Making Decisions (True or False)

1. Making Decisions(True or False) Relational Operators > greater than < less than >= greater than or equal to <= less than or equal to == equal to != not equal to Note: Remember to put spaces around operators for readability.

2. Boolean Expressions Given: a = 4, b = -3, c = 0, d = 8 Evaluate the following boolean expressions as True or False Ex. a < b means is 4 < -3 and the answer is false a – b <= d – 1 ? a + b > c ? d/4 != c ? d – 2*a == c ?

3. Comparison of non-integer Numbers Remember that floating point numbers may not be stored exactly. Avoid comparing the EQUALITY of two non-integer expressions. Use <= or >= where feasible.

4. Value of True & False The boolean value ‘true’ is stored as 1. The boolean value ‘false’ is stored as 0. Boolean expressions evaluate to either ‘true’ or ‘false’ (i.e. 1 or 0) ONLY.

5. Single-sided alternative (if… ) Syntax: if (test condition) { stuff to execute if test condition is true } OR (if there is only one statement to execute) if (test condition) statement to execute; Note: The body of the single-sided alternative is indented. As a matter of preference braces {} may be used even if there is only one statement.

6. Examples int x, y, z; cout << “Enter three integers: “; cin >> x >> y >> z; if (x != 0) z = y / x; ________________________________________ (What is wrong here?) What about ? if (x = 0) cout >> “You cannot divide by 0”;

7. Double Sided Alternative if (test condition) { statements to execute if the test condition evaluates to true } else { statements to execute if the test condition evaluates to false } or if (test condition) statement to execute (true leg); else statement to execute (false leg); Note: DO NOT mix {} and no {} even if one side of the double-sided alternative has only one statement. This is a style requirement!

8. More Truth • true and false are boolean values and bool variables can be initialed Ex. bool ok; ok = false; • A relational value that is true has value 1 • A relational value that is false has value 0 • In if (test condition) structures, if the test condition evaluates to anything other than 0, it is considered true for decision making purposes. • Because real numbers are not necessarily stored exactly you should never use == to compare two floats, two doubles, etc.

9. File Open Errors Filestream objects can be tested like a Boolean expression. This fact can be used to make sure that a file is actually opened successfully before the program tries to use its contents. ifstream fin; fin.open(“inputfile.txt”); if (fin) { process the file as the open was successful } else { display a message that the input file failed to open }

10. Nested if Statements Where appropriate, ‘if’ statements of all kinds can be nested inside another ‘if’ statement. Note the indentation used in the syntax example. Syntax: if (test condition 1) { if (test condition2) { … } } else { … }

11. Careful! • Indentation becomes even more critical • Braces MUST be paired correctly • Can become confusing so documentation of logic may be needed • Check to see if multi-layered embedded ‘if’ statements can be simplified with an if … else if …. else structure (next slide)

12. if … else if … else if … else This structure may be useful if there are a series of conditions to be met. Note that it is just a cleaner appearing permutation of nested ifs. Syntax: if (test condition 1) { … statement(s) to be executed if test condition 1 is true } else if (test condition 2) { … statement(s) to be executed if test condition 1 is false and test condition 2 is true } else if (test condition 3) { … statement(s) to be executed if all previous test conditions are false and test condition 3 is true } else //optional { …statement(s) to be executed if all previous test conditions are false }

13. Boolean Flags A ‘flag’ is a value that is used to signal when a particular condition occurs in a program. • Typically a flag is a boolean (bool) variable or named constant • A flag can also be of type int since 0 is false and all other int values are considered true