1. Arrays CS177 (Week 06)

2. Announcements ● Project 2 due today ● Project 3 will be posted tomorrow

3. Questions?

4. Definitions ● Homogeneous – If you have an array A, then A[0] = 0 and A[1] = “string” is not valid ● Static – Size has to be specified when creating the array

5. Creating an array ● int[] age; – Creates a variable 'age' which can be used to reference an array of int type. No data can be stored in it ● age = new int[10]; – Now this variable can be used to store data. This array can have 10 int values. ● These two instructions are equivalent to ● int[] age = new int[10];

6. Accessing data in an array ● To store a value in an array at some position, do : age[position] = value; – So, if you want the first value in the array 'age' as '1', then do : age[0] = 1; ● Note args was an array of String type. ● Same as args, if you want to access a value in 'age', you write age[position] ● Remember that in java most of the time numbering starts with 0. i.e. the first value in 'age' is age[0] and not age[1]

7. cont... ● This is one way of initializing and assigning values to an array: ● String[] days = {"Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday", "Sunday"}; ● In this example, days is an array of size 7.

8. Length of array ● To get the length of String str, we used str.length() ● To get length of an array vector, we use vector.length ● Note, there are no parentheses after length

9. default value ● When you declare an array of type int, double, boolean or char, java assigns a default value to all the elements of the array. ● The default value is 0 for int and double, false for boolean and '\0' for char ● For others (like String), you have to explicitly assign some value

10. loops on arrays ● One of the most used combinations ● An example for summing all arguments provided by user (java Sum 2 3 4 5 12 3) ● int sum = 0; ● for(int i = 0; i < args.length; i++) ● sum += Integer.parseInt(args[i]); ● Note that its a standard practice that you use array.length even if you know the length of array.

11. Swapping values ● Suppose you have 2 variables int a and int b. And you want to swap their values. Below is a sample code: ● int a = 5; ● int b = 6; ● int temp; ● temp = a; //temp = 5 ● a = b; //a = 6 ● b = temp; //b = 5

12. Swapping values ● Similarly, in case of arrays, if we want to swap element i and j, it will be : ● int temp; ● temp = vector[i]; ● vector[i] = vector[j]; ● vector[j] = temp;

13. 2D arrays ● Suppose you want to store coordinates of a car at 10 different times. How do we do that? ● We use 2 dimensional arrays ● int[][] coordinates; ● coordinates = new int[10][2]; ● Or, equivalently, int[] coordinates = new int[10][2];

14. Sample 2D array ● Below is a code to take data from user to fill the coordinates array; ● for(int i = 0; i < coordinates.length; i++) { ● coordinates[i][0] = StdIn.readInt(); ● coordinates[i][1] = StdIn.readInt(); ● }

15. Another example from lecture int[][] table = new int[5][10]; int label = 1; for(int i = 0; i < 5; i++) for(int j = 0; j < 10; j++) { table[i][j] = label; label++; } ● What is the value of table[i][j]? (i * 10 + j + 1)

16. cont... ● Let us go though the loop sequentially: ● Initially, i = 0 and j = 0 and label = 1 then table[0][0] = 1 ● Then i = 0, j = 1; label = 2, so table[0][1] = 2 ● So on, then i = 0; j = 9; label = 10, so table[0][1] = 10 ● Then i = 1, j = 0, label = 11, so table[1][0] = 11; ● So on, then i = 1 = 1, j = 9, label = 20, so table[1][9] = 20

17. cont.. ● What happens when we swap the two for loops: ● for(int j = 0; j < 10; j++) ● for(int i = 0; i < 5; i++) { ● table[i][j] = label; ● label++; ● } ● table[i][j] = j * 5 + i = 1

18. Liar ● There is nothing called multidimensional arrays ● Its actually arrays of arrays (of arrays....) ● Think of int[] as some datatype say intA, then int[][] is actually intA[], i.e. Array of data type int[] ● So, actually, you can decide size of one dimension and then separately decide size of other dimension

19. Ragged arrays ● Example: ● int[][] pharma = new int[5][]; ● for(int i = 0; i < 5; i++) ● pharma[i] = new int[i+1];

20. cont.... ● Result:

21. Use of ragged arrays ● Suppose you want to store courses students are doing. ● Fixed number of students, but each student can be doing different number of courses ● Ragged arrays save space.

22. >2 dimensions? ● Example of 3D array: ● int[][][] rubiksCube = new int[3][3][3]; ● int count = 1; ● for(int i = 0; i < rubiksCube.length; i++) ● for(int j = 0; j < rubiksCube[i].length; j++) ● for(int k = 0; k < rubiksCube[i][j].length; k++) ● rubiksCube[i][j][k] = count++;

23. cont.. ● Notice rubriksCube[i][j].length and rubriksCube[i].length ● As we discussed earlier, multidimensional arrays are arrays of arrays, so rubriksCube[i] is an array and so is rubriksCube[i][j] ● Is rubriksCube[i][j][k] an array?

24. (dis)advantages of multidimensional arrays ● Sometimes, having higher dimensions are troublesome ● In general, with an addition of a dimension, you include another loop. 5 dimension means 5 loops. 5 loops means messy code ● Linus Torvalds (The father of awesome Linux) says: If you are using more than 3 loops, you are doing it wrong. ● But sometimes you do need more loops. ● How many elements are there int[5][10[100]?

25. Questions