Chapter 4
Selection
Structures:
if and switch
Statements
Instructor:
Yuksel / Demirer
Control Structures in C
• Control structures control the flow of execution in a
program or function.
• Compound statement, written as a group of statements
bracketed by { and } is used to specify a sequential
flow.
• There are three kinds of execution flow:
– Sequence:
• the execution of the program is sequential.
– Selection:
• A control structure which chooses alternative to execute.
– Repetition:
• A control structure which repeats a group of statements.
• We will focus on the selection control structure in this
chapter.
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4-2
Sequence
Entry point
{
statement1;
statement2;
statement3;
…
statementn;
}
statement1
statement2
statementn
Exit point
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4-3
Repetition
Entry point
statement1
statement2
statementn
Exit point
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4-4
Selection
Entry point
selection
statement1
statement2
Exit points
A selection control structure chooses which alternative to execute
by testing the value of key variables.
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4-5
Condition
• Condition: an expression that is either false
(represented by 0) or true (usually represented
by 1)
• It is a criteria.
• Example: rest_heart_rate > 75
• Formats:
variable relational-operator variable
variable relational-operator constant
variable equality-operator variable
variable equality-operator constant
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4-6
Conditions
• A program may choose among alternative
statements by testing the value of key variables.
– e.g., if( your_grade > 60 )
printf(“you are passed!”);
• Condition is an expression that is either false
(represented by 0) or true (represented by 1).
– e.g., “your_grade > 60” is a condition.
• Conditions may contain relational or equality
operators, and have the following forms.
– variable relational-operator variable (or constant)
– variable equality-operator variable (or constant)
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Operators Used in Conditions
Operator
Meaning
Type
<
Less than
Relational
>
Greater than
Relational
<=
Less than or equal to
Relational
>=
Greater than or equal to
Relational
==
Equal to
Equality
!=
Not equal to
Equality
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4-8
Examples of Conditions
Operator
Condition
<=
x <= 0
<
Power < MAX_POW
==
mom_or_dad == ‘M’
!=
num != SENTINEL
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Meaning
x less than or equal
to 0
Power less than
MAX_POW
mom_or_dad
equal to ‘M’
num not equal to
SENTINEL
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Logical Operators
• There are three kinds of logical operators.
– &&: and
– ||: or
– !: not
• Logical expression is an expression which uses
one or more logical operators, e.g.,
– (temperature > 90.0 && humidity > 0.90)
– !(n <= 0 || n >= 100).
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4-10
The Truth Table of Logical Operators
Op 1
nonzero
nonzero
0
0
Op 2
nonzero
0
nonzero
0
Op 1 && Op2
Op 1 || Op2
1
0
0
0
1
1
1
0
Op 1
! Op 1
nonzero
0
0
1
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4-11
Operator Precedence
• An operator’s
precedence determines
its order of evaluation.
• Unary operator is an
operator that has only
one operand.
– !, +(plus sign), -(minus
sign), and &(address of)
– They are evaluated second
only after function calls.
Operator
Precedence
function calls
! + - &
highest
* / %
+ < <= >= >
== !=
&&
||
=
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lowest
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Example
x
-5
power MAX_POW
1024
1024
y
7
item
1.5
MIN_ITEM m_o_d num
-999.0
‘M’
999
SENTINEL
999
Conditions:
1. x<=0
2. Power < MAX_POW
3. x>=y
4. Item> MIN_ITEM
5. m_o_d == ‘M’
6. num != SENTINEL
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4-13
Examples
•
•
•
•
•
salary < MIN_SALARY || dependent > 5
temperature > 90.0 && humidity > 0.90
n >= 0 && n <= 100
n <= 0 && n <= 100
!(0 <= n && n <= 100)
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Example
• x: 3.0, y: 4.0, z: 2.0, flag: 0
• Expressions:
–
–
–
–
! flag
x + y / z <= 3.5
! flag || (y + z >= x – z)
! (flag || (y + z >= x – z))
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Evaluation for !flag || (y + z
>= x - z)
Evaluation
tree
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The result of this
expression is true
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Comparing Characters
• We can also compare characters in C using the
relational and equality operators.
Expression
Value
‘9’ >= ‘0’
1 (true)
‘a’ < ‘e’
1 (true)
‘Z’ == ‘z’
0 (false)
‘a’ <= ‘A’
system dependent
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4-17
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4-18
DeMorgan’s Theorem
• DeMorgan’s theorem gives us a way of transforming a
logical expression into its complement.
