MATH AND RANDOM CLASSES

 The need for random numbers occurs frequently when writing software.  The Random class, which is part of the java.util class, represents a pseudorandom number generator. A random number generator picks a number at random out of a range of values.

 A pseudorandom number generator performs a series of complicated calculations, based on an initial seed value, and produces a number.  Though they are technically not random (because they are calculated), the values produced appear random.

 The statement double r = Math.random();  Produces a random real number in the range of 0.0 to 1.0, where 0.0 is included and 1.0 is not.  On the AP exam you will be expected to write algebraic expressions involving Math.random() that represent linear transformations of the original interval 0.0<= x <1.0.

 Examples: Produce a random real value x in the range 0.0 <= x <6.0. double x = 6 * Math.random(); Produce a random real value x in the range 2.0 <= x < 3.0. double x = Math.random() + 2; Produce a random real value x in the range 4.0 <= x < double x = 2 * Math.random() + 4; In general, to produce a random real value in the range lowValue <= x < highValue. double x = (highValue – lowValue) * Math.random() + lowValue;

 Using a cast to int, a scaling factor, and a shifting value, Math.random() can be used to produce random integers in any range.  Example: Produce a random integer, from 0 to 99. int number = (int) (Math.random() * 100; In general, the expression (int) (Math.random() * k) Produces a random int in the range 0,1…, k-1, where k is called the scaling factor. Note that the cast to int truncates the real number Math.random() * k.

 Example 2 Produce a random integer, from 1 to 100. int num = (int) (Math.random() * 100 ) + 1; In general, if k is a scaling factor, and p is a shifting value, the statement int n = (int) (Math.random() * k) + p; Produces a random integer n in the range p, p+1, …, p + (k-1). Produce a random integer from 5 to 24. int num = (int)(Math.random() * 20) + 5; There are 20 possible integers from 5 to 24, inclusive.

MATH CLASSES  The Math class provides alarge number of basic mathematical functions that are often helpful in making calculations.  The Math class is defined in the java.lang package  All the methods in the Math class are static methods, which means they can be invoked through the name of the class in which they are defined.

 The methods of the Math class return values, which can be used in expressions as needed.  The following statement computes the absolute value of the number stored in total, adds it to the value of count raised to the fourth poser and stores the result in the variable value.  Value = Math.abs(total) + Math.pow(count,4);

SOME METHODS OF THE MATH CLASS.  static int abs (int num) – Returns the absolute value of num.  static double exp(double power) – returns the value e raised to the specific power.  static double pow(double num, double pow) – returns the value num raised to the specified power.  static double random() – returns a random number between 0.0 (inclusive) and 1.0(exclusive)  static double sqrt (double num) – returns the square root of the num, which must be postiive