1. Arrays Wellesley College CS230 Lecture 02 Thursday, February 1
Handout #9
Reading: Downey: Chapter 10
Problem Set: Assignment #1 due Tuesday, February 13

2. Array Operations
int[] 9 1 7 2 1 3 8 4 6 a
Java arrays are homogeneous: all slots must contain the same kind of element. E.g., an array with type int[] is an array of integers.
Java arrays are fixed length: a.length → 5
Get the contents of a slot via a[index] (“a sub index”): a[0] → 9, a[4] → 6, a[5] → ArrayIndexOutOfBoundsException
Set the contents of a slot via a[index]= expression; a[0] = 2; a[1] = a[2] + a[3]; a[i]++ means a[i] = a[i] + 1; and a[i]-- means a[i] = a[i] – 1;
Create an integer array with num slots via new int[num]: int [] a = new int[5]; // All slots contain 0 by default a[0]=9; a[1]=7; … ; a[4]=6; Shorthands: int [] a = new int[] {9,7,1,8,6}; int [] a = {9,7,1,8,6};

3. Arrays Elements can be of Any Type
boolean[] b1 = new boolean[4];
boolean[] b2 = {true,false,true};
boolean[]
boolean[]
false 1
false 2
false 3
false
true 1
false 2
true b1 b2
char[]
‘c’ 1
‘s’ 2
‘2’ 3
‘3’ 4
‘0’
char[] c = {’c’,’s’,’2’,’3’,’0’}; c
String[] s1 = new String[3];
String[] s2 = {“I”, “am” “Sam”};
String[]
String[] 1 2 1 2 s2 s1
“I”
“am”
“Sam”

4. Arrays of Arrays An irregular 2-dimensional array A regular 2D array
int[][] aa1 = {{9,7,1,8,6}, {4,0}, {}, {5,2,3}};
int[][] aa2 = {{5,2,7,6},
{9,0,8,4},
{7,3,2,5}};
int[][]
int[]
aa2 1 7 2 1 3 8 4 6
int[][]
int[] 9 1 2 2 7 3 6
int[] 5 4 1
int[] 1 1 2 8 3 4 1
aa1
int[] 9 2
int[] 1 3 2 3 5 2
int[] 7 5 1 2 2 3 3
OR int[][] aa2 = new int[3][4];
aa2[0][0] = 5; aa2[0][1] = 2; …

5. Arrays Diagrams Illustrate Sharing
int[][]
int[] 1 7 2 1 3 8 4 6
aa1[3] = aa1[0];
aa1[0][2] = aa1[0][3] + aa1[0][4];
aa1[3][0] = aa1[3][1] + aa1[3][2];
System.out.println(aa1[3][0]); 9
int[] 4 1 1
aa1
int[] 2
int[] 5 1 2 2 3 3

6. A Simple Array Iteration: Summation
int[]
Iteration Table 9 1 7 2 1 3 8 4 6
nums i
ans 1 9 2 16 3 17 4 25 5 31
Want IntArray.sum(nums) → 31
// Assume this is in the class IntArray public static int sum (int [] a) { }

7. What does mystery() do? nums IntArray.mystery(nums) → ???9 1 7 2 1 3 8 4 6
nums
IntArray.mystery(nums) → ???
// Assume this is in the class IntArray
public static String mystery (int [] a) {
if (a.length == 0) // The empty array is a special case
return "{}";
else {
String result = "{" + a[0];
for (int i = 1; i < a.length; i++) { // Why is the first index 1, not 0?
result = result + "," + a[i]; }
return result + "}";

8. Testing sum(): Approach 1
Suppose we want java IntArray sum to give the following output: sum({9,7,1,8,6}) = sum({4,0}) = sum({}) = sum({5,2,3}) = 10
// Assume this is in the class IntArray
public static void testSum (int [] a) {
System.out.println("sum(" + toString(a) + ") = " + sum(a)); }
public static void main (String [] args) {
if (args.length == 1) { if (args[0].equals(“sum”)) {
testSum(new int[] {9,7,1,8,6}); testSum(new int[] {4,0});
testSum(new int[] {}); testSum(new int[] {5,2,3});
} else … // Testing of other methods goes here
} else System.out.println(“usage: IntArray <name>”);

9. Testing sum(): Approach 2
public static void main (String [] args) {
int[][] arrays = {{9,7,1,8,6}, {4,0}, {}, {5,2,3}};
if (args.length == 1) { if (args[0].equals(“sum”)) {
} else … // Testing of other methods goes here
} else System.out.println(“usage: IntArray <name>”); }

10. Testing sum(): Approach 3
Suppose we want java IntArray sum to give the following output: sum({8,17,1,22,6}) = 54
// Assume this is in the class IntArray // Returns an integer array based on strings in strs[1..(strs.length-1)]
public static int[] makeIntArray (String[] strs) { }
public static void main (String [] args) {
if (args.length >= 1) { // Use >= to allow extra arguments
if (args[0].equals(“sum”)) {
testSum(makeIntArray(args));
} else … // Testing of other methods goes here
} else System.out.println(“usage: IntArray <name> <int1> … <intn> ”);

11. Array Reversal Before: nums After: nums
int[] 9 1 7 2 1 3 8 4 6
Before:
nums
IntArray.reverse(nums);
int[] 6 1 8 2 1 3 7 4 9
After:
nums
// Assume this is in the class IntArray public static void reverse (int [] a) { }

12. Copying Array Reversal
int[] 9 1 7 2 1 3 8 4 6
Before:
nums
int[] c = IntArray.copyReverse(nums);
After:
int[]
int[] 9 1 7 2 1 3 8 4 6 6 1 8 2 1 3 7 4 9
nums c
// Assume this is in the class IntArray public static int[] copyReverse (int [] a) { }

13. Column Summation Want IntArray.sumColumns(aa2) → {21, 5, 17, 15}
(assume input is a regular 2D array) 1 2 2 7 3 6 5
int[] 1 2 8 3 4
aa2 1 9
int[] 1 3 2 3 5 2 7

14. Insertion Sort 9 1 7 2 1 3 8 4 6 7 1 9 2 1 3 8 4 6 1 1 7 2 9 3 8 4 6 1 1 7 2 8 3 9 4 6 1 1 6 2 7 3 8 4 9