quote).

The string "" is known as the empty string: it is the string with 0 characters in it.

Strings can contain escape characters in them, such as \n, \t, \\, and \". For example, \n represents a newline character. When a newline is printed it makes the cursor go to the next line. For example, println("hello\nthere") prints this:

hello
there

We can also use assign strings to variables. For example:

String greeting = "Hello there!";
println(greeting);

The println function prints a string on the console; in the Processing IDE this is the small black window at the bottom of the editor window.

String is an example of a type. Declaring a variable to be of type String ensures that Processing only allows it to store strings. For example:

String s = "apple";
s = 55; // error: can't assign a number to a String

Notice also that String begins with a capital S. If you wrote string instead you’d get an error.

Processing provides numerous special functions for dealing with strings. For now we will look at only a few of the most useful ones.

The Length of a String

To get the number of characters in a string call the .length() function. For example:

String fruit = "lemon or apple";
println(fruit.length());  // prints 14

14 is printed because there are exactly 14 characters in the string lemon or apple. Note that spaces count as characters.

You can also call length() directly on a string literal, although it is rare in practice:

println("Main Window".length());  // prints 11

Be careful with strings that contain escape characters. An escape character counts as one character, even though it is written with more then one symbol. For example:

String s = "a\nb";
println(s.length());  // prints 3

The string "a\nb" contains exactly three characters (not 4!): a, \n, and b.

The empty string has length 0:

String s = "";
println(s.length());  // prints 0

Be careful not to confuse the empty string with a string that contains a space:

String s = " ";       // s contains a space character
println(s.length());  // prints 1

Testing if Two Strings are the Same

Sometimes you need to know if two string variables, say s and t, are equal. By equal we mean that they have the same characters in the same order. The .equals function gives the answer:

if (s.equals(t)) {
   println("s and t are the same");
} else {
   println("s and t are different");
}

String comparisons are case-sensitive, i.e. .equals considers "apple" and "Apple" to be different.

Warning

Unfortunately, Processing also lets you write code like this:

if (s == t) {
   println("s and t are the same");
} else {
   println("s and t are different");
}

This does not test if s and t are the same strings. While == is how you compare primitive values (like ints and floats), it does not work correctly for objects like String. For strings, s == t returns true just when the variable s and the variable t refer to the same internal memory address. While this might sometimes give the same results as using .equals, there are no guarantees. Generally, comparing strings with == is rarely useful, and so it is almost always a mistake.

Concatenation: Combining Strings

Processing lets you create new strings by adding two or more strings together. This is known as string concatenation. For example:

String firstName = "Donald ";
String lastName = "Knuth";
String fullName = firstName + lastName;  // "Donald Knuth"

Adding strings together is one way to create formatted output:

String name = "Jay";
println("Hello " + name + "! How are you today?");
// prints: "Hello Jay! How are you today?

Processing also lets you concatenate primitive values (such as floats) to strings. For instance:

float x = 250;
println("x = " + x);  // prints: "x = 250"

The expression "x = " + x is interesting because it is a float added to a String. Processing automatically converts the float x to a String, and then performs concatenation.

You can use the += operator to append a string to the end of a string. For example:

String muppet = "Bert";
muppet += " and Ernie";  // "Bert and Ernie"

Characters

Processing also has a data type called char that represents single characters. For example, these are all character literals:

'A'   'a'   '7'   '='   '\''   '\n'

There is no such thing as an empty character, i.e. '' is an error.

While characters might look like strings, they are not: 'a' and "A" are completely different types, and so you can’t, for instance, directly compare them.

Here’s a simple example of comparing two characters:

char a = 'q';
char b = 'Q';

if (a == b) {
   println("same");
} else {
   println("different");
}

You must use == to compare characters, not .equals (that would give an error).

You can access individual charactes of a String using the charAt function, e.g.:

String s = "apple";
char firstChar = s.charAt(0);   // 'a'
char secondChar = s.charAt(1);  // 'p'
char thirdChar = s.charAt(2);   // 'p'

The number given to charAt is the index of the character you would like to retrieve. The first character of a string is always at index location 0, the second is at index location 1, and so on.

There’s a lot more that we could say about characters (and strings). However, we will not be using characters much, if at all, in this course. Strings of length 1 work well enough most of the time.

Questions

1. What is a string?
2. What is the empty string?
3. Where does println(s) print s?
4. What is the newline character?
5. What does println("1\n2\n3") print?
6. How do you determine the length of a string s?
7. How do you test if the strings s and t are the same (i.e. they have the same characters in the same order)?
8. Why is comparing strings with == a problem?
9. What is string concatenation?
10. What is the value of the expression "1" + "2"?
11. What is the value of the expression "1" + 2?
12. Give an example of how the += operator works with strings.
13. Explain the difference between 'a' and "a".
14. How would you test if the characters (both of type char) x and y are equal?

Programming Questions

1. Write a program that makes a ball bounce around the screen and modify it so that every time you click on the screen with the mouse, this message is immediately printed on the console window:

   mouse click at (x, y)
   

   Replace x and y with the coordinates of where the mouse pointer actually was when it was clicked.

   Make sure the console message is exactly the same format as in the example!