CMPT 102
Introduction to Scientific
Computer Programming
Conditional Statements
Control Structures
© Janice Regan, CMPT 102, Sept. 2006
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Flowcharts
 Flowcharts use some basic symbols

To start or end a function

To contain calculations

To make decisions

To connect different parts of
an algorithm
© Janice Regan, CMPT 102, Sept. 2006
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Control Structures
 Three methods of processing a program
 In sequence
 Branching
 Looping
 Branch: Altering the flow of program
execution by making a selection or choice
 Loop: Altering the flow of program
execution by repetition of a particular
block of statement(s)
© Janice Regan, CMPT 102, Sept. 2006
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Selection Based on Branching
 One-Way Selection
 One alternative: “Things to do” if a condition is true
 C implementation: if statement
 Two-Way Selection
 Two alternatives: “Things to do” if the condition is true,
a different “things to do” if the condition is false
 C implementation: if-else statement
 Compound Statements
 Allows a group of statements to be considered as a
block
 That block can replace a single statement in any
selection structure
© Janice Regan, CMPT 102, Sept. 2006
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Selection Based on Branching
 Multiple Selections
 A series of different “things to do” if a series
of conditions hold
 C implementation: if-elseif-else structure
 Alternate C implementation: Nested if
statements
 Switch Structures
 A series of different “things to do” depending
on the value of a particular variable
© Janice Regan, CMPT 102, Sept. 2006
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Decisions
 Use a decision statement when an action is to be taken




only if a particular condition holds
The condition which must hold may be logical or
relational expression or a Boolean variable. The value of
the condition must be True or False
Each possible path through a condition statement will
contain a sequence of steps to be executed
The condition and the sequences of steps that are
executed for each outcome of the condition statement
form a selection structure.
A selection structure is a type of control structure
© Janice Regan, CMPT 102, Sept. 2006
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Flowchart: one way selection
 A simple decision
uses a decision
box to hold the
condition (Boolean
value: relational or
logical expression)
 The sequence of
statements is held
in a sequence box
or boxes
© Janice Regan, CMPT 102, Sept. 2006
condition
T
Statement 1;
⋮
Statement n;
F
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Example of one-way
selection
 if statements
 An action is taken only if a particular condition
is true. The action is described by a single C
statement.
 Example:
if (examScore < 50)
fprintf(“Printing email address of failing
student so they can be contacted to arrange
tutoring: email is %s”, emailAddress);
© Janice Regan, CMPT 102, Sept. 2006
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Compound/Block Statement
 Only one statement following the if statement is
part of the if control structure
 Must use a compound statement, implemented
in C as { }, to include more than one statement
in a list of things to do if the condition is true

A compond statement can also be called a "block"
statement
 You should always use a block statement { }
 Even if the block contains just one statement
 Using the { } if there is only one statement is a style
decision, not a requirement of C
© Janice Regan, CMPT 102, Sept. 2006
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One way selection
 Do a series of actions only if a given condition holds
 If the condition does not hold skip the actions
if ( condition )
{
/* Series of actions to be taken */
/* when the condition is TRUE */
action 1;
⋮
action n;
}
© Janice Regan, CMPT 102, Sept. 2006
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Compound Statement in Action
 Note indenting in this example:
if (myScore > yourScore)
{
printf(“My score was higher than yours “);
difference = myScore – yourScore;
printf(“I got %d more points than you did”,
difference)
}
© Janice Regan, CMPT 102, Sept. 2006
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Flowchart: two way selection
 A selection structure
uses a decision
box, and sequence
boxes. There may
be multiple
sequence boxes
along each path
condition
F
T
Statement 1;
⋮
Statement n;
Statement 1;
⋮
Statement n;
© Janice Regan, CMPT 102, Sept. 2006
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Example of two-way selection
 if-else statements
 Choice of two alternate statements based
on condition expression
 Example:
if (examScore > 50)
myCourseGrade = “PASS”;
else
myCourseGrade = “FAIL”;
© Janice Regan, CMPT 102, Sept. 2006
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Compound/Block Statement
 Only one statement following the if statement is
part of the if control structure
 Only one statement following the else statement
is part of the control structure
 Must use a compound statement { } to include
more than one statement
 Style Pointer: Each block (after the if and after
the else) should have block statement even if
the block contains just one statement
© Janice Regan, CMPT 102, Sept. 2006
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Two way selection
 Complete one of two possible series of actions


First series of actions is complete if condition is true
Second series of actions is completed if condition is false

if (condition)
{
//Series of actions to be taken when the condition is TRUE
action 1;
⋮
action n;
}
else
{
// Series of actions to be taken when the condition is FALSE
action 1;
⋮
action n;
}
© Janice Regan, CMPT 102, Sept. 2006
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if-else Statement Syntax
Selection structure for C
 Formal syntax:
 if (<boolean_expression>)
<yes_statement>
else
<no_statement>
 Note each alternative is only ONE statement!
 To have multiple statements execute in
either branch  use compound statement
© Janice Regan, CMPT 102, Sept. 2006
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Flowchart for multiple selection
Statement 1;
T
condition
F
condition2
T
⋮
Statement n;
Statement 1;
⋮
Statement n;
F
Statement 1;
⋮
Statement n;
© Janice Regan, CMPT 102, Sept. 2006
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Example of Multiple selection
 if-else statements