– The complement of expr1 && expr2 is comp1 ||
comp2, where comp1 and comp2 are the complement of
expr1 and expr2, respectively.
– The complement of expr1 || expr2 is comp1 &&
comp2.
• e.g., age > 25 && (status == ‘S’ ||
status ==‘D’)
is equal to
!(age <=25 || (status != ‘S’) &&
status != ‘D’)
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4-19
More examples
• in_range = (n > -10 && n < 10)
• is_letter = (‘A’ <= ch && ch <= ‘Z’) ||
(‘a’ <= ch && ch <= ‘z’)
• even = (n % 2 ==0)
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4-20
The if Statement
• The if statement is the primary selection control
structure.
• Syntax: if (condition) statement;
else statement;
• An example of two alternatives:
if ( rest_heart_rate > 56 )
printf(“Keep up your exercise program!\n”);
else
printf(“Your heart is in excellent health!\n”);
• An example of one alternative:
if ( x != 0.0 )
product = product * x;
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4-21
IF() THEN {} ELSE {}
if condition
{compound_statement_1 } // if condition is true
else
{ compound_statement_2 } // if condition is false
Example:
if (crash_test_rating_index <= MAX_SAFE_CTRI) {
printf("Car #%d: safe\n", auto_id);
safe = safe + 1;
}
else {
printf("Car #%d: unsafe\n", auto_id);
unsafe = unsafe + 1;
}
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4-22
Flowchart
• Flowchart is a diagram that shows the step-bystep execution of a control structure.
– A diamond-shaped box represents a decision.
– A rectangular box represents an assignment
statement or a process.
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4-23
Flowcharts of if Statements with (a) Two
Alternatives and (b) One Alternative
Decision
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Decision
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Nested if Statements
• Nested if statement is an if statement with
another if statement as its true task or false
task.
• e.g.,
if (road_status == ‘S’)
if (temp > 0) {
printf(“Wet roads ahead!\n”);
}else{
printf(“Icy roads ahead!\n”);
}
else
printf(“Drive carefully!\n”);
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4-25
An Example for the Flowchart of
Nested if Statements
Another if statement
Main if statement
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IF() THEN {} ELSE {}
• When the symbol { follows a condition or else, the C
complier either executes or skips all statements
through the matching }
• In the example of the previous slide, if you omit the
braces enclosing the compound statements, the if
statement would end after the first printf call.
• The safe = safe + 1; statement would always
be executed.
• You MUST use braces if you want to execute a
compound statement in an if statement.
• To be safe, you may want to always use braces, even
if there is only a single statement.
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4-27
No {}?
if (rest_heart_rate > 56)
printf("Keep up your exercise program!\n");
else
printf("Your heart is in excellent health!\n");
• If there is only one statement between the {} braces,
you can omit the braces.
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4-28
One Alternative?
• You can also write the if statement with a single
alternative that executes only when the condition is
true.
if ( a <= b )
statement_1;
• Another form – seldom used, but still allowed
if ( a == b )
else
statement_2;
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4-29
Nested if Statements
• So far we have used if statements to code decisions with one
or two alternatives.
• A compound statement may contain more if statements.
• In this section we use nested if statements (one if statement
inside another) to code decisions with multiple alternatives.
if (x > 0)
num_pos = num_pos + 1;
else
if (x < 0)
num_neg = num_neg + 1;
else
num_zero = num_zero + 1;
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4-30
Multiple-Alternative Decisions
• If there are many alternatives, it is better to use the
syntax of multiple-alternative decision.
• Syntax:
if (condition1)
statement1
else if (condition2)
statement2
…
else if (conditionn)
statementn
else
statemente
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4-31
An Example of Multiple-Alternative
Decisions
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4-32
Short Circuit Evaluation (1)
• Short-circuit (minimal) evaluation - C stops evaluating a
logical expression as soon as its value can be determined.
– if the first part of the OR expression is true then the overall
condition is true, so we don’t even look at the second part.
– if the first part of the AND expression is false then the
overall condition is false, so we don’t even look at the
second part.
– Example:
if ((a<b) && (c>d)) if (a<b) is false, (c>d) is not evaluated.
if ((e!=f) || (g<h)) if (e!=f) is true, (g<h) is not evaluated.
– This can be significant for your program performance.
