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BITP 1113 PROGRAMMING TECHNIQUE
Lecture 6 – Selection
FTMK, UTeM – Sem 1 2013/2014
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LEARNING OUTCOMES At the end of this lecture, you should
be able to use relational operators and expressions to
compare numeric and char values control the flow of the program using if,
nested if, if/else if and switch statement use conditional operator to express if/else
statement
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RELATIONAL OPERATORS Used to compare numbers to determine
relative order Operators:
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> 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 Boolean expressions – 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; Assigns 0 for false, 1 for true Do not confuse = and ==
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THE IF STATEMENT Allows statements to be conditionally
executed or skipped over Models the way we mentally evaluate
situations: "If it is raining, take an umbrella." "If it is cold outside, wear a coat."
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FLOWCHART FOR EVALUATING A DECISION
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THE IF STATEMENT General Format:
if (expression)statement;
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THE IF STATEMENT-WHAT HAPPENSTo evaluate:
if (expression) statement;
If the expression is true, then statement is executed.
If the expression is false, then statement is skipped.
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IF STATEMENT IN PROGRAM 4-2
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Continued…
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IF STATEMENT IN PROGRAM 4-2
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FLOWCHART FOR PROGRAM 4-2 LINES 21 AND 22
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IF STATEMENT NOTES Do not place ; after (expression) Place statement; on a separate line
after (expression), indented:if (score > 90)
grade = 'A'; Be careful testing floats and doubles
for equality 0 is false; any other value is true
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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 block of code
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THE IF/ELSE STATEMENT Provides two possible paths of
execution Performs one statement or block if the expression is true, otherwise performs another statement or block.
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THE IF/ELSE STATEMENT General Format:
if (expression)statement1; // or block
elsestatement2; // or block
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IF/ELSE-WHAT HAPPENSTo evaluate:
if (expression) statement1;else statement2;
If the expression is true, then statement1 is executed and statement2 is skipped.
If the expression is false, then statement1 is skipped and statement2 is executed.
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THE IF/ELSE STATEMENT AND MODULUS OPERATOR IN PROGRAM 4-8
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FLOWCHART FOR PROGRAM 4-8 LINES 14 THROUGH 18
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TESTING THE DIVISOR IN PROGRAM 4-9
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Continued…
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TESTING THE DIVISOR IN PROGRAM 4-9
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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
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FLOWCHART FOR A NESTED IF STATEMENT
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NESTED IF STATEMENTS From Program 4-10
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NESTED IF STATEMENTS Another example, from Program 4-1
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USE PROPER INDENTATION!
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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 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”
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IF/ELSE IF FORMATif (expression)
statement1; // or blockelse if (expression)
statement2; // or block . . // other else ifs .
else if (expression) statementn; // or block
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THE IF/ELSE IF STATEMENT IN PROGRAM 4-13
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USING A TRAILING ELSE TO CATCH ERRORS IN PROGRAM 4-14
The trailing else clause is optional, but it is best used to catch errors.
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This trailing else catches invalid test scores
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FLAGS Variable that signals a condition Usually implemented as a bool variable Can also be an integer
The value 0 is considered false Any nonzero value is considered true
As with other variables in functions, must be assigned an initial value before it is used
Example:31
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FLAGS - EXAMPLE
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// salesQuotaMet as flag
bool salesQuotaMet = false;
if (sales > QUOTA_AMOUNT) salesQuotaMet = true;else salesQuotaMet = false;
if (salesQuotaMet) cout << "You have met your sales quota!\n";
// is equivalent toif (salesQuotaMet == true) cout << "You have met your sales quota!\n";
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LOGICAL OPERATORS Used to create relational expressions
from other relational expressions Operators, meaning, and explanation:
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&& 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
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int x = 12, y = 5, z = -4; (x > y) && (y > z) true
(x > y) && (z > y) false
(x <= z) || (y == z) false
(x <= z) || (y != z) true
!(x >= z) false
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THE LOGICAL && OPERATOR IN PROGRAM 4-15
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THE LOGICAL || OPERATOR IN PROGRAM 4-16
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THE LOGICAL ! OPERATOR IN PROGRAM 4-17
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LOGICAL OPERATOR-NOTES ! has highest precedence, followed by &&, then ||
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)
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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 || grade >= 100) cout << "Invalid grade";
Cannot use mathematical notation:if (0 <= grade <= 100) //doesn’t work!
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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 statements
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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
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VALIDATING USER INPUT Input validation: inspecting input data
to determine whether it is acceptable Bad output will be produced from bad
input Can perform various tests:
Range Reasonableness Valid menu choice Divide by zero
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INPUT VALIDATION IN PROGRAM 4-19
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COMPARING CHARACTERS Characters are compared using their ASCII
values 'A' < 'B'
The ASCII value of 'A' (65) is less than the ASCII value of 'B'(66)
'1' < '2' The ASCII value of '1' (49) is less than the ASCI
value of '2' (50) Lowercase letters have higher ASCII codes
than uppercase letters, so 'a' > 'Z'44
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RELATIONAL OPERATORS COMPARE CHARACTERS IN PROGRAM 4-20
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COMPARING STRING OBJECTS Like characters, strings are compared
using their ASCII values
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string name1 = "Mary";string name2 = "Mark";name1 > name2 // truename1 <= name2 // falsename1 != name2 // true
name1 < "Mary Jane" // true
The characters in each string must match before they are equal
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RELATIONAL OPERATORS COMPARE STRINGS IN PROGRAM 4-21
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THE CONDITIONAL OPERATOR Can use to create short if/else
statements Format: expr ? expr : expr;
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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
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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 operator
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THE CONDITIONAL OPERATOR IN PROGRAM 4-22
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THE SWITCH STATEMENT Used to select among statements from
several alternatives In some cases, can be used instead of if/else if statements
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SWITCH STATEMENT FORMATswitch (expression) //integer{ case exp1: statement1;case exp2: statement2;...case expn: statementn;default: statementn+1;
}
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THE SWITCH STATEMENT IN PROGRAM 4-23
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SWITCH STATEMENT REQUIREMENTS1. 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 WORKS1) expression is evaluated2) 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 exit a switch statement If it is left out, the program "falls
through" the remaining statements in the switch statement
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BREAK AND DEFAULT STATEMENTS IN PROGRAM 4-25
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BREAK AND DEFAULT STATEMENTS IN PROGRAM 4-25
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USING SWITCH IN MENU SYSTEMS switch statement is a natural choice
for menu-driven program: display the menu then, get the user's menu selection use user input as expression in switch
statement use menu choices as expr in case
statements
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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
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INNER BLOCK VARIABLE DEFINITION IN PROGRAM 4-29
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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. When in inner block, outer definition is not
available Not a good idea
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TWO VARIABLES WITH THE SAME NAME IN PROGRAM 4-30
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ASK YOURSELF Do you know how to use relational
operators and evaluate relational expressions?
Can you describe the difference between if,nested if, if/else if statement and a series of if statements?
Do you know how to use switch statement?
What is a flag and how does it work? Can you explain how logical operators
work?
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