The Selection Control Structure 2.0. 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; // consequentelse

statement2; // optional alternate consequentFrom 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 FB. 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 FWhat 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.).Likewiseelse {// begin male Male++; Console.Write (“Male”);} // end maleNote 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.csNote 1: switch condition and case label listNote 2: break. What does it do?Note 3: default. Why?Limitation of switch?

Only designed for ordinal values.

Meaning?