1. CS2011 Introduction to Programming I Arrays (I)
Chengyu Sun
California State University, Los Angeles

2. Example: Grades
Find the average of the grades, and display the ones that are above average
3 students: grade1, grade2, grade3
10 students: grade1, grade2, …, grade10
100 students??

3. Create An Array Declare an array variable Allocate an empty array
Populate the array

4. Declare An Array Variable
Type of the elements
in the array
Indicate that the variable
is an array
Name of the
variable
int[] grades; or
int grades[];
All elements in an array must be of the same type

5. Allocate An Empty Array
Create a new array
of given size
Size of the array
grades = new int[30];
Type of the elements in the array
(must match the type in array declaration)
The size of an array is fixed and cannot be changed
At this point the array is empty

6. Populate An Array Array index grades[0] = 85; grades[1] = 100;
Array index starts from 0 and goes up to size – 1
IndexOutOfBound error
array[index] is just like a variable
Assign it a value
Access its value

7. What Happens in Each Step
Memory
Memory
Memory
grades
grades
grades 85
100
Declare a variable
Allocate space for the array
Point the variable to the array
Put values into the array

8. Array Variable vs. The Actual Array
An array variable only holds a reference to array, e.g. grades is a variable that contains a reference to an array of 30 int elements

9. Array Initializer No need to specify array size
A list of values
int[] numbers = { 10, 20, 25 };
String[] words = { "hi", "you" };
No need to specify array size
Must be in the same statement where the array is declared

10. Iterate Through An Array …
for( int i=0 ; i < grades.length ; ++i )
System.out.println( grades[i] );
The size of the array is stored in a variable in the array called length
for loop is a natural fit for arrays as array size is fixed and known

11. … Iterate Through An Array
A variation of the for loop is call foreach loop, as in "for each element in the array, do something"
for( int grade : grades )
System.out.println( grade );

12. Grades Example Create an array of size 10
Populate the array with 10 random numbers between 80 and 100
Use a method to print the array
Pass an array as a parameter
Calculate the average grade
Display the grades that are above average

13. Command-Line Arguments
What's the args in the main() method?
Prints out args in main() and try running the program like the following:
java <Program> hi you 100
Specify command line arguments in Eclipse: Run Configurations

14. More Array Examples Find the largest/smallest element
Find the index of the largest/smallest element
Find if a value exists in an array
Sequential search
Binary search a sorted array