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Chapter 4
Making Decisions
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Topics
4.1 Relational Operators4.2 The if Statement4.3 Flags4.4 Expanding the if Statement4.5 The if/else Statement4.6 The if/else if Statement4.7 Using a Trailing else4.8 Menus4.9 Nested if Statements
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Topics
4.10 Logical Operators4.11 Checking Numeric Ranges with Logical Operators4.12 Validating User Input4.13 More About Variable Definitions and Scope4.14 Comparing Strings4.15 The Conditional Operator4.16 The switch Statement4.17 Testing for File Open Errors
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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
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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 true7 <= 5 is false
if x is 10, then x == 10 is true, x != 8 is true, and x == 8 is false
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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 trueNote: computer will assign 1 to truth,
but 1 is not the only value regarded as true
Do not confuse = and ==
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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;
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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;
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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.
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if statement – what happens
expression
statement
expressionis true
expressionis false
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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';
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if statement notes
Don’t test floats, doubles for equalitydouble a=1.5, b=1.5;
a +=0.0000000000001;
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
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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
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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
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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 10000.00
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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
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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.
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if/else – what happens
expression
statement1
expressionis true
expressionis false
statement2
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if/else
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 50000.00 in which case it assigns 0.20 to commission
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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”
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if/else if format
if (expression) statement1; // or block
else if (expression) statement2; // or block . . // other else ifs .
else if (expression) statementn; // or block
Example: prog 4-7, sample input: 70 Prog 4-8, sample input:70
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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
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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
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Menu-driven program organization
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
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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';
}
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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.
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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
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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)
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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
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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 ) cout<<“Your are very good!”
Example
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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
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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
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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
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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
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4.16 The switch Statement
Used to select among statements from several alternatives
May be used instead of if/else if statements
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switch statement format
switch (expression) //integer
{
case exp1: statement1;
case exp2: statement2;
...
case expn: statementn;
default: statementn+1;
}
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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
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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, the 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:
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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;break;
...case expn: statementn;
break;default: statementn+1;
}
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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
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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 infile.open("test.txt");
if (infile.fail()){
cout << "File open failure!";}
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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
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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[10] = "George";
char yourName[10] = "Georgia";
if (strcmp(myName,yourName) < 0)
cout << myName << " comes before "
<< yourName << " in the alphabet";
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Comparing Strings
Example: Prog. 4-16 Prog. 4-17
The two following statements perform the same operationif( strcmp(firstString, secondString) == 0)
if( ! strcmp(firstString, secondString) ) Sorting strings
Example: prog. 4-18
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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 betested
2nd Expression:Executes if firstexpression is true
3rd Expression:Executes if the firstexpression is false
a = x > 100 ? 0 : 1; if ( x>100 )a=0;
else a=1;
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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 operatorcout << “Your grade is “
<< (score < 60 ? “Fail.” : “Pass.”);
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