Cosc 1436 chapter 4
4.6 The if / else if Statement
* Tests a series of conditions until one is found to be true * Often simpler than using nested if/else statements 8 Can be used to model thought processes such as: "If it is raining, take an umbrella, else, if it is windy, take a hat, else, take sunglasses"
4.5 Nested if Statements
* An if statement that is nested inside another if statement ** Nested if statements can be used to test more than one condition
Relational Expressions (cont)
* Can be assigned to a variable: result = x <= y; ** Assigns 0 for false, 1 for true *** Do NOT confuse = and ==
4.13 The Conditional Operator
* Can use to create short if / else statements * Format expr ? expr : expr ' x <0 ? y = 10 : z = 20 ; look at slide for the rest of the info ****slide 60*****
Menu-Driven Program Organization
* Display list of numbered or letterd choices for actions * Prompt user to make selection * Test user selection in expression * if a match, then execute code for action * if not, then go on to next expression
4.11 Validating User Input
* Input validation: inspecting input data to determine wheather it is acceptable * Bad output will be produced from bad input * Can perform various test: a) Range b) Reasonableness c) Valid menue choice d) Divide by zero
4.10 Menus
* Menu-driven program: Program execution controlled by user selecting from a list of actions * Menu: List of choices on the screen * Menus can be implemented using if/else if statement
4.4 The if / else Statement
* Provides two possible paths of execution * Performs one statement or block if the expressions is true, otherwise performs another statement or block.
Nested if Statements
From Program 4-10 (ex.) see slide 29, 30
The if/else statement (con)
General Format: if (expression) statement1; // or block else statement2; // or block
If statement (cont) General Format:
if (expression) statement;
switch Statement Format
switch (expression) // integer { case exp1: statement1; case exp2: statement2; ... case expn: statementn; default: statementn+1; } *****see slide 65*****
*Sec 4.2) The IF statement
1) Allows statements to be conditionally executed or skipped over 2) Models the way we mentally evaluate situations: a) "if it is raining, take an umbrella." b) "if it is cold outside, wear a coat."
If statement Notes
1) Do not place ; after ( expression) 2) Place statement; on a separate line after (expression) , indented: if (score > 90) grade = 'A' ; 3) Be Careful Testing floats and doubles for equality 4) 0 is false; and other value is true
switch Statement-How it Works
1) expression is evaluated 2) The value of expression is compared against exp1 through expn. 3) If expression matches value expi, program branches to the statement following expi and continues to the end of the switch 4) If no matching value is found, the program branches to the statement after default:
switch Statement Requirements
1) expression must be an integer variable or an expression that evaluates to an integer value 2) exp1 through expn must be constant integer expressions or literals, and must be unique in the switch statement 3) default is optional but recommended
The Conditional Operator
* The value of a conditional expression is a) The value of the second expression if the first expression is true b) The value of the third expression if the first expression is false * Parentheses () may be needed in an expression due to precedence of conditional operator
4.3 Expanding the if Statement
* To execute more than one statement as part of an if statement, enclose them in { } : if (score > 90) { grade = 'A'; cout << "Good Job! \n"; } * { } creates a clock of code
Break Statement
* Use to exit a switch statement * If it is left out, the program "falls through the remaining statements in the switch statement
Relational Operators
* Used to compare numbers to determine relative order ** Operators: > Greater than < Less than >= Greater than or equal to <= Less than or equal to == Equal to != NOT equal to
4.9 Checking Numeric Ranges with Logical Operators
* Used to test to see if a value falls inside a range: if (grade >= 0 && grade <= 100) cout << "Valid grade"; * Can also test to see if value falls outside of range: if (grade <= 0 l l grade >= 100) cout << "Invalid grade"; * Cannot use mathematical notation: if (0 <= grade <= 100) // DOESN't Work!
4.7 Flags
* Variable that signals a condition * Usually implemented as a bool variable * Can also be an integer a) The value 0 is considered false b) Any nonzero value is considered true * As with other variables in functions, must be assigned an initial value before it is used
Variables with the Same Name
* Variables defined inside { } have local or block scope * When inside a block within another block, can define variables with the same name as in the outer block. a) When in inner block, outer definition is not available b) Not a good idea
Using switch in Menu Systems
* switch statement is a natural choice for menu-driven program: a) display the menu b) then, get the user's menu selection c) use user input as expression in switch statement d) use menu choices as expr in case statements
Comparaing string Objects
*Like characters, strings are compared using their ASCII values string name1 = "Mary"; string name2 = "Mark"; ***The characters in each string must match before they are equal*** name1 > name2 // true name1 <= name2 // false name1 !=name2 // true name1 < "Mary Jane" // true
4.15 More About Blocks and Scope
*Scope of a variable is the block in which it is defined, from the point of definition to the end of the block * Usually defined at beginning of function * May be defined close to first use
4.8 Logical Operators
*Used to create relational expressions from other relational expressions * Operators, meaning, and explanation: SEE SLIDE 40 , and 41 for the rest of the information prog 4-15 slide 42 prog 4-16 slide 43 prog 4017 slide 44
4.14 The switch Statement
*Used to select among statements from several alternatives * In some cases, can be sued instead of if / else if statements
Logical Operator-Notes
1) ! has highest precedence, followed by then l l 2) If the value of an expression can be determined by evaluating just the sub-expression on left side of a logical operator, then the sub-expression on the right side will not be evaluated (short circuit evaluation)
Relation Expressions (cont)
Boolean expressions - true or false Ex. 12 ? 5 is true 7 <= 5 is false if x is 10, then x== 10 is true, x!= 8 is true, and x == 8 is false
4.12 Comparing Characters and Strings
Comparing Characters 1) Characters are compared using their ASCll values 2) 'A' < 'B' 3) "1" < "2" a) The ASCII value of '1' (49) is less than the ASCI value of '2' (50) 4) Lowercase letters have higher ASCII codes than uppercase letters, so 'a' > 'Z'
USE PROPER INDENTATION1
SEE SLIDE 31
Relational Operators compare Strings in Prog 4-21
See slide 58
The if statement-What Happens
To evaluate: if (expression) statement; 1) If the expression is true, then statement is executed. 2) If the expression is false, then statement is skipped
if / else - What Happens
To evaluate; if (expression) statement1; else statement2; 1) If the expression is true, then statement1 is executed and statement2 is skipped 2) If the expression is false, then statement1 is skipped and statement2 is executed.
If statement inf Program 4-2
see slide 12, 13, 14
The if/else statement and Modulus Operator ins Program 4-8
see slide 22, 23,
Testing the Divisor in Program 4-9
see slide 24, 25,
Flowchart for a Nested if Statement
see slide 28,
If / else if Format
see slide 34, prog 4-13 see slide 35 prog 4-14 see slide 36
Input Validation in Prog 4019
see slide 53
Relational Operators Compare Characters in Prog 4-20
see slide 56
The Conditional Operator in Prog 4-22
see slide 62
The switch Statement in Prog 4-23
see slide 66
break and default statements Prog 4-25
see slide 70, 71
Inner Block Variable Definition in Prog 4-29
see slide 75,
Two Variables with the Same Name in Prog 4-30
see slide 77
Flow chart for evaluating a Decision
see slide number 8 and 9