EGR 2261 Unit 4
Control Structures I: Selection
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Read Malik, Chapter 4.
Homework #4 and Lab #4 due next
week.
Quiz next week.
Control Structures
• A program’s execution can proceed:
–
–
–
–
In sequence.
Selectively (branch): making a choice.
Repetitively (iteratively): looping.
By calling a function.
• We’ll look at selection (branching) in Chapter 4, at
repetition (iteration) in Chapter 5, and at function
calls in Chapter 6.
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Control Structures (cont’d.)
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Logical Expressions
• Selection or repetition requires evaluation of a
logical (Boolean) expression that evaluates to a
logical (Boolean) value of true or false.
– Example of a Boolean expression: “8 is greater than 3.”
• Most logical expressions contain one or more
relational operators:
– Allow comparisons.
– Require two operands (binary).
– Next slide lists the relational operators.
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Relational Operators
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Relational Operators and the Simple
Data Types
• Relational operators can be used with all three
simple data types (int, double, and char) :
• Comparing int values:
Example: 8 < 15 evaluates to true
Example: 6 != 6 evaluates to false
• Comparing double values:
Example: 2.5 == 5.8 evaluates to false
Example: 5.9 <= 7.5 evaluates to true
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Relational Operators and the Simple
Data Types (cont’d.)
• Comparing char values :
– Example: 'R' > 'T’
– Result depends on the order of the characters in the ASCII
code.
– From the table in Appendix C (next slide) we see that the
ASCII code for 'R' is 82 and the ASCII code for 'T’ is
84. Therefore 'R' > 'T’ evaluates to false.
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Relational Operators and the
string Type
• Relational operators can also be applied to strings.
– Strings are compared character by character,
starting with the first character.
– Comparison continues until either a mismatch is
found or all characters are found equal.
– If two strings of different lengths are compared
and they're the same through the last character of
the shorter string, the shorter string is less than
the longer string.
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Relational Operators and the
string Type (cont’d.)
• Suppose we have the following declarations:
string
string
string
string
string
str1
str2
str3
str4
str5
=
=
=
=
=
"Hello";
"Hi";
"Air";
"Bill";
"Big";
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Relational Operators and the
string Type (cont’d.)
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Relational Operators and the
string Type (cont’d.)
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Relational Operators and the
string Type (cont’d.)
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One-Way Selection
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One-Way Selection (cont’d.)
• One-way selection syntax:
• statement is executed if the value
of the expression is true.
• But statement is skipped if the value is false;
program goes to whatever comes next.
– statement is any C++ statement. It could be a cin
statement, a cout statement, an assignment statement, ….
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One-Way Selection: Example
•Note indenting, which is for readability.
Compound Statements
• A compound statement (or block) is a group of
statements enclosed in braces:
• Such a compound statement can be used anywhere a
single statement can be used.
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One-Way Selection with a
Compound Statement: Example
Two-Way Selection
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Two-Way Selection (cont’d.)
• Two-way selection syntax:
• If expression is true, statement1 is executed;
otherwise, statement2 is executed.
– statement1 and statement2 are any C++ statements.
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Two-Way Selection: Example
Two-Way Selection with
Compound Statements: Example
Multiple Selections: Nested if
• Nesting: one control statement is located within
another.
• An else is associated with the most recent if that
has not been paired with an else.
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Multiple Selections: Nested if
(cont’d.)
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Comparing if…else Statements with a
Series of if Statements
• The following slide shows another (less efficient) way
to accomplish the same task.
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Comparing if…else Statements with a
Series of if Statements (cont’d.)
• This code accomplishes the same task as the code on
the previous slide, but it’s not as efficient.
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bool Data Type and Logical
(Boolean) Expressions
• The data type bool has logical (Boolean) values
true and false.
• bool, true, and false are reserved words.
• The identifier true has the value 1.
• The identifier false has the value 0.
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The int Data Type and Logical
(Boolean) Expressions
• Early versions of C++ did not provide a built-in bool
data type.
• Instead, logical values were stored in variables of the
data type int, with a value of 0 representing false
and any non-zero value representing true.
