2. 1 Conditions Logical Expressions Selection Control Structures Chapter 5

3. 2 The Judge says, "Listen up!" Who was George Boole and will he help me decide anything? IF I cannot figure this out, THEN I will smash this infernal machine !

4. 3 Flow of Control *Defn => The order in which the computer executes statements in a program *Control Structure *Statements are normally executed in a sequential flow *Control structure statements alter the normal, sequential flow

5. 4 Selection or Branching Statements *Cause the computer to choose between two alternative actions ? stmt 1 stmt 2 FalseTrue

6. 5 Logical Expressions *Also called “Boolean” expressions *Usually the “condition” which is checked *Examples: ? stmt 1 stmt 2 FalseTrue

7. 6 Boolean Data *Some languages (Pascal) have a Boolean type with actual values TRUE and FALSE *C and C++ accomplish this with int values * FALSE with the int value 0 (zero) *TRUE with any nonzero value Does anything get printed?

8. 7 A Boolean Constant *It is possible to create your own Boolean constants  Use another preprocessor statement, the #define Is anything printed?

9. 8 Relational Operators *Used to evaluate how quantities are related *Typically they are mathematical inequality symbols = *Note => =< *Some variations in C *equal = = *not equal != Examples

10. 9 Logical Operators *Used to combine logical (Boolean) expressions *Operators *AND use && *OR use || *NOT use ! *&& and || are binary operators *two operands * ! is a unary operator *one operand Is anything printed?

11. 10 Short-Circuit Evaluation *Consider *If the first condition is FALSE, no need to check second condition (why?) *if we do check the second, it is a run time error (why?) *“Short-circuit evaluation” *evaluate in L to R order *evaluation stops as soon as final truth value can be determined

12. 11 Precedence of Operators ! * / % + - >= = = ! = && | | = High precedence Low precedence See also page A1, Appendix B

13. 12 Changing English Statements to Logical Expressions *We say “x greater than 10 and less than 20” *But *Computer Syntax is different

14. 13 Proper Logical Expressions *We are really wanting to AND two comparisons *(x greater than 10) AND (x less than 20) Is anything printed?

15. 14 Interesting Phenomenon with Floating Point Values *Calculated values may be algebraically equal but evaluate differently This is due to conversion to and from binary, internal round off.

16. 15 Relational Operators with Floating Point Values *Do not compare floating point numbers for equality *Operands for = = must match bit for binary bit *Algebraically equal is not bitwise equal *Instead, compare for closeness

17. 16 The If-Else Form *Note the syntax *The condition expression is usually a comparison *It must be enclosed in parentheses *C++ does not use the “then” key word *Semicolons ; follow statements if (abs (n1 - n2) < 0.0001) cout << "close" << endl; else cout << "not equal" << endl; if (abs (n1 - n2) < 0.0001) cout << "close" << endl; else cout << "not equal" << endl; ? stmt 1 stmt 2 FalseTrue

18. 17 Blocks -- Compound Statements *For the “then” or “else” portion of the statement we may wish multiple statements *Use curly brackets around the block of statements if (abs (n1 - n2) < 0.0001) { cout << "may not be equal but, " ; cout << "close" << endl; } else cout << "not equal" << endl; if (abs (n1 - n2) < 0.0001) { cout << "may not be equal but, " ; cout << "close" << endl; } else cout << "not equal" << endl;

19. 18 The If (only) Form *The “else” portion is optional *If the condition is false nothing is done ? stmt 1 FalseTrue if (n1 == n2) cout << "equal" << endl; if (n1 == n2) cout << "equal" << endl; Example:

20. 19 A Common Mistake *Using the = operator (assignment) instead of the = = (comparison for equality) *Actual result … 5 assigned to x, then that true value used to determine path through the if statement if (x = 5) cout << “ x = 5”; else cout << “x not equal to 5”; What gets printed?

21. 20 The Nested IF IF

22. 21 Nested IF *Syntax calls for a “statement” after the if ( … ) *That statement can be any kind of statement *(List statements we know about) *It can be an if statement cout cin assignment if if (x 5) cout << “hi mom”;

23. 22 The Dangling Else *How to determine which if the else goes with *Example: if (abs (x - 7)) if (x < 7) cout << “x approaches 7 from left”; else cout << “x approaches 7 from the right”; else cout << “x not close to 7”; Rule :An else goes with the closest unmatched if ? ?

24. 23 The Dangling Else  Rule : an else goes with the closest unmatched if  Consider … how do you force an else to go with a previous if ? if (x < y) if (y > 3) cout 3”; else cout << “message about x and y”; if (x < y) if (y > 3) cout 3”; else cout << “message about x and y”; if (x < y) { if (y > 3) cout 3”; } else cout << “message about x and y”; if (x < y) { if (y > 3) cout 3”; } else cout << “message about x and y”; Use { curly brackets } to nest the statements

25. 24 Testing the State of an I/O Stream *The name of the input stream (used by itself) returns a value *returns a 0 if it is NOT successful *it returns a NON zero value if it IS successful

26. 25 Testing the State of an I/O Stream *When reading a file (a named input stream) we wish to know when it reaches the end *Since the name returns a 0 or non-0, this can be used as a Boolean value *Used to control program sequencing, control a file reading loop

27. 26 Algorithm Walk-Through *Checking an algorithm (module) for correctness *Write down what is supposed to be true *before an algorithm runs *after the algorithm runs *Check the source code to make sure these pre- and post-conditions are as specified

28. 27 Implementation Phase *Use a “code walkthrough” *think of it as “playing computer” *execute the module by hand, changing values, etc. to verify *Use an execution trace *use the watch window *Test control structures *use data, program options that will execute each branch of the program at least once

29. 28 Testing and Debugging Hints *Beware of confusing *the assignment = with the equals = = *the bitwise & with the logical && *the bitwise | with the logical | | *Use =, NEVER =  Don’t forget { } around blocks of statements controlled by an if or an else *Echo print input data, test for bad data