Multi-Dimensional Arrays
Rectangular & Jagged
Plus: More 1D traversal
Array Recall
• Creating a 2D array of doubles:
•
•
•
•
•
•
double[][] grid = new double[3][4];
3 rows: indices 0 to 2, 4 columns: indices 0 to 3
grid: reference to entire array
grid[0]: reference to 1st row
grid[0][2]: third element in first row
all entries initialized to 0.0
• 1D array of Point objects:
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•
•
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Point[] points = new Point[3];
points[0] = new Point(2, 5);
points[1] = new Point(0, 0);
points[2] = new Point(4, 7);
Printing 2D Arrays
• Arrays.toString() does not work well for 2D
• Produces a String representation of each
element, but for a 2D array, each element is an
array.
• Use Arrays.deepToString() instead:
double[][] grid = new double[3][4];
grid[0][1] = 5;
grid[2][3] = 7;
System.out.println(Arrays.deepToString(grid));
Output:
[[0.0, 5.0, 0.0, 0.0], [0.0, 0.0, 0.0, 0.0], [0.0, 0.0, 0.0, 7.0]]
Jagged Arrays
Multi-dimensional Arrays:
Number of elements in a row can vary
int[][] arr = new int[3][];
arr[0] = new int[2]; // 2 columns in row 0
arr[1] = new int[4]; // 4 columns in row 1
arr[2] = new int[3]; // 3 columns in row 2
To print this array:
for(int i = 0; i < arr.length; i++) {
// print row i
for(int j = 0; j < arr[i].length; j++)
System.out.print(arr[i][j] + “\t”);
System.out.println();
}
Jagged Array Example
• Create the following array, and the print it:
jag
1
1 2
1 2 3
• Output (with tabs between values in a row):
1
1 2
1 2 3
Tallying Values with Arrays
• Problem: Write a method
mostFrequentDigit that returns the digit
value that occurs most frequently in a number.
– Example: The number 669260267 contains:
one 0, two 2s, four 6es, one 7, and one 9.
mostFrequentDigit(669260267) returns 6.
– If there is a tie, return the digit with the lower value.
mostFrequentDigit(57135203) returns 3.
based on notes from Reges and Stepp, Building Java Programs
Multi-Counter Problem
• We could declare 10 counter variables ...
int counter0, counter1, counter2, counter3, counter4,
counter5, counter6, counter7, counter8, counter9;
• But a better solution is to use an array of size 10.
– The element at index i will store the counter for digit value
i.
– Example for 669260267:
index
value
0
1
2
3
4
5
6
7
8
9
1
0
2
0
0
0
4
1
0
0
– How do we build such an array? And how does it help?
based on notes from Reges and Stepp, Building Java Programs
An Array of Tallies
// assume n = 669260267
int[] counts = new int[10];
while (n > 0) {
// pluck off a digit and add to proper
counter
int digit = n % 10;
counts[digit]++;
n = n / 10;
}
index
0
1
2
3
4
5
6
7
8
9
value
1
0
2
0
0
0
4
1
0
0
based on notes from Reges and Stepp, Building Java Programs
Tally Solution
// Returns the digit value that occurs most frequently in n.
// Breaks ties by choosing the smaller value.
public static int mostFrequentDigit(int n) {
int[] counts = new int[10];
while (n > 0) {
int digit = n % 10; // pluck off a digit and tally it
counts[digit]++;
n = n / 10;
}
// find the most frequently occurring digit
int bestIndex = 0;
for (int i = 1; i < counts.length; i++) {
if (counts[i] > counts[bestIndex]) {
bestIndex = i;
}
}
}
return bestIndex;
based on notes from Reges and Stepp, Building Java Programs
Array Histogram Question
• Given a file of integer exam scores, such as:
82
66
79
63
83
Write a program that will print a histogram of stars
indicating the number of students who earned each
unique exam score.
85: *****
86: ************
87: ***
88: *
91: ****
based on notes from Reges and Stepp, Building JavaPrograms
Array Histogram
// Reads a file of test scores and shows a histogram of score distribution.
import java.io.*;
import java.util.*;
public class Histogram {
public static void main(String[] args) throws FileNotFoundException {
Scanner input = new Scanner(new File("midterm.txt"));
int[] counts = new int[101];
// counters of test scores 0 - 100
while (input.hasNextInt()) {
// read file into counts array
int score = input.nextInt();
counts[score]++;
// if score is 87, then counts[87]++
}
}
}
for (int i = 0; i < counts.length; i++) {
// print star histogram
if (counts[i] > 0) {
System.out.print(i + ": ");
for (int j = 0; j < counts[i]; j++) {
System.out.print("*");
}
System.out.println();
}
}
based on notes from Reges and Stepp, Building Java Programs
Exercises
What is the output?
public class Mystery {
public static void main(String[] args) {
int x = 1;
int[] a = new int[2];
mystery2(x, a);
System.out.println(x + “ “ + Arrays.toString(a));
x--;
a[1] = a.length;
mystery2(x, a);
System.out.println(x + “ “ + Arrays.toString(a));
}
public static void mystery2(int x, int[] list) {
list[x]++;
x++;
System.out.println(x + “ “ + Arrays.toString(list));
}
}
Exercise
Output?
int[][] x = {{1, 2, 3}, {1, 2}, {1}};
int sum = 0;
for(int i = 0; i < x.length; i++) {
sum += x[i].length;
}
System.out.println(sum);