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Types in Java

Types in Java. 8 Primitive Types byte, short, int, long float, double boolean Char Also some Object types: e.g. “String” But only single items. What if we want to store lots of items – e.g. midsem marks?. Arrays. Arrays are data structures that can hold a series of values

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Types in Java

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  1. Types in Java • 8 Primitive Types • byte, short, int, long • float, double • boolean • Char • Also some Object types: e.g. “String” • But only single items. What if we want to store lots of items – e.g. midsem marks?

  2. Arrays • Arrays are data structures that can hold a series of values • An array uses an index to access a particular value

  3. Primitive Type Syntax double number = 3;

  4. Array Syntax double[ ] number;

  5. Array Syntax double[ ] number = {1,2,3} ; String s[ ] = {"This", "is", "an", "array"};

  6. Array Syntax double[ ] number; Can also initialize with the “new” construct: double [ ] number = new double[3];

  7. Array Implementation Every time an array is initialized, using new, or by {… }, • a contiguous block of memory is assigned for it • the array name (the identifier number) is assigned the start address (In Java, this address is the reference for this variable). • number[3] gets an address 3 shifted from the start

  8. Parallel Arrays • int[ ] rollNum= {6016, 6024, 6078}; • double[ ] midsemMarks = {61.7, 54 , 74.2 }; • String[ ] name = {"AbhishekA", "AbhishekS", "Ankit"}; • In practice, parallel arrays are hard to maintain. • Better is to define an object with data: rollNum, marks, and name, and define arrays of these objects

  9. Printing an Array • class ArrayPrint • { • public static void main (String arg[]) • { • double numbers[] = {1,2,3}; • double numbers2[] = {1,2,3,4}; • String s[] = {"This", "is", "an", "array"}; • printArray (s,4); • } • public static void printArray (String[] arr, int length) • { for(int j=0;j<length;j++) • System.out.println("Element "+j+"= "+arr[j]); • } • }

  10. Array Length Array objects have some predefined variables e.g. length String s[ ] = {"This", "is", "an", "array"}; System.out.println("Length of s: "+ s.length); Dot Syntax: If x is an object, then x.y tells you that y is some property computed for the object x.

  11. Finding the average class Average { public static void main (String arg[]) { float dailyRainFall[] = {12.3, 13.5, 4.2, 2.4, 1.1, 0, 10.8}; int i; float average = 0; for (i=0; i<7; i++) { average += dailyRainFall[i]; } average /= 7; System.out.println (“Average rain fall: ” + average + “ mm”); } }

  12. Finding the average for (i=0; i<7; i++) { average += dailyRainFall[i]; } average /= 7; Everytime I have another day of rain, I need to change the number “7” in my program Instead, I can use for (i=0; i<dailyRainFall.length; i++) { average += dailyRainFall[i]; } average /= dailyRainFall.length;

  13. class ArrayPrint class ArrayPrint { public static void main (String arg[]) { String s[ ] = {"This", "is", "a", "test", "array"}; printArray(s); System.out.println( "s[0] length is: "+s[0].length() ); } public static void printArray (String[] arr) { for(int j=0;j<arr.length;j++) System.out.println("Element "+j+"= "+arr[j]); } // exercise: define lengthOfStringArray (String[] s) }

  14. Finding maximum class Maximum { public static void main (String arg[]) { int n = 100; double numbers[] = new double[n]; Initialize (numbers, n); System.out.println (“Maximum: ” + FindMax (numbers, n)); }

  15. Finding maximum public static void Initialize (double numbers[], int length) { int i; for (i=0; i<length; i++) { numbers[i] = Math.sin(2*Math.PI/(i+1)) + Math.cos(2*Math.PI/(i+2)); } }

  16. Finding maximum public static double FindMax (double numbers[], int length) { double max = numbers[0]; int i; for (i=1; i<length; i++) { if (numbers[i] > max) { max = numbers[i]; } } return max; } } // end class

  17. Methods to return more than one parameter • Want to identify both maximum marks, and also who has this maximum marks • Need to return two values from FindMax • Use a 2-element array as return type

  18. Finding max and max index class Maximum { public static void main (String arg[]) { int n = 100; double numbers[] = new double[n]; double result[]; Initialize (numbers, n); result = FindMax (numbers, n); System.out.println ("Maximum: " + result[0] + ", Position: " + (int)result[1]); }

  19. Finding max and max index public static void Initialize (double numbers[], int length) { int i; for (i=0; i<length; i++) { numbers[i] = Math.sin(2*Math.PI/(i+1)) + Math.cos(2*Math.PI/(i+2)); } }

  20. Finding max and max index public static double[] FindMax (double numbers[], int length) { double result[] = {numbers[0], 0}; int i; for (i=1; i<length; i++) { if (numbers[i] > result[0]) { result[0] = numbers[i]; result[1] = i; } } return result; } } // end class

  21. Array Variables • Array variables are typed by their array declaration • Is this legal? int[] rollNum= {6016, 6024, 6078}; int[] a = {1,2,3,4}; a = rollNum; • Now the part of memory previously accessed through “a” is lost. “a” and “rollNum” now point to the same place in memory.

  22. Passing Arrays • The method findAverage() is defined thus: public static double findAverage (double[] arr) { for (int i = 1; i<arr.length; i++) { arr[0] += arr[i]; } return (arr[0] / arr.length) ; // will this work? } • Q. I invoke this method on the array numbers[ ]. findAverage(numbers). Does numbers[0] remain what it was or does it change? • Since method arguments in Java are passed by reference – so numbers[0] has the same address as arr[0] inside the method; and modifications to arrays inside the method are reflected outside the method

  23. class ArrayMidsems class ArrayMidsems { public static void main (String arg[]) { int[] rollNum= {6016, 6024, 6078}; double[] midsemMarks = {61.7, 54 , 74.2 }; String[] name = {"Abhishek A", "Abhishek S", "Ankit"}; System.out.println (getMarks(rollNum, midsemMarks, 6024)); } public static void printArray (String[] arr, int length) { for(int j=0;j<length;j++) System.out.println("Element "+j+"= "+arr[j]); } public static double findMax (double numbers[]) { double max = numbers[0]; for (int i=1; i<numbers.length; i++) { if (numbers[i] > max) max = numbers[i]; } return max; } public static double findAverage (double[] arr) { double sum = 0; for (int i = 0; i<arr.length; i++) { sum += arr[i]; } return (sum / arr.length) ; } /* gets the marks of i-th student */ public static double getMarks (double[] marks, int i) { if (i > marks.length-1) { System.out.println ("Array index out of bounds"); return -999; } return marks[i]; } /* returns the marks for student with target rollNum */ public static double getMarks (int[] n, double[] marks, int target) { if (n.length !=marks.length) { System.out.println ("Array lengths don't match!"); return -999; } for (int j=0; j< n.length; j++) { if (n[j] == target) return marks[j]; } return -999; } } // end ArrayMidsems

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