1. CSCI1402: Lecture 2 Week 6 Dr. David A. Elizondo Centre for Computational Intelligence School of Computing Office: Gateway 6.61 email: elizondo@dmu.ac.uk

2. Short Circuit Logical Operators  Normal & (AND) and | (OR) operands will always evaluate each operand.  Short circuit && (AND) || (OR) will evaluate the second operand only when necessary (more efficient).  In an AND operation, if the 1st operand is false, the outcome is false, no matter what value the 2nd operand.  In an OR operation, if the 1st operand is true, the outcome of the operation is true no matter what the value of the second operand.

3. Short Circuit Logical Operators public class SCops { public static void main (String args[]) { int n, d, q; n = 10; d = 2; if (d != 0 && (n % d) == 0) System.out.println(d + “ is a factor of “ + n); d = 0; // now, set d to zero // Since d is zero, the 2 nd operand is not evaluated if (d != 0 && (n % d) == 0) System.out.println(d + “is a factor of ” + n); // Now, try same thing without short circuit operator // This will cause a divide by zero error. if (d != 0 & (n % d) == 0) System.out.println(d + “is a factor of ” + n); } Now both expressions are evaluated allowing a division by 0 Short circuit operator prevents division by 0

4. Selection – switch statements  It provides a multi way branch.  It enables a program to select among several alternatives.  Switch is generally more efficient than nested ifs  The value of an expression is successfully tested against a list of constants. When a match is found, the statement sequence associated with that match is executed.

5. Selection – switch statements  General form of a switch statement: switch(expression) { case constant1: statement sequence break; case constant2: statement sequence break; case constant3: statement sequence break;. default: statement sequence }

6. Selection – switch statements  The switch expression must be of type char, byte, short, or int. (floating-point expressions are not allowed).  Often the expression controlling the switch is simply a variable.  The case constants must be literals of a type compatible with the expression.  No two case constants in the same switch can have identical values.

7. Selection – switch statements  The default statement sequence is executed if no case constant matches the expression.  The default is optional; if not present, no action takes place if all matches fail.  When a match is found, the statements associated with that case are executed until the break is encountered or, in the case of default or the last case, until the end of the switch is reached.

8. Selection – switch statements  Sample program: public class SwitchDemo { public static void main(String args []){ int i; for (i=0;i<10;i++) switch(i) { case 0: System.out.println(“i is zero”); break; case 1: System.out.println(“i is one”); break; case 2: System.out.println(“i is two”); break; case 3: System.out.println(“i is three”); break; case 4: System.out.println(“i is four”); break; default: System.out.println(“i is five or more”); }

9. Selection – switch statements  Program output: i is zero i is 0ne i is two i is three i is four i is five or more

10. The break statement  It is “technically” optional (most applications of the switch will use it).  When encountered within a statement sequence of a case, the break statement causes program flow to exit from the entire switch.  If a break statement does not end the statement sequence associated with a case, then all statements at and following the matching case will be executed until a break (or the end of the switch) is encountered.

11. No break sample program class Nobreak { public static void main(String args[]) { int i; for(i=0;i<=5;i++) { switch(i) { case 0: System.out.println(“i is less than one”); case 1: System.out.println(“i is less than two”); case 2: System.out.println(“i is less than three”); case 3: System.out.println(“i is less than four”); case 4: System.out.println(“i is less than five”); } System.out.println(); } The case statements fall through here

12. No break sample program  Program output i is less than one i is less than two i is less than three i is less than four i is less than five i is less than two i is less than three i is less than four i is less than five

13. No break sample program  Program output i is less than three i is less than four i is less than five i is less than four i is less than five  Execution continues into the next case if no break statement is present

14. Empty cases  You can have empty cases: switch(i) { case 1: case 2: case 3: System.out.println(“i is 1, 2, or 3”); break; case 4: System.out.println(“i is 4”); break; }  If i has the value 1,2, or 3, the 1 st println() statement executes. If it is 4, the 2 nd println() statement executes.

15. Nested switch statements  It is possible to have a switch as part of the sequence of an outer switch.  This is called a nested switch.  Even if the case constants of the inner and outer switch contain common values, no conflicts will arise.

16. Nested switch statements  Program sample: switch(ch1) { case ‘A’ : System.out.println(“This is part of an outer switch”); switch(ch2) { case ‘A’ : System.out.println(“This is part of an inner switch”); break; case ‘B’ : //... } // end of inner case break; case ‘B’ : //... }