What are they? Why do we need them? Casts What are they? Why do we need them? Uses Scanner Copyright © 2005-2009 Curt Hill

Mixed Mode Arithmetic Multiple types in one arithmetic expression 3.5 * 2 A real and integer constant Most programming languages allow some form of mixed mode C and C++ are quite liberal Java somewhat more restrictive Copyright © 2005-2009 Curt Hill

Better Example Suppose: Scanner inp = new Scanner(System.in); int a=2, b = inp.nextInt(); double d = inp.nextDouble(); d = d*b – a; // Legal a = b*d – a; // Not legal What is the difference? One loses information and the other does not! Copyright © 2005-2009 Curt Hill

Conversions Java will perform certain conversions automatically Converting an integer value to a double value is automatic Doubles are stronger This conversion does not lose anything The conversion from double to int will not be done automatically Copyright © 2005-2009 Curt Hill

When is the conversion done? Consider the following: int a=2, b=3; double d = 3; d = a/b; What do we get? From an algebra view: 2/3 should be .6666… Actually we get zero Copyright © 2005-2009 Curt Hill

Why zero? Java only looks at the immediate pair of operands and refuses to look ahead So it first does a/b, which is 2/3 Since both are integers, we do integer division This gives a zero Copyright © 2005-2009 Curt Hill

Observations Java is very short sighted Even though the resulting expression of: d = a/b*d; is double, it does no conversion until needed It avoids some thornier problems by doing so Copyright © 2005-2009 Curt Hill

So what is the fix? How do we get 2/3 instead of zero? A cast A cast is a conversion of type from one type to another The variable type is not changed! We extract the value and change the type of that value Copyright © 2005-2009 Curt Hill

The Java Cast The form of a cast is the type in parenthesis This is a unary operator, it takes the operand on its right and converts the value into the appropriate type: (double)a/b It has very high precedence so it occurs before the division Copyright © 2005-2009 Curt Hill

The 2/3 problem solved We have two ways to accomplish what we wanted: d = (double)a/b; d = a/(double)b; These work, but this does not: d = (double)(a/b); The damage is already done Copyright © 2005-2009 Curt Hill

More on casts The above example helped us to strengthen a weak type (int) into a stronger type (double) We can also go the other way: int a=2, b=inp.nextInt(); double d = inp.nextDouble(); a = b*d – a; // Not legal a = (int) (b*d-a); // Legal Copyright © 2005-2009 Curt Hill

Rounding How is a double converted into an int? The pentium may round in four ways: Towards zero (truncation) Toward positive infinity (ceiling) Towards negative infinity (floor) Towards closest (regular rounding) Java only believes in truncation! (int)0.99999 is zero! How do we round? Simple: add 0.5 prior to the cast (int)(0.9999 + 0.5) is (int)(1.4999) is 1 Copyright © 2005-2009 Curt Hill

Dollars and Cents Suppose we hold a monetary value in a double Many manipulations will produce fractional cents Loan payment calculations Interest calculations Since we cannot split the penny, how do we round? Copyright © 2005-2009 Curt Hill

Rounding Dollars and Cents We may only round to an integer, so we first make the quantity an integer number of cents, not dollars Multiply by 100 Add the 0.5 Cast Divide by 100 Like so: d = (int)(d*100 + 0.5)/100.0; Copyright © 2005-2009 Curt Hill

Precedence Chart Again Highest to lowest ( ) - + casts (Unary) * / % + - (Binary arithmetic) = Yes, there are quite a few more! Copyright © 2005-2009 Curt Hill