Choice of two alternate statements based
on condition expression

Example:
if (examScore > 80)
myCourseGrade = “A”;
else if (examScore > 60)
myCourseGrade = “C”;
else
myCourseGrade = “FAIL”;
© Janice Regan, CMPT 102, Sept. 2006
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Compound/Block Statement
 Only one statement following the if statement is




part of the if control structure
Only one statement following the else statement is
part of the control structure
Only one statement following the else if statement
of the control structrue
Must use a compound statement { } to include
more than one statement
Style Pointer: Each block should have block
statement even if it contains one statement
© Janice Regan, CMPT 102, Sept. 2006
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Multiple Selections (else if)
if (condition1)
{
// Series of actions to be taken when condition 1 is TRUE
action 1;
action n;
}
else if (condition 2)
{
// actions to be taken when condition 1 is FALSE and condition2 is TRUE
action 1;
action n;
}
else
{
// Series of actions to be taken when condition 1and condition 2 are FALSE
action 1;
action n;
}
© Janice Regan, CMPT 102, Sept. 2006
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Multiway if-else: Example
 Not new, just different indenting
 Avoids "excessive" indenting
 Syntax:
© Janice Regan, CMPT 102, Sept. 2006
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CMPT 102 Programming Style
 When using decision structures (if statements) you should
always use the { } regardless of whether you use a single
or multiple statements in each branch of the if statement.
 The opening and closing brackets for the block should
each be written on their own line. (examples follow)
 WHY?
 As your code evolves it is common to add statements
(functionality) within a decision statement.
 When you add statements and forget to add the {} to
indicate the extent of the block unexpected things
happen. The resulting problems can be difficult to find.
© Janice Regan, CMPT 102, Sept. 2006
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Defining the condition
 Each decision statement is based upon a condition
 The condition is an expression with a logical value (true
or false)
 The condition is a boolean expression and may be
 A relational expression (a type of logical expression)



Two numerical values combined using a binary relational
operator (a simple relational expression)
A more complex relational expression
Another type of logical expression




Two logical values combined with a binary logical operator
One logical value
One logical value operated on by a unary logical operator
A more complex logical expression
© Janice Regan, CMPT 102, Sept. 2006
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Binary Relational Operators in C




<
<=
>
>=
less than
less than or equal to
greater than
greater than or equal to
Binary Equality Operators in C


==
!=
equal to
not equal to

Evaluated left to right
© Janice Regan, CMPT 102, Sept. 2006
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Binary Logical Operators


&&
||
Logical AND
Logical OR
Unary Logical Operators

!
Not
 Evaluated left to right
 Arguments of logical operators have values of
true or false
© Janice Regan, CMPT 102, Sept. 2006
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Truth Table &&
The && (And) operator
EXPRESSION1
EXPRESSION2
T
T
F
F
© Janice Regan, CMPT 102, Sept. 2006
T
F
T
F
EXPRESSION1 &&
EXPRESSION2
T
F
F
F
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Truth Tables ||
The || (Inclusive Or) operator
EXPRESSION1
EXPRESSION2
T
T
F
F
© Janice Regan, CMPT 102, Sept. 2006
T
F
T
F
EXPRESSION1 ||
EXPRESSION2
T
T
T
F
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Truth Tables ^
The ^ (Exclusive or) operator
EXPRESSION1
EXPRESSION2
T
T
F
F
© Janice Regan, CMPT 102, Sept. 2006
T
F
T
F
EXPRESSION1 ^
EXPRESSION2
F
T
T
F
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Truth Tables !
The ! (Not) operator
EXPRESSION1
! EXPRESSION1
T
F
© Janice Regan, CMPT 102, Sept. 2006
F
T
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Precedence of operators in C









( ) [] .
++ -- (pre) + - ! ~(unary)
* / %
+ < <= > >=
== !=
&&
||
= += -= *= /= %=
© Janice Regan, CMPT 102, Sept. 2006
innermost first
(right to left)
(right to left)
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Precedence Examples
 Arithmetic before logical

x + 1 > 2 || x + 1 < -3 means: (x + 1) > 2
|| ( x + 1) < -3
 Short-circuit evaluation


(x >= 0) && (y > 1)
Be careful with increment operators!

(x > 1) && x<y++
 In C Boolean values (true and false) are represented as
integers


All non-zero integer values  true
Zero value  false
© Janice Regan, CMPT 102, Sept. 2006
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Expressions with relational operators
 Value of a relational expression (expression including a
relational or binary equality operator) is true or false.
 Arguments of a relational operator are numerical (or
character)
 A<C
 (A + B) >= C
Let A=9, B=5, C=2
 A * B <= C
 A % C == A % B
 A != -C
 A>B
© Janice Regan, CMPT 102, Sept. 2006
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Expressions: logical operators
 Value of a logical expression (expression
including a logical operator) is true or false.
 Arguments of the logical operator are also true
or false
Let A=9, B=5, C=2 Then
 (C < B) && A >= C
 (A < B) || A > C
 !(A < B) || B<A
 A < B && B < C++
© Janice Regan, CMPT 102, Sept. 2006
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Common Pitfalls in C




Operator "=" vs. operator "=="
One means "assignment" (=)
One means "equality" (==)
These two operators are VERY different
 Example of common error:
if (x = 12) Note operator used!
Do_Something
else
Do_Something_Else
© Janice Regan, CMPT 102, Sept. 2006
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