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Learn how to declare, access, and manipulate arrays in Java programming. Understand the concept of indexing, allocation/deallocation, and practical use cases with Java examples.

georgemcdonald
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  1. Announcements • Assignment P2 is due on Thursday • Assignment P3 is handed out today • Prelim on Monday the 19th. Coming soooooooooon. Lecture 10

  2. Today’s Topics • Review • Arrays • Allocation/Deallocation • Subscripting • Use of arrays to store large collections of data • Several examples Lecture 10

  3. 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 Lecture 10

  4. Hypothetical Problem • Input: zero or more grades from 0 to 100 preceded by the number of grades5 90 85 40 89 12 • 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? Lecture 10

  5. Use arrays instead! g 90 0 • 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 1 85 2 40 3 89 4 12 Lecture 10

  6. 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 Lecture 10

  7. Another example h 1 • h.length is the number of array elements in array h -- here, h.length is 323 0 1 54 2 -32 3 -101 4 82 . . . 322 93 Lecture 10

  8. h i j k Some notation segment number of values in it empty when g[h..i-1] i-h h = i g[i..j] j+1-i j = i-1 g[j+1..k-1] k-(j+1) j = k g Lecture 10

  9. Declaring an array variable null • 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 null Lecture 10

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

  11. More on declaration and accessing • What happens in the statemente[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. Lecture 10

  12. 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 Lecture 10

  13. 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; } } Lecture 10

  14. 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]); } Lecture 10

  15. 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) } Lecture 10

  16. // 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;} Lecture 10

  17. // 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; } Lecture 10

  18. // 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; } Lecture 10

  19. 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 Lecture 10

  20. Is a given array a palindrome? // Return the value of the statement “array b is a palindrome” staticpublic 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; } Lecture 10

  21. 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 Lecture 10

  22. Initializer Lists Note curly braces • You can initialize the elements of an array just as you can initialize a variable • Exampleint[] 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 Lecture 10

  23. 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); Lecture 10

  24. 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 parametersDeclaration: q(int x)Call: q(a[3]) Lecture 10

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