 Chapter 4 Making Decisions

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Chapter 4 Making Decisions

Topics 4.1 Relational Operators 4.2 The if Statement 4.3 Flags
4.4 Expanding the if Statement 4.5 The if/else Statement 4.6 The if/else if Statement 4.7 Using a Trailing else 4.8 Menus 4.9 Nested if Statements

Topics 4.10 Logical Operators
4.11 Checking Numeric Ranges with Logical Operators 4.12 Validating User Input 4.13 More About Variable Definitions and Scope 4.14 Comparing Strings 4.15 The Conditional Operator 4.16 The switch Statement 4.17 Testing for File Open Errors

4.1 Relational Operators Used to compare numeric values to determine relative order Relational Operators: > Greater than < Less than >= Greater than or equal to <= Less than or equal to == Equal to != Not equal to

Relational Expressions
All the relational operators are binary: two operands; All the relational operators have left-to-right associativity: left-to-right order on which an operator works with its operands Relational expressions are Boolean (i.e., evaluate to true or false) Examples: 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

Relational Expressions
Can be assigned to a variable: result = x <= y; Relational expressions have higher precedence than the assignment operator Assigns 0 for false, 1 for true Note: computer will assign 1 to truth, but 1 is not the only value regarded as true Do not confuse = and ==

Ex. Assume x is 10, y is 7, and z,a, and b are ints or bools. What’s the outcome of each of following statements z= x < y; cout << (x > y); a = x >= y; cout << (x <= y); b = y != x; P149 Table 4-4, p161 P150 p161, Checkpoint,4.1, 4.4

4.2 The if Statement The if statement can cause statements to execute only under certain conditions Models the way we mentally evaluate situations: “If it is raining, take an umbrella.” Format: if (expression) statement;

if statement – what happens
To evaluate: if (expression) statement; If (expression) is true, then statement is executed. If (expression) is false, then statement is skipped.

if statement – what happens
expression expression is false expression is true statement

if statement notes Example: Program 4-2,
Do not place ; after (expression) Place statement; on a separate line after (expression), indented: if (score > 90) grade = 'A';

if statement notes Don’t test floats, doubles for equality
double a=1.5, b=1.5; a += ; if( a==b ) cout<<“Both have the same value”; else cout<<“They have different values”; 0 is false; any other value is true if(value) cout <<“It is true”; Example Program 4-3

4.3 Flags Variable that signals a condition Often implemented as bool
As with other variables in functions, must be assigned an initial value before it is used Example

4.4 Expanding the if Statement
To execute > 1 statement as part of an if statement, enclose them in { }: if (score > 90) { grade = 'A'; cout << "Good Job!\n"; } { } creates a block of code

Ex. Write an if statement that:
Assign 0 to x if y is equal to 20. Assign .20 to commission if sales is greater than or equal to Checkpoint, p174,

4.5 The if/else Statement The if/else statement will execute one group of statements if the expression is true, or another group of statements if the expression is false Allows choice between statements if (expression) is true or false Format: if (expression) statement1; // or block else statement2; // or block

if/else – what happens To evaluate:
if (expression) statement1; else statement2; If (expression) is true, then statement1 is executed and statement2 is skipped. If (expression) is false, then statement1 is skipped and statement2 is executed.

if/else – what happens expression expression expression is false
is true statement1 statement2

if/else Ex: write an if/else statement that
Example: program 4-6, Ex: write an if/else statement that assign 1 to x if y is equal to 100. Otherwise it should assign 0 to x Assign 0.10 to commission unless sales is greater than or equal to in which case it assigns 0.20 to commission Checkpoint, p178:

4.6 The if/else if Statement
The if/else if statement is a chain of if statements. They perform their tests, one after the other, until one is found to be true Also models thought processes: “If it is raining, take an umbrella, else, if it is windy, take a hat, else, take sunglasses”

if/else if format if (expression) statement1; // or block
else if (expression) statement2; // or block . . // other else ifs statementn; // or block Example: prog 4-7, sample input: 70 Prog 4-8, sample input:70

4.7 Using a Trailing else A trailing else, placed at the end of an if/else if statement, provide default action when none of (expression) is true Provides default statement/action Used to catch invalid values, other exceptional situations Example: using trailing else

4.8 Menus Use if/else if statements to create menu-driven program
Menu: list of choices on the screen Menu-driven program: program execution controlled by user selecting from a list of actions

Display list of numbered or lettered 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) Example: Prog. 4-9 [Class 8]

4.9 Nested if Statements A nested if statement is an if statement in the conditionally executed code of another if statement. Can be used to evaluate > 1 data item or condition: if (score < 100) { if (score > 90) grade = 'A'; }