• Can still use the int data type to manipulate logical
(Boolean) values, but it’s more convenient to use the
bool data type.
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Logical (Boolean) Operators and
Logical Expressions
• Logical (Boolean) operators: let you combine logical
expressions.
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The ! (Not) Operator
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The && (And) Operator
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The || (Or) Operator
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Precedence of Some Operators
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Associativity of Operators
• Of the operators shown on the previous slide,
operators with the same precedence are evaluated
from left to right. (They have left-to-right
associativity.)
• As always, parentheses can override the normal
order of evaluation.
• Recall that the textbook’s Appendix B (next slide)
shows precedence and associativity of all C++
operators.
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Precedence of Operators (cont’d.)
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Precedence of Operators (cont’d.)
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Precedence of Operators (cont’d.)
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Short-Circuit Evaluation
• Short-circuit evaluation: evaluation of a logical
expression stops as soon as the value of the
expression is known.
• Example:
(age >= 21) || ( x == 5)
(grade == 'A') && (x >= 7)
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Three Common Mistakes
•The next three slides discuss three common
mistakes:
1. Comparing floating-point numbers for
equality.
2. Ignoring the associativity of relational
operators.
3. Confusing equality (==) with assignment
(=).
Comparing Floating-Point Numbers
for Equality: A Precaution
• Comparison of floating-point numbers for equality
may not behave as you would expect.
– Example:
• 1.0 == 3.0/7.0 + 2.0/7.0 + 2.0/7.0
evaluates to false.
• Why? 3.0/7.0 + 2.0/7.0 + 2.0/7.0 =
0.99999999999999989
• Solution: either avoid doing this, or use a tolerance
value instead:
– Example: if fabs(x – y) < 0.000001
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Associativity of Relational Operators:
A Precaution
What we really want here is
if (0 <= num && num <= 10)
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Associativity of Relational Operators:
A Precaution (cont’d.)
• Suppose num = 5:
• Next, suppose num = 20:
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Confusion Between the Equality (==)
and Assignment (=) Operators
• C++ allows you to use any expression that can be
evaluated to either true or false as an expression
in the if statement:
if (x = 5)
cout << "The value is five." << endl;
• This statement is syntactically correct, but it’s almost
certainly not what the programmer intended.
• Using = in place of == is a silent killer:
– It is not a syntax error.
– It is a logical error.
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Program Style and Form (Revisited):
Indentation
• A properly indented program:
– Shows the natural grouping of statements.
– Helps you spot and fix errors quickly.
• Insert a blank line between statements that are
naturally separate.
• Two commonly used styles for placing braces:
– On a line by themselves.
– Or left brace is placed after the expression, and the right
brace is on a line by itself.
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Alternatives to if and if…else
•Using if statements and if…else
statements, you can handle any decisionmaking situations that your programs may
require.
•But C++ does offer two other ways to do the
same sort of thing. You may wish to use these
instead of if and if…else:
•The conditional operator ?:
•The switch structure
Conditional Operator ?:
• The conditional operator looks like this: ?:
– It’s a ternary operator: takes 3 arguments.
• Syntax:
expression1 ? expression2 : expression3
• If expression1 is true, the result of the entire
expression is expression2.
– Otherwise, the result is expression3.
• Concise and efficient, but hard to read.
• See next slide for example.
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Conditional Operator ?: (cont.)
• Example:
max = (a >= b) ? a : b;
• The preceding statement is equivalent to the
following code:
if (a >= b)
max = a;
else
max = b;
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switch Structures
• Syntax is shown at right.
• (expression) is evaluated first,
and it must evaluate to an integer.
• Value of expression determines
which corresponding action is taken.
• Expression is sometimes
called the selector.
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switch Structures (cont’d.)
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switch Structures (cont’d.)
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switch Structures (cont’d.)
• One or more statements may follow a case label.
• Braces are not needed to turn multiple
statements into a single compound statement.
• When a case value is matched, all statements
after it execute until a break is encountered.
• The break statement may or may not appear
after each statement.
• switch, case, break, and default are
reserved words.
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