Multidimensional Arrays Histograms CSC 1401: Introduction to Programming with Java Week 11 – Lecture 1 Wanda M. Kunkle

2 1-Dimensional vs. 2-Dimensional Arrays Recall how we declared and created a 1-D array: Recall how we declared and created a 1-D array: int values[] = new int[5]; int values[] = new int[5]; We declare and create a 2-D array in much the same way: We declare and create a 2-D array in much the same way: int table[][] = new int[10][6]; int table[][] = new int[10][6]; The above Java statement creates a 2-D integer array with 10 rows and 6 columns, essentially a 10 x 6 matrix The above Java statement creates a 2-D integer array with 10 rows and 6 columns, essentially a 10 x 6 matrix Note: The above 2-D array code can also be written: int [][] table = new int [10][6]; Note: The above 2-D array code can also be written: int [][] table = new int [10][6];

3 1-Dimensional Array Recall how we initialized a 1-D array: Recall how we initialized a 1-D array: Initial values known Initial values known values[0] = 25; values[1] = 11; values[2] = 38; values[3] = 3; values[4] = 17; values[0] = 25; values[1] = 11; values[2] = 38; values[3] = 3; values[4] = 17; int values[] = {25, 11, 38, 3, 17}; int values[] = {25, 11, 38, 3, 17};

4 1-Dimensional Array Recall how we initialized a 1-D array: Recall how we initialized a 1-D array: Initial values not known Initial values not known Initialize element contents to 0 to “play it safe” Initialize element contents to 0 to “play it safe” int values[] = new int[5]; for (int i = 0; i < values.length; i++) values[i] = 0; int values[] = new int[5]; for (int i = 0; i < values.length; i++) values[i] = 0; We’ll assign real values to the array later. We’ll assign real values to the array later. The array “knows” how big it is.

5 2-Dimensional Array In Lab 6, we created 2-dimensional figures (triangle, arrow) using nested loops. In Lab 6, we created 2-dimensional figures (triangle, arrow) using nested loops. Similarly, we generally initialize 2- dimensional arrays using nested loops. Similarly, we generally initialize 2- dimensional arrays using nested loops. Code to initialize the 2-D array table to all 0s appears in the next slide. Code to initialize the 2-D array table to all 0s appears in the next slide.

6 2-Dimensional Array Typical initialization routine Typical initialization routine Initialize array elements to 0 Initialize array elements to 0 int[][] table = new int[10][6]; int row, column; for (row = 0; row < 10; row++) for (column = 0; column < 6; column++) table[row][column] = 0; int[][] table = new int[10][6]; int row, column; for (row = 0; row < 10; row++) for (column = 0; column < 6; column++) table[row][column] = 0;

7 2-Dimensional Array Alternate initialization routine Alternate initialization routine Initialize array elements to values specified by the programmer (as in the Tictactoe program in Lab 11 which employs a 3 x 3 character array) Initialize array elements to values specified by the programmer (as in the Tictactoe program in Lab 11 which employs a 3 x 3 character array) char [][] tictac = { {'X', 'X', 'O'}, {' ', 'O', ' '}, {'X', 'O', 'X'} }; char [][] tictac = { {'X', 'X', 'O'}, {' ', 'O', ' '}, {'X', 'O', 'X'} };

8 Sample Program The following program uses 2-D arrays to implement a table showing how interest rates affect bank balances: The following program uses 2-D arrays to implement a table showing how interest rates affect bank balances: InterestTable.java InterestTable.java InterestTable.java

9 Histograms Since Lab 11 employs histograms, I thought it wise to briefly discuss them in case any one of you is not familiar with them. Since Lab 11 employs histograms, I thought it wise to briefly discuss them in case any one of you is not familiar with them. According to the Wikipedia*: According to the Wikipedia*: “In statistics, a histogram is a graphical display of tabulated frequencies. A histogram is the graphical version of a table that shows what proportion of cases fall into each of several or many specified categories.” *This is the definition of a histogram most relevant for Lab 11; this and other definitions can be viewed at: “In statistics, a histogram is a graphical display of tabulated frequencies. A histogram is the graphical version of a table that shows what proportion of cases fall into each of several or many specified categories.” *This is the definition of a histogram most relevant for Lab 11; this and other definitions can be viewed at: displayfrequenciescategories displayfrequenciescategories

10 Die Rolling Simulation A program that simulates the rolls of a die is suitable for graphically displaying frequencies, such as the number of times a 1 is rolled, the number of times a 2 is rolled, etc. A program that simulates the rolls of a die is suitable for graphically displaying frequencies, such as the number of times a 1 is rolled, the number of times a 2 is rolled, etc. A sample program that does just that is: DieRollingSimulation.java A sample program that does just that is: DieRollingSimulation.java DieRollingSimulation.java

11 Lab 11 Now let’s look at how the preceding discussion relates to today’s lab. Now let’s look at how the preceding discussion relates to today’s lab.