Notes on coding nested ifs
An else matches the nearest if that does not have an else: if (score < 100) if (score > 90) grade = 'A'; else ...// goes with second if, // not first one Proper indentation helps greatly Important: which if statement each else belongs to Example Ex: write nested if statements that perform the following test: if amount1 is greater than 10 and amout2 is less that 100, display the greater of the two. Ex: Checkpoint p ,

4.10 Logical Operators Logical operators connect two or more relational expressions into one or reverse the logic of an expression. Operators, meaning, and explanation: && AND New relational expression is true if both expressions are true || OR New relational expression is true if either expression is true ! NOT Reverses the value of an expression – true expression becomes false, and false becomes true

Logical Operators - examples
int x = 12, y = 5, z = -4; (x > y) && (y > z) (x > y) && (z > y) (x <= z) || (y == z) (x <= z) || (y != z) !(x >= z)

Logical Operators - precedence
! has highest precedence, followed by &&, then || a<b || y== z && m> j ! Operator has higher precedence than many of the C++ operators ! (x > 2) is not same as !x > 2 && and || operators rank lower in precedence than the relational operators (a>b) && (x<y) is the same as a> b && x<y

Logical Operators - short circuit evaluation
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) if ( csGrade > 90 && mathGrade > 90 ) cout<<“Your are very good!” if ( csGrade > 90 || mathGrade > 90 ) Example

4.11 Checking Numeric Ranges with Logical Operators
Logical operators are effective for determining if a number is in or out of a range Used to test to see if a value falls into a range: if (grade >= 0 && grade <= 100) cout << "Valid grade"; Can also test to see if value falls outside of range: if (grade < 0 || grade > 100) cout << "Invalid grade"; Cannot use mathematical notation: if (0 <= grade <= 100) //doesn’t work!

Ex: write an if statement that prints the message:
“The number is valid” if the variable speed is within the range 0 through 200 “The number is not valid” if the variable speed is within outside the range 0 through 200 Ex. Checkpoint p ,

4.12 Validating User Input Input validation: inspecting data to a program to determine if it is acceptable Bad output will be produced from bad input Can perform various tests: Range Reasonableness Valid menu choice Divide by zero Example: Program 4-12

4.13 More About Variable Definitions 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

Still More About Variable Definitions and Scope
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. When in inner block, outer definition is not available Not a good idea Example

4.16 The switch Statement Used to select among statements from several alternatives May be used instead of if/else if statements

switch statement format
switch (expression) //integer { case exp1: statement1; case exp2: statement2; ... case expn: statementn; default: statementn+1; }

switch statement requirements
1) expression must be an integer variable or an expression that evaluates to an integer value exp1 through expn must be constant integer expressions or literals, and must be unique in the switch statement default is optional but recommended

switch statement – how it works
1) expression is evaluated The value of expression is compared against exp1 through expn. If expression matches value expi, the program branches to the statement following expi and continues to the end of the switch If no matching value is found, the program branches to the statement after default:

break statement Used to stop execution in the current block
Also used to exit a switch statement Useful to execute a single case statement without executing the statements following it Example: Prog 4-19 Prog 4-20 Prog 4-21 switch (expression) //integer { case exp1: statement1; break; case exp2: statement2; ... case expn: statementn; default: statementn+1; }

Using switch with a menu
switch statement is a natural choice for menu-driven program: display menu get user input use user input as expression in switch statement use menu choices as expr in case statements Example: Prog 4-22 Do checkpoint p :

4.17 Testing for File Open Errors
Can test a file stream object to detect if an open operation failed: infile.open("test.txt"); if (!infile) { cout << "File open failure!"; } Can also use the fail member function if (infile.fail())

4.14 Comparing Strings Can not use relational operators with character strings Must use the strcmp function to compare C-strings strcmp compares the ASCII codes of the characters in the strings. Comparison is character-by-character

Comparing Strings strcmp(str1, str2): compares strings str1 and str2 (ASCII code) returns 0 if the strings are the same, negative number if str1 < str2, positive number if str1 > str2 char myName = "George"; char yourName = "Georgia"; if (strcmp(myName,yourName) < 0) cout << myName << " comes before " << yourName << " in the alphabet"; [Class 9]

Comparing Strings Example:
Prog. 4-16 Prog. 4-17 The two following statements perform the same operation if( strcmp(firstString, secondString) == 0) if( ! strcmp(firstString, secondString) ) Sorting strings Example: prog. 4-18 Do checkpoint p211:

4.15 The Conditional Operator
Can use to create short if/else statements Format: expr ? expr : expr; Ternary operator x<0 ? y=10 : z=20; First Expression: Expression to be tested 2nd Expression: Executes if first expression is true 3rd Expression: Executes if the first expression is false if ( x>100 ) a=0; else a=1; a = x > 100 ? 0 : 1;

The Conditional Operator
The value of a conditional expression is The value of the second expression if the first expression is true 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 cout << “Your grade is “ << (score < 60 ? “Fail.” : “Pass.”);