Download Powerpoint 3: Strings and arrays

Arrays and Strings
A way to make oodles of variables,
and a deeper look at classes
Variables vs. arrays
• The variables we’ve
looked at so far are
all primitive types
• One variable holds
one value
• An array holds
several variables of
the same type
• Its components are
numbered, starting
with 0
• One array variable
holds multiple values
Declaring and assigning arrays
• We use brackets [] right after the variable
type to indicate that we are declaring an
array
• We use the word “new” to create new arrays
• We use index numbers within the brackets to
refer to individual components of an array
int[] fibonacci;
fibonacci = new int[5];
fibonacci[0]
fibonacci[1]
fibonacci[2]
fibonacci[3]
fibonacci[4]
=
=
=
=
=
1;
1;
2;
3;
5;
Assigning values to arrays
1
1
2
3
5
[0]
[1]
[2]
[3]
[4]
fibonacci[0] fibonacci[1] fibonacci[2] fibonacci[3] fibonacci[4]
Arrays and FOR loops
• It is often useful to use arrays and FOR
loops together for assigning values to
arrays and for outputting values of
arrays
int c;
int[] naturals;
naturals = int[5];
for ( c=0; c<5; c++) {
naturals[c] = c+1;
}
for ( c=0; c<5; c++ ) {
Std.out.println(naturals[c]);
}
Use arrays!
• Write a program that asks the user for
their five favorite numbers, and store
those numbers in an array.
• Modify your program to ask the user
for a number n, and then ask the user
for their n favorite numbers, and store
those numbers in an array.
Multi-dimensional arrays
• The arrays we have examined so far are
only one-dimensional arrays
• You can create arrays in two, three, or
more dimensions.
• Remember, the more dimensions your
array is, the more memory they will
require!
Declaring and assigning multidimensional arrays
• We declare and assign multi-dimensional
arrays the same way as one-dimensional
arrays
• We use multiple sets of brackets to indicate
the desired number of dimensions
int[][] grid;
grid = new int[2][3]
Assigning values to multidimensional arrays
[0][0]
[1][0]
[2][0]
[0][1]
[1][1]
[2][1]
Arrays and FOR loops
• It is often useful to use nested FOR
loops to assign values to multidimensional arrays
int x,y;
int[][] multtable;
multtable = int[10][10];
for ( x=0; x<10; x++) {
for ( y=0; y<10; y++ ) {
multtable[x][y] = (x+1)*(y+1);
}
}
What is a string?
• A string is any
sequence of text,
numbers, or text and
numbers together
• A substring of a
string is any
sequence of text
and/or numbers
contained within the
larger string
Strings in Java
• In Java, a string is an object variable
• We use a class built into the Java language
called “String”
• We call the String class a standard class
Declaring and assigning strings
• We use the word “new” and the constructor
method of the String class to create new
strings
int days;
days = 31;
String name;
name = new String(“Matthew”);
String automaton;
automaton = new String(Std.in.readLine());
Adding strings
• We can “add” strings together using a plus
sign
String firstName;
String lastName;
String fullName;
firstName = new String(“William”);
lastName = new String(“Gates”);
fullName = new String(firstName + “ “ + lastName);
Outputting strings
• We can output strings using the
Std.out.println command
String firstName;
String lastName;
String fullName;
firstName = new String(“William”);
lastName = new String(“Gates”);
fullName = new String(firstName + “ “ + lastName);
Std.out.println(fullName);
What makes strings special?
• When we create a
string, we are
creating an instance
of the standard
class String
• Therefore, we use
methods in the
standard class to
find out information
about our string
• Think of the
standard class
String as a rubber
stamper
• Each time we make a
new string, it’s like
making a stamp with
all the properties of
the original
Useful methods
Method
name
Input
type
Output
type
Action
s.length()
none
int
returns the number of
characters in s
s.charAt(n)
int
char
returns the character
at position n in s
s.substring(n)
int
String
returns the substring
from position n to the
end of s
s.substring(n,m)
int, int
String
returns the substring
from position n to
position m-1
Using string methods
import extra.*;
public class NameReader {
public static void main (String args[]) {
String name;
int x;
Std.out.println(“What is your name?”);
name = new String(Std.in.readLine);
x = name.length();
Std.out.println(“Your name has “ + x + “ letters.”);
}
}
Use some strings!
• Write a program
that asks the user
for his or her first
name
• The program should
store that name in a
string and determine
the first letter of
the name and print
that letter
• Modify your program
to find the last
letter of your user’s
first name