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content/ta/fall2017/cpsc220/oct23.md

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+# Lecture for October 23
+
+## Two-Dimensional Arrays
+
+You can think of a two dimensional array as a grid that is organized by rows then columns.
+
+To declare a two dimensional array do the following
+
+```java
+int[][] grid  = new int[5][5];  // Declares a 2D array of 5 rows and 5 columns
+```
+
+You can have as many dimensions as you want. For graphics a 3-dimensional array would render 3D points.
+
+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.
+
+Strings are essentially a char array with extra methods attached. We can imitate an array of strings with a 2D char array.
+
+```java
+char[][] helloWorld = new char[5][5];
+hello[0][0] = 'h';
+hello[0][1] = 'e';
+hello[0][2] = 'l';
+hello[0][3] = 'l';
+hello[0][4] = 'o';
+hello[1][0] = 'w';
+hello[1][1] = 'o';
+hello[1][2] = 'r';
+hello[1][3] = 'l';
+hello[1][4] = 'd';
+
+```
+
+## Nested-For Loops
+
+To access the elements in a 2D array, you need to use a nested for-loop. 
+
+Here is how you print out the hello world example above
+
+```java
+for (int row = 0; row < helloWorld.length; row++) {
+  for (int col = 0; col < helloWorld[row].length; col++) {
+      System.out.print(helloWorld[row][col]);
+  }
+  System.out.print(" ")
+}
+```
+
+The code above prints out "hello world"
+
+
+
+## 2D Arrays in methods
+
+You can write a get like method in the following way
+
+```java
+public static void get(int[][] array, int row, int col) {
+    return array[row][col];
+}
+```
+
+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.
+
+```java
+public static void insert(int[][] array, int row, int col, int numToInsert) {
+    array[row][col] = numToInsert;
+}
+```
+
+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.
+
+```java
+public static void insert(const int[][] array, int row, int col, int numToInsert) {
+    array[row][col] = numToInsert; // This line will throw an errror
+}
+```
+