Transcript Intro to Arrays

Arrays
Collections
• We would like to be able to keep lots of
information at once
• Example: Keep all the students in the
class
• Grade each one without writing each one
out
• Collections are data types that hold groups
of data
Arrays
• An array is a data type that keeps data in
a line.
• All the elements in the array have to be
the same type.
• Arrays aren’t primitives, but their syntax
isn’t really like objects
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Array Indices
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• Elements in an array are numbered for
convenience
• Numbering starts at 0, like Strings
Syntax for Arrays
• Declare an array: type of data in the array,
followed by square brackets.
int[ ] numbers;
Car[ ] myCars;
• Create an array: use word new and the number
of elements in the array
numbers = new int[7];
myCars = new Car[5];
• Can combine the two statements:
Car [ ] myCars = new Car[5];
• Notice there are no parentheses!!!
Array Elements
• Use square brackets to access individual
array elements
• int x = myNums[4] means put the number
in index 4 into variable x.
• Shortcut for filling up an array:
• double[ ] myList = {1.9, 3.4, -3.2, 0.0};
• Using this way you don’t need the word
new.
Array Elements
• Also put things in an array with element
notation:
• myNumbers[4] = 5;
• Access the length of any array with .length
• int len = myNumbers.length
• No parentheses!!!
Examples
• Make an array of 100 integers, where
each number in the array is equal to twice
its index value
• Sum up all numbers in an array
• Write a method that reverses an array.
• Find the middle number in an array of ints
• Find the biggest number in an array of ints
Objects in Arrays
• Arrays are capable of holding primitive as
well as object types.
• Creating an array of objects does not
create the objects in the array.
• Sometimes the length of the array is not
the same as the number of elements in the
array that you want to use.
Visualizing an Array of Objects
• Create the array
Car[ ] myCars = new Car[5];
myCars
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• Create the objects
for(int i = 0; i< myCars.length; i++){
myCars[i] = new Car();
}
Passing Arrays to Methods
• Arrays can be input or output to methods
just like any other data type
• Arrays are passed to methods by
reference, like objects
• Arrays must be copied explicitly
int[ ] nums1 = {1,2,3};
int[ ] nums2 = nums1;
nums1[1] = 5;
System.out.println(nums1[1]);
System.out.println(nums2[1]);
More Exercises
• Create an array of 50 BankAccounts. Then
deposit $100 in each.
• Take an array of Car objects and return the Car
with the lowest gas mileage. (Assume there is a
getGasMileage() method).
public Car lowestGasMileage(Car[] myCars)
• Take an array of Students and return the student
object whose name is given. Assume there is a
getName() method.
public Student hasName(Student[] students, String name)
Multidimensional Arrays
• We can also create arrays of arrays
(matrices)
• int[ ][ ] matrix = new int[2][4];
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Initializing Shortcut
• int[ ][ ] matrix = { {0,3,2,4},{8,9,2,1} };
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Multidimensional Arrays
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myNumbers
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myNumbers[1][2] = 1
myNumbers.length is the number of rows
myNumbers[0] is the first row
myNumbers[0].length is the number of
elements in the first row (same as number
of columns)
Array Patterns
• Access every element in an array:
for(int i = 0; i<array.length; i++){
int x = array[i];
}
• Access every element in a matrix:
for(int i = 0; i<array.length; i++){
for(int j = 0; j< array[i].length; j++){
int x = array[i][j];
}
}
Exercises
• Create a multiplication table
(mult[a][b] = axb)
public int[][] multTable(int a, int b){
• Add all the elements in a table
public int addUp(int[][]myTable){
• Print the indices of a number in the table
public void findIndices(int target, int[][] table){