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78 lines
2.1 KiB
Markdown
78 lines
2.1 KiB
Markdown
# Lecture for October 23
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## Two-Dimensional Arrays
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You can think of a two dimensional array as a grid that is organized by rows then columns.
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To declare a two dimensional array do the following
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```java
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int[][] grid = new int[5][5]; // Declares a 2D array of 5 rows and 5 columns
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```
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You can have as many dimensions as you want. For graphics a 3-dimensional array would render 3D points.
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It doesn't have to be inherently visual though, you can have the n-dimensional array look at the interaction between n different variables. For example, the relationships to different questions in a survey.
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Strings are essentially a char array with extra methods attached. We can imitate an array of strings with a 2D char array.
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```java
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char[][] helloWorld = new char[5][5];
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hello[0][0] = 'h';
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hello[0][1] = 'e';
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hello[0][2] = 'l';
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hello[0][3] = 'l';
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hello[0][4] = 'o';
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hello[1][0] = 'w';
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hello[1][1] = 'o';
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hello[1][2] = 'r';
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hello[1][3] = 'l';
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hello[1][4] = 'd';
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```
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## Nested-For Loops
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To access the elements in a 2D array, you need to use a nested for-loop.
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Here is how you print out the hello world example above
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```java
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for (int row = 0; row < helloWorld.length; row++) {
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for (int col = 0; col < helloWorld[row].length; col++) {
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System.out.print(helloWorld[row][col]);
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}
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System.out.print(" ")
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}
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```
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The code above prints out "hello world"
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## 2D Arrays in methods
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You can write a get like method in the following way
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```java
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public static void get(int[][] array, int row, int col) {
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return array[row][col];
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}
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```
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Arrays in Java are pass by reference not pass by value. Meaning that if you change the array within the method then it will change outside the method.
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```java
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public static void insert(int[][] array, int row, int col, int numToInsert) {
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array[row][col] = numToInsert;
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}
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```
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To make it not be able to change the array inside the method, use the keyword `const` inside the method header. To code below will throw a compiler error.
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```java
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public static void insert(const int[][] array, int row, int col, int numToInsert) {
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array[row][col] = numToInsert; // This line will throw an errror
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}
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```
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