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4-33
Short Circuit Evaluation (2)
if (lname == “Smith”) && (ssn == 12345678)
if (ssn == 12345678) && (lname == “Smith”)
• What is the difference?
• AND operator
– You want to get “false” as soon as possible, since it finishes
comparisons
– i.e. the “most selective” test should be placed at the beginning.
• OR operator
– You want to get “true” as soon as possible, since it finishes
comparisons
– i.e. the “least selective” test should be placed at the beginning
and the “most selective” test at the very end.
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4-34
Multiple-Alternative Decision Form of
Nested if
• Nested if statements can become quite complex. If there are more than
three alternatives and indentation is not consistent, it may be difficult for
you to determine the logical structure of the if statement.
• You can code the nested if as the multiple-alternative decision described
below:
if ( condition_1 )
statement_1
else if ( condition_2 )
statement_2
.
.
.
else if ( condition_n )
statement_n
else
statement_e
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4-35
Example
• Given a person’s salary, we want to calculate the tax due by
adding the base tax to the product of the percentage times the
excess salary over the minimum salary for that range.
Salary Range
Base tax
Percentage of Excess
0.00
15
15,000.00 – 29,999.99
2,250.00
18
30,000.00 – 49,999.99
5,400.00
22
50,000.00 – 79,999.99
11,000,00
27
80,000.00 – 150,000.00
21,600.00
33
0.00 – 14,999.99
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4-36
if ( salary < 0.0 )
tax = -1.0;
else if ( salary < 15000.00 )
tax = 0.15 * salary;
else if ( salary < 30000.00 )
tax = (salary – 15000.00)*0.18 + 2250.00;
else if
tax =
else if
tax =
else if
tax =
else
tax =
( salary < 50000.00)
(salary – 30000.00)*0.22 + 5400.00;
( salary < 80000.00)
(salary – 50000.00)*0.27 + 11000.00;
( salary <= 150000.00)
(salary – 80000.00)*0.33 + 21600.00;
-1.0;
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4-37
Order of Conditions in a MultipleAlternative Decision
• When more than one condition in a multiplealternative decision is true, only the task following the
first true condition executes.
• Therefore, the order of the conditions can affect the
outcome.
• The order of conditions can also have an effect on
program efficiency.
• If we know that salary range 30,000 - 49,999 are much
more likely than the others, it would be more efficient
to test first for that salary range. For example,
if ((salary>30,000.00) && (salary<=49,999.00))
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4-38
Nested if Statements with More Than One
Variable
• In most of our examples, we have used nested if
statements to test the value of a single variable.
• Consequently, we have been able to write each nested if
statement as a multiple-alternative decision.
• If several variables are involved in the decision, we
cannot always use a multiple-alternative decision.
• The next example contains a situation in which we can
use a nested if statement as a ”filter” to select data that
satisfies several different criteria.
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4-39
Example
• The Department of Defense would like a program that
identifies single males between the ages of 18 and 26,
inclusive.
• One way to do this is to use a nested if statement whose
conditions test the next criterion only if all previous
criteria tested were satisfied.
• Another way would be to combine all of the tests into a
single logical expression
• In the next nested if statement, the call to printf
executes only when all conditions are true.
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4-40
Example
/* Print a message if all criteria are met.*/
if ( marital_status == ’S’ )
if ( gender == ’M’ )
if ( age >= 18 && age <= 26 )
printf("All criteria are met.\n");
• or we could use an equivalent statement that uses a single if with
a compound condition:
/* Print a message if all criteria are met.*/
if ((maritial_status == ’S’) && (gender == ’M’) &&
(age >= 18 && age <= 26))
printf("All criteria are met.\n");
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4-41
Common if statement errors
if crsr_or_frgt == ’C’
printf("Cruiser\n");
• This error is that there are no ( ) around the condition, and this is
a syntax error.
if (crsr_or_frgt == ’C’);
printf("Cruiser\n");
• This error is that there is a semicolon after the condition. C will
interpret this as there is nothing to do if the condition is true.
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4-42
If Statement Style
• All if statement examples in this lecture have the true
statements and false statements indented. Indentation
helps the reader but conveys no meaning to the
compiler.
• The word else is typed without indentation on a
separate line.
• This formatting of the if statement makes its meaning
more apparent and is used solely to improve program
readability.
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4-43
Tracing an if Statement
• A critical step in program design is to verify that an algorithm or
C statement is correct before you spend extensive time coding or
debugging it.
• Often a few extra minutes spent in verifying the correctness of an
algorithm saves hours of coding and testing time.
