Download Two-Dimensional Arrays: Declaration, Initialization, and Manipulation and more Lecture notes Mathematics in PDF only on Docsity!
Lesson 35…..Two-Dimensional Arrays
Consider the following array (3 rows, 2 columns) of numbers:
22 23 24 25 26 27
Let’s declare our array as follows:
int a[ ] [ ] = new int [3] [2];
Subscript convention: Notice that in both mathematics and computer science, designations for a two dimensional array (subscripted variable) conventionally have rows first and columns second. Just think of RC Cola, …RC (rows, columns).
Initializing a two-dimensional array: Now let’s initialize the a array, i.e. store values in the various positions. There are three ways to do this:
The first way: int a[ ] [ ] = new int [3] [2]; //declaration a[0] [0] = 22; //initialization from here on down a[0] [1] = 23; a[1] [0] = 24; a[1] [1] = 25; a[2] [0] = 26; a[2] [1] = 27;
The second way: int a[ ] [ ] = { {22, 23}, {24, 25}, {26, 27} }; //Notice that declaration and initialization // must both take place on the same line. The third way: int a[ ] [ ] = new int[][] { {22, 23}, {24, 25}, {26, 27} };
How many rows and columns? Determine the number of rows and columns in a two-dimensional array (sometimes called a matrix or subscripted variables ) as follows:
For the matrix above, a.length returns a value of 3…the numbers of rows.
For the matrix above, a[0].length returns a value of 2…the number of columns in row 0 a[1].length returns a value of 2…the number of columns in row 1 a[2].length returns a value of 2…the number of columns in row 2
“Ragged” arrays: The previous discussion seems redundant, since all rows have 2 columns. So we begin to wonder if it’s possible for various rows to have different number of columns? Is it really possible to produce a “ragged” looking array with uneven rows? The answer is, “Yes,” even though it’s highly unusual and seldom used.
Suppose we want the following matrix structure:
X X X X X X X X X
Here’s the code that would declare such an array:
int a[ ] [ ] = new int[3] [ ]; //array has 3 rows, unspecified number of columns a[0] = new int[4]; //row 0 has 4 columns a[1] = new int[2]; // row 1 has 2 columns a[2] = new int[3]; // row 2 has 3 columns
Incidentally, the first line of code above ( int a[ ] [ ] = new int[3] [ ]; ) could be equivalently replaced with the following; however, the former is preferred: int a[ ] [ ] = new int[3] [ 0 ]; //3 rows, unspecified number of columns
While on the subject of working with a single row of a two-dimensional array, consider an array a of three rows declared as was done above. How would we pass a single row of this array to a method called myMethod in which each element would be initialized?
a[2] = new int[3]; //Row 2 has 3 columns. Before passing a[2] below, we must //have specified the number of columns. myMethod(a[2]); // Call the method and pass the row with index 2.
... public void myMethod(int [] x) //Notice how we receive the row { x[0] = 36; // Initialize the three columns. x[1] = 101; x[2] = -45; }
Automatic initialization of arrays: As with all one-dimensional numeric arrays, all elements of two-dimensional arrays are also automatically initialized to 0 until specific values are given. int abc[][] = new int[20][30]; System.out.println(abc[5][22]); // 0
Using the Arrays class: In Lesson 19 (page 3), several methods of the Arrays class were discussed. Below we present their equivalents as used with two-dimensional arrays. int a[][] = { {3, 9, 2, 1}, {5, 7, 6, 0} }; int b[][] = { {0, 2, 8, 4}, {3, 9, 2, 1} };
System.out.print(a[row][col] + “\t”); } System.out.println(“ ”); }
- What is printed by the following? Arrays.sort(a[0]); System.out.println(Arrays.binarySearch(a[0],5));
- What is printed by the following? Arrays.sort(a[0]); System.out.println( Arrays.binarySearch(a[0],0) );
- Show what the matrix a would look like after the following code executes: for (int row = 0; row < a.length; row++) { for(int col = 0; col < a[row].length; col++) a[row][col] = row * col; }
- Show what the matrix a would look like after the following code executes: Arrays.fill(a[2], -156);
- Must all two-dimensional arrays have the same number of columns in each row?
In the remaining problems, some of the code might not compile or will give a run-time exception. If that’s the case then state, “Won’t compile” or “Run-time exception.”
- What is printed by the following? double d[][] = new double[8][25]; System.out.println(d[4][2]);
- If x and y both represent two-dimensional int arrays and have identically the same elements, what does the following print? System.out.println( Arrays.equals(x,y) );
- Is it possible to sort z (a two-dimensional array) with Arrays.sort(z);?
- Is it possible to use one of the sort methods of the Arrays class to sort a single row (index
- of the two-dimensional matrix g? If so, show the code that will accomplish this.
Two-Dimensional Arrays… Contest Type Problems
- Which of the following is the resulting array after running the code to the right?
A. 0 0 0 0 1 1 1 1 2 2 2 2
B. 0 1 2 3 0 1 2 3 0 1 2 3
C. 1 2 3 4 1 2 3 4 1 2 3 4
D. 1 1 1 1 2 2 2 2 3 3 3 3
E. None of these
int [][] zorro = new int[3] [4]; for(int row=0; row<zorro.length; row++) { for(int col=0; col<zorro[row].length; col++) { zorro[row][col] = col + 1; } }
- Which of the following is the resulting array after running main in the Tester class to the right?
A. 4 5 6 7 0 1 2 3 -1 0 1 2
B. 5 6 7 8 1 2 3 4 0 1 2 3
C. 6 7 8 9 2 3 4 5 1 2 3 4
D. 1 1 1 1 2 2 2 2 3 3 3 3
E. None of these
public class Tester { public static void main(String args[]) { int z[][] = { {5,6,7,8}, {1,2,3,4}, {0,1,2,3} }; MatrixManip f = new MatrixManip( ); f.adjust(z); } }
public class MatrixManip { … public void adjust(int[][] mat) { for(int p=0; p<mat.length; p++) for(int q=0; q<mat[p].length; q++) --mat[p][q]; } … }
