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

2. Selection  Sometimes we may want an instruction, or set of instructions, to be executed only if a particular condition is true.  An example is a simple computerized guessing game that would be suitable for small children. In the first version of the game, the program asks the player for a letter between A and Z. If the player chooses the same letter as the one stored in the program, a message “**Right** is displayed on the screen.  The logic of this program is as follows:  Get a letter from the player at the keyboard  IF this letter is equal to the letter stored in the program THEN display **Right**

3. Selection – if statement  The if conditional statement can execute selectively a part of a program. if (condition) statement; Here condition is a Boolean expression (true or false). If condition is true, then the statement is executed. If condition is false, then the statement is bypassed.

4. Selection – if statement  Example: if (10 < 11) System.out.println(“10 is less than 11”); Since 10 < 11, the conditional expression is true, and println() will execute. if (10 < 9) System.out.println(“This won’t be displayed”); Since 10 is not < 9, the println will not be executed.

5. Selection – if statement  Complete form of the if statement: if(condition) statement; else statement; The targets of the if and else are single statements

6. Selection – if statement  General form of the if statement: if(condition) { statement; } else { statement; }  The targets of both if and else are blocks of statements  If the conditional expression is true, the target of the if will be executed; otherwise, if it exists, the target of the else will be executed.

7. Selection – if statement if (number > = 0) System.out.println(“+”);elseSystem.out.println(“-”); write out + write out - true false number >= 0

8. Selection – if statement  Example: public class Guess { public static void main(String args[]) throws java.io.IOException { char ch, answer = ‘K’; System.out.println(“I’m thinking of a letter between A and Z.”); System.out.println(“Can you guess it: ”); ch = (char) System.in.read(); // read a character from the KB if (ch == answer) System.out.println(“*** RIGHT ***“); }

9. Selection – if statement  This program prompts the player and then reads a character from the KB.  Using an if statement, it then checks that character against the answer, which is K in this case.  If K was entered, the message is displayed. When you try this program K must be entered in uppercase.

10. Selection – if statement  Enhancing the if statement: public class Guess2 { public static void main(String args[]) throws java.io.IOException { char ch, answer = ‘K’; System.out.println(“I’m thinking of a letter between A and Z.”); System.out.println(“Can you guess it: ”); ch = (char) System.in.read(); // read a character from the KB if (ch == answer) System.out.println(“*** RIGHT ***“); else System.out.println(“... Sorry, you’re wrong.”); }

11. Nested ifs  A nested if is an if statement that is the target of another if or else.  Nested if s are very common in programming.  IMPORTANT:  IMPORTANT: an else statement always refers to the nearest if statement that is within the same block as the else, and not already associated with an else.

12. Nested ifs  Example: if (i == 10) { if(j < 20) a = b; if(k > 100) c = d; else a = c; // this else refers to if (k < 100) } else a = d; // this else refers to if(i == 10)  The final else is not associated with if (j < 20), because it is not in the same block.

13. Nested ifs  Enhancements to the guess program public class Guess3 { public static void main(String args[]) throws java.io.IOException { char ch, answer = ‘K’; System.out.println(“I’m thinking of a letter between A and Z.”); System.out.println(“Can you guess it: ”); ch = (char) System.in.read(); // read a character from the KB if (ch == answer) System.out.println(“*** RIGHT ***“); else { System.out.print(“... Sorry, you’re wrong.”); // a nested if if (ch < answer) System.out.println(“too low”); else System.out.println(“too high”); }

14. Nested ifs  Sample run of enhanced guess program: I’m thinking of a letter between A and Z. Can you guess it: Z...Sorry, you’re too high

15. The if-else-if Ladder  if-else-if ladder: if (condition) statement; else if (condition) statement; else if (condition) statement;. else statement;

16. The if-else-if Ladder  The conditional expressions are evaluated from top downward.  As soon as a true condition is found, the statement associated with it is executed, and the rest of the ladder is bypassed.  If none of the conditions is true, the final else statement will be executed.  The final else often acts as the default condition: if all conditional tests fail, the last else statement is performed.

17. The if-else-if Ladder  Example: public class Ladder { public static void main(String args[]) { int x; for(x=0;x<6;x++) { if(x==1) System.out.println(“x is one”); else if(x==2) System.out.println(“x is two”); else if(x==3) System.out.println(“x is three”); else if(x==4) System.out.println(“x is four”); else System.out.println(“x is not between 1 and 4”); } Default statement

18. The if-else-if Ladder  Program output: x is not between 1 and 4 x is one x is two x is three x is four x is not between 1 and 4  Default else statement is executed only if none of the preceding if statements succeeds.

19. Relational and Logical Operators  Relational operators: relationships than values can have with one another.  Logical operators: ways in which true and false values can be connected together.  Relational operators produce true and false results

20. Relational operators OperatorMeaning == (!!! Different than =)*Equal to !=Not equal to >Greater than <Less than >=Greater than or equal <=Less than or equal

21. Logical Operators OperatorMeaning &AND |OR ^XOR ||Short circuit OR &&Short circuit AND !NOT

22. Relational Operators Example public class RelOps{ public static void main(String args []) { int i, j; i = 10; j = 11; if (i < j) System.out.println(“i < j”); if (i <= j) System.out.println(“i <= j”); if (i != j) System.out.println(“i != j”); if (i == j ) System.out.println(“this won’t execute”); if (i >= j) System.out.println(“this won’t execute”); if (i > j) System.out.println(“this won’t execute”); }

23. Relational Operators Example  Output of the program: i <j i <=j i !=j

24. Relational Operators Example  Output of the program: i <j i <=j i !=j

25. Logical Operators Example public class LogOps{ public static void main(String args []) { boolean b1, b2; b1 = true; b2 = false; if (b1 & b2) System.out.println(“this won’t execute”); if (!(b1 & b2)) System.out.println(“!(b1 & b2) is true”); if (b1 | b2) System.out.println(“b1 | b2 is true”); if (b1 ^ b2) System.out.println(“b1 ^ b2 is true”); }

26. Logical Operators Example  Output of the program: !(b1 & b2)is true b1 | b2is true b1 ^ b2is true