• A hand trace or desk check is a careful, step-by-step simulation
on paper of how the computer executes the algorithm or
statement.
• The results of this simulation should show the effect of each
step’s execution using data that is relatively easy to process by
hand.
• Sections 4.4 and 4.5 in the text have great step-by-step examples
of using if statements to solve problems.
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4-44
The switch Statement
• The switch statement is used to select one of several
alternatives when the selection is based on the value of
a single variable or an expression.
switch (controlling expression) {
case label1:
If the result of this controlling
statement1
expression matches label1, execute
break;
staement1 and then break this switch
case label2:
statement2
block.
break;
…
case labeln:
statementn
break;
If the result matches none of all
default:
labels, execute the default statementd.
statementd;
}
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4-45
Switch statements
• The switch statement is a better way of writing a
program when a series of if-elseif occurs.
• The switch statement selects one of several alternatives.
• The switch statement is especially useful when the
selection is based on the value of a single variable or of
a simple expression
– This is called the controlling expression
• In C, the value of this expression may be of type int
or char, but not of type double.
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4-46
An Example of a switch Statement
with Type char Case Labels
class is a char variable.
Two or more cases can execute
the same statement.
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4-47
Case Study (1/2)
• Write a program that
prompts the user to input
the boiling point in degree
Celsius.
• The program should output
the substance
corresponding to the boiling
point listed in the table.
• The program should output
the message “substance
unknown” when it does not
match any substance.
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Substance Boiling point
Water
100°C
Mercury
357°C
Copper
1187°C
Silver
2193°C
Gold
2660°C
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Case Study (2/2)
• Examples of the scenario of your program.
Please input: 357
The substance is Mercury.
Please input: 3333
Substance unknown.
• You can determine the substance within a range
of boiling points to get bonus (e.g., +5 degrees).
– Please refer to pages 207-208 in the text book.
Please input: 359
The substance is Mercury.
• You can apply any technique in this chapter.
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4-49
Figure 4.12 Example of a switch Statement
with Type char Case Labels
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4-50
Explanation of Example
• This takes the value of the variable class and compares it to
each of the cases in a top down approach.
• It stops after it finds the first case that is equal to the value of
the variable class.
• It then starts to execute each line of the code following the
matching case till it finds a break statement or the end of the
switch statement.
• If no case is equal to the value of class, then the default case
is executed.
– default case is optional. So if no other case is equal to the value of the
controlling expression and there is a default case, then default case is
executed. If there is no default case, then the entire switch body is
skipped.
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4-51
Remember !!!
• The statements following a case label may be one or more C
statements, so you do not need to make multiple statements
into a single compound statement using braces.
• You cannot use a string such as ”Cruiser” or ”Frigate” as a
case label.
– It is important to remember that type int and char values may be
used as case labels, but strings and type double values cannot be
used.
• Another very common error is the omission of the break
statement at the end of one alternative.
– In such a situation, execution ”falls through” into the next alternative.
• Forgetting the closing brace of the switch statement body is
also easy to do.
• In the book it says that forgetting the last closing brace will
make all following statements occur in the default case, but
actually the code will not compile on most compilers.
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4-52
Nested if versus switch
• Advantages of if:
– It is more general than a switch
• It can be a range of values such as x < 100
– A switch can not compare Strings or doubles
• Advantages of switch:
– A switch is more readable
• Use the switch whenever there are ten or fewer case
labels
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4-53
Common Programming Errors
• Consider the statement:
if (0 <= x <= 4)
• This is always true!
– First it does 0 <= x, which is true or false so it evaluates to 1 for true and 0 for
false
– Then it takes that value, 0 or 1, and does 1 <= 4 or 0 <= 4
– Both are always true
• In order to check a range use (0 <= x && x <= 4).
• Consider the statement:
if (x = 10)
• This is always true!
– The = symbol assigns x the value of 10, so the conditional statement evaluates
to 10
– Since 10 is nonzero this is true.
– You must use == for comparison
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4-54
More Common Errors
• Don’t forget to parenthesize the condition.
• Don’t forget the { and } if they are needed
• C matches each else with the closest unmatched if, so be careful
so that you get the correct pairings of if and else statements.
• In switch statements, make sure the controlling expression and
case labels are of the same permitted type.
• Remember to include the default case for switch statements.
• Don’t for get your { and } for the switch statement.
• Don’t forget your break statements!!!
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