1. CS100Lecture 101 Announcements Assignment P2 is due on Thursday Assignment P3 is handed out today Prelim on Monday the 19th. Coming soooooooooon.

2. CS100Lecture 102 Today’s Topics Review Arrays –Allocation/Deallocation –Subscripting –Use of arrays to store large collections of data –Several examples

3. CS100Lecture 103 Review of iteration For loops –for(j = 0; j < NUM; j++) System.out.println(j); –What do you think for(;;) does, for example? Invariants break and continue statements

4. CS100Lecture 104 Hypothetical Problem Input: zero or more grades from 0 to 100 preceded by the number of grades 5 9085408912 Task: read grades and compile information about them –print them in reverse order –print them in increasing order –print a histogram So, need to read in all the values before processing, need as many variables as grades... How to do this using the Java we know so far?

5. CS100Lecture 105 Use arrays instead! An array is an ordered list of values. Each value is stored at a position in the array The number referring to the position is called an index In Java, array indices begin at 0 40 85 89 12 90 g 0 1 2 3 4

6. CS100Lecture 106 Deconstruction of this array g[0] is 90 g[1] is 85 g[2] is 40 g[3] is 89 g[4] is 12 g.length is the number of elements in the array, g.length is 5 In “g[4]”, 4 is the index or subscript

7. CS100Lecture 107 Another example h.length is the number of array elements in array h -- here, h.length is 323 -32 54 -101 82 1 h 0 1 2 3 4... 93 322

8. CS100Lecture 108 Some notation segment number of values in it empty when g[h..i-1] i-h h = i g[i..j] j+1-ij = i-1 g[j+1..k-1] k-(j+1) j = k h i j k g

9. CS100Lecture 109 Declaring an array variable int[] g; g float[] averages; averages Employee[] employees; employees Declaring a variable does not “allocate” or create the array of elements; it only declares a variable that can contain a reference to an array of elements. An array is much like a class in that an array variable contains a reference to an array. null

10. CS100Lecture 1010 How to get the space? g = new int[5]; g Employee[] e = new Employee[206]; e 0 1 2 3 4 null null null null null... null null 0 1 2 3 4 … 204 205

11. CS100Lecture 1011 More on declaration and accessing What happens in the statement e[3] = new Employee(“Millett”, 1999); Given allocated array e, we can reference e[0], e[1] …., but we can also use expressions as array indices: e[2*b], e[i], etc.

12. CS100Lecture 1012 Using array elements Suppose g is an array of integers Then, g[i] where i is in range can be used just as an integer variable –g[i] = 3; a = g[i]*2; c.setX(g[i]); –System.out.println(“value is ” + g[i]); Note that arrays are objects -- therefore have the same ‘call by value’ conventions Java does bounds checking -- throws exception if out of bounds

13. CS100Lecture 1013 Example program with arrays // Read in a list of integer grades, preceded by the // number of grades, and print in them in reverse order int n= Integer.parseInt(stdin.readLine()); // number of grades int[ ] g= new int [n];// g[0..n-1] are the grades // Read in the grades int i= 0; while (i != g.length) { g[i]= Integer.parseInt(stdin.readLine()); i= i+1; } // Print grades in reverse order int k= n; while (k > 0) { System.out.println(g[k - 1]); k = k-1; } }

14. CS100Lecture 1014 Rewrite previous with for loops int n= Integer.parseInt(stdin.readLine()); // number of grades int[ ] g= new int [n];// g[0..n-1] are the grades // Read in the grades for(int i = 0; i < g.length; i++) { g[i]= Integer.parseInt(stdin.readLine());} // Print grades in reverse order for (int k = n; k > 0; k--) { System.out.println(g[k - 1]); }

15. CS100Lecture 1015 Histogram example -- explanation Program scheme to print “histogram” of grades // Read in a list of grades in the range 0..100, preceded // by the number of grades, and // print out how many times each grade appears int n= in.readInt(); // number of grades int[ ] f= new int [101]; // f[i] will be the no. of times grade i appears // Initialize frequencies f[0..100] to 0. // Read in the grades and make up array f. // Print the grades and their frequencies (print only the grades in 0..10 // that appeared at least once in the input) }

16. CS100Lecture 1016 // Read in a list of grades in the range 0..100, preceded by the number // of grades, and print out how many times each grade appears int n= in.readInt(); // number of grades int[ ] f= new int [101]; // f[i] will be the no. of times grade i appears // Initialize frequencies f[k] to 0. int k= 0; while (k != f.length) {f[k]= 0; k= k+1;}

17. CS100Lecture 1017 // Read in the grades and make up array f int i= 0; // Inv: i grades have been read in and // each f[k], for 0<=k<=100, contains // the no. of times grade k was read in while (i != f.length) { int grade= Integer.parseInt(stdin.readLine() ); f[grade]= f[grade] + 1; i= i+1; }

18. CS100Lecture 1018 // Print the grades and their frequency (print only the // grades in 0..100 that appeared at least once in the // input) int i= 0; // Inv: the grades 0..i-1 that occurred at least // once have been printed while (i != f.length) { if (f[i] != 0) System.out.println(“grade: ” + i + “ frequency: ” + f[i]); i= i+1; }

19. CS100Lecture 1019 Palindromes A palindrome is a word that reads the same backwards and forwards. –The empty string –A –AA –ABBA –NOON The following are palindromes if blanks and punctuation are ignored –able was I ere I saw elba –a man a plan a canal panama

20. CS100Lecture 1020 Is a given array a palindrome? // Return the value of the statement “array b is a palindrome” static public bool isPalindrome(char[ ] b) { int i= 0; int j= b.length; // Invariant: b is a palindrome iff b[i..j-1] is. In other // words, b[j..length-1] is the reverse of b[0..i-1] while ( j - i >1) { j= j-1; if (b[i] != b[j]) return false; i= i+1; } // {b[i..j-1] has 0 or 1 elements, so it’s a palindrome} return true; }

21. CS100Lecture 1021 Odd Syntax Thing int[] grades; is equivalent to int grades[]; Be careful however: int a, b, c; int[] x, y, z; int r[], s, t[]; Best to associate array brackets with the type not the variable name

22. CS100Lecture 1022 Initializer Lists You can initialize the elements of an array just as you can initialize a variable Example int[] grades = {88, 92, 65, 77, 33}; grades is now an array of 5 integers with values as shown Such an initializer list can only be used when the array is first declared Note curly braces

23. CS100Lecture 1023 Arrays of objects Remember: Java allows arrays of more than ints Suppose you would like an array of strings: String[] words = new String[25]; We could declare an array of employees, as we saw previously: Employee[] workers = new Employee[25]; workers[3] = new Employee(“Millett”, 1999); workers[3].setSalary(50000);

24. CS100Lecture 1024 Arrays as parameters Similar to when passing objects as parameters Declaration: z(int[] list) Call: z(a) where a is declared int[] z can change elements of the array, but not array reference itself Can also pass elements of the array as parameters Declaration: q(int x) Call: q(a[3])