The Selection Control Structure Questions on Program 3? 1.Review. 2. Truth-tables. 3. Forms of the if statement. 4. The switch statement.

0. Questions on Program 3? §1) Note: you do not have to enter the date as a string—you can use separate, numeric, variables. §2) To check if something is evenly divisible, you can use modulus e.g. § if ((X % 100) == 0) Console.Write (X + “ is evenly divisible by 100”); 3) Check your answer for “normal” and “unusual” cases. Normal: 1996 (leap year) 1999 (Not a leap year). Unusual: 1700, 1800, 1900 and 2100 are not leap years, but 1600, 2000, and 2400 are leap years

1. Review. §The selection or branching code is implemented by the if statement. §if (condition) //antecedent § statement1; // consequent §else statement2; // optional alternate consequent §From last time: §Why is testing more complex once ifs are used? §How many times will we need to run the program?

Review (cont.). §What are conditions? §A condition is an expression which…. §Types. §The 3 types of condition are: §1. Relational (meaning?) §2. Logical (meaning?) §3. Boolean (meaning?)

2.Truth-tables. §How do we evaluate conditions? That is, how do we tell when they will be true, when false? §We need a method which shows every possible combination of T’s and F’s: Truth tables. §Let X, Y, Z stand for conditions. §E.g. X could be (Age >= 18) §Y could be (Count > 0), etc.

Truth-tables (cont.). §A.Negation (!). ! X § F T § T F §B.Conjunction (&&). X && Y § T T T § T F F § F F T § F F F

Truth-tables (cont.). §C.Disjunction (| |). X | | Y § T T T § T T F § F T T § F F F §What if we had: X && (Y | | Z)? §How many rows on the truth-table? §How do we know? Formula.

Truth-tables (cont.). §Equivalencies. §2 conditions are equivalent if, and only if: §They have the same pattern of T’s and F’s under their main connective (logical operator). §But what is “the main connective”? §Examples: §What is !(X | | Y) equivalent to? §What is !(X && Y) equivalent to?

Truth-tables (cont.). §Why does correct logic matter to the programmer? §Consider the following coding error: §if ((Grade != ‘A’) | | (Grade != ‘B’) | | (Grade != ‘C’) | | (Grade != ‘D’) ) Console.Write(“Fail”); Who fails? Why?

Truth-tables (cont.). §Therefore, we should code: §if ((Grade != ‘A’) && (Grade != ‘B’) && (Grade != ‘C’) && (Grade != ‘D’) ) Console.Write (“Fail”); §Precedence of logical operators: §1) ! §2) && §3) | |

Full precedence. §1) ! + - (as one-place operators) §2) *, /, % §3) +, - (as binary operators) §4), >= §5) ==, != §6) && §7) || §8) = (assignment)

Truth-tables (cont.). §Short-circuit evaluation. §When the compiler evaluates A | | B it can stop with A if……. §But it will have to check both if….. §When the compiler evaluates A && B it can stop with A if……. §But it will have to check both if…..

3. Forms of the if statement. §A.Simple if (no else, no blocks). §if (Month_Number = = 1) Console.Write (“January”); B. if…else. §if (Count ! = 0) Average = Total / Count; else Average = 0;

Forms of the if statement (cont.). §C.Compound/block ifs. §These allow the use of compound statements = a series of statements enclosed in { } §Can be used in antecedent/consequent. §if (Gender = = ‘F’) { // begin female § Female++; § Console.Write ( “Female”); §} // end female

Forms of the if statement (cont.). §Compound/block ifs (cont.). §Likewise §else {// begin male § Male++; § Console.Write (“Male”); §} // end male §Note use of commenting

Forms of the if statement (cont.). §D.Nested ifs. §An if inside an if (the guard in Monty Python and the Holy Grail: if,if, if,…er..) §if (Age >5) if (Age <= 18) Console.Write (“School”); else Console.Write (“College or job”);

Forms of the if statement (cont.). §Nested ifs (cont.). §What does the else match with? Why? §E.The problem of the dangling else. §if (Grade != ‘F’) if (Grade == ‘D’) Console.Write( “Academic probation”); else Console.Write(“Failing.”);

Forms of the if statement (cont.). §Who will fail? §Why? §Solution? Terminate nested if. §if (Grade != ‘F’) { if (Grade == ‘D’) Console.Write( “Academic probation”); } //end else Console.Write(“Failing.”);

4. The switch statement. §See example: Switch.cs §Note 1: switch condition and case label list §Note 2: break. What does it do? §Note 3: default. Why? §Limitation of switch? Only designed for ordinal values. Meaning?