CPS120: Introduction to
Computer Science
Decision Making in Programs
Decision Making In
Computers

A circuit quite simply allows one out of two
choices to be made depending on its inputs
 When decisions are made in a computer program,
they are simply the result of a computation in
which the final result is either TRUE or FALSE
 The value zero (0) is considered to be FALSE by
C++. Any positive or negative value is considered
to be TRUE
Programming & Decisions
 Practically
all computer programs, when
modeled with a flowchart, demonstrate that
branching occurs within their algorithms.
Selection Statements

The if statement allows the program to test the state
of the program variables using a Boolean expression
Selection Statements
Flow of
control of
if statement
Selection Statements
Selection Statements
case Statement

For convenience, many high-level languages
include a case (or switch) statement
 Allows us to make multiple-choice decisions
easier, provided the choices are discrete
Decision Making in C++
1.
2.
3.
4.
if statement
switch statement
? conditional operator statement
goto statement
Using Relational Operators

Relational operators provide the tools
with which programs make decisions
with true and false evaluations
==
equal to NOTE: this is two equals symbols next
to each other, not to be confused with the assignment
operator, =
>
greater than
<
less than
>=
greater than or equal to
<=
less than or equal to
!=
not equal to
Using Logical Operators

When complex decisions must be coded into an
algorithm, it may be necessary to "chain together"
a few relational expressions (that use relational
operators)
 This is done with logical operators (also called
Boolean operators.)
&&
||
!
is the logical AND operator
is the logical OR operator
is the logical NOT operator
Truth Tables

Use this truth table to determine the results of the
logical operators. In this table, 1 represents TRUE
and 0 represents FALSE.
 Note that the ! symbol (the logical NOT operator)
changes a TRUE to a FALSE.
AND
A
0
0
1
1
B
0
1
0
1
A && B
0
0
0
1
OR
A
0
0
1
1
B
0
1
0
1
A || B
0
1
1
1
NOT
A
0
1
!A
1
0
Order of Logical Operations

Logical operators may be mixed within
evaluation statements but the following order of
preference must be respected:
1.
2.
3.

NOT operator (!)
AND operator (&&)
OR operator (||)
Short-circuit evaluation in C++ allows the
compiler to quickly evaluate a logical expression
in some cases without having to completely
evaluate each part of the expression
Complete order of operations

The complete order of operations including all
of the arithmetic, relational, and logical
operators including all of the basic arithmetic,
relational, & logical operators is:
*, /, %
+, <, >, <=, >=, ==, !=
!
&&
||
Compare and Branch
 A program
can instruct a computer to
compare two items and do something
based on a match or mismatch which, in
turn, redirect the sequence of programming
instructions.
 There
are two forms:
 IF-THEN
 IF-THEN-ELSE
Levels of Complexity for if
 Simple
if statement
 if … else statement
 Nested if … else statement
 else..if ladder
IF-THEN
Entry
Test
condition p
Exit
false
true
True
statement a
Use the “IF” structure
 Practically
all computer languages have
some sort of if structure. In C++, the if
structure is a one-way selection structure:
if (number == 3)
{
cout << "The value of number is 3";
}

IF structures use a control expression to
determine if the code in the braces is to be
executed or not
Compound Conditionals
 You
must use the AND operator (&&) to
form a compound relational expression:
if (0 < number && number < 10)
{
cout << number << " is greater
than 0 but less than 10." << endl;
}
Coding IF Structures
 Place
a semicolon at the end of each
statement within the braces, which can
contain many executable statements
 Use curly braces around the body of an
IF structure even if there is only one
statement
IF…ELSE
Entry
Test
condition p
false
“false”
statement a
true
Exit
“true”
statement a
General Form
if (test expression)
{
True-block statements;
}
else
{
False-block statements;
}
next statement;
The If … Else statement

Two-way selection structure since either the block
of code after the "if" part will be executed or the
block of code after the "else" part will be executed

The “If" part is executed if the control expression
is TRUE while the "else" part is executed if the
"if" part is FALSE, guaranteeing one part of the
expression to be executed or the other
if… else if Ladder: General Form
if (condition 1)
statement 1;
else if (condition 2)
statement 2;
else if (condition 3)
statement 3;
else if (condition n)
statement n;
else
default statement;
statement x;
if/else if statement


As soon as a true condition is found, the statement associated
with it is executed and control is transferred to the statement
after the ladder
The else clause is optional just as it is with an if statement.
if (number > 10)
{
cout << number << " is greater than 10." << endl;
}
else if (number <= 9 && number >= 6)
{
cout << number << " is greater than 5
and less than 10." << endl;
}
else
{
cout << number << "must be less than 6." << endl;
}
number = number + 1;
Nested If Statements
 If
structures and if/else statements can
be nested within one another in order to
model complex decision structures.
 Use
the braces and semicolons properly
when coding such structures.
General Form
if (test condition 1)
{
// true-block1 statements
if (test condition 2)
{
true-block2 statements;
}
else
{
false-block2 statements;
}
}
else
{
false-block1 statements;
}
Switch Structure

The switch structure is a multiple-selection
structure that allows even more complicated
decision statements than a two-way if/else
structure allows.

It chooses one of the "cases" depending on the
result of the control expression.

Only variables with the INT or CHAR data types
may be used in the control expressions (i.e.
parentheses) of switch statements.


Single quotes must be used around CHAR variables
Single quotes are NOT used around the integer values
Sample Switch Structure
switch (character_entered)
{
case ‘A’ :
cout << “The character entered was A.\n”;
break;
case ‘B’:
cout << “The character entered was B.\n”;
default:
cout << “Illegal entry\n”;
}
Switch Structure
 The
break statement must be used within
each case if you do not want following
cases to evaluate once one case is found.
When the break statement is executed
within a switch, C++ will execute the next
statement outside of the switch statement.
 The default case, if present, will result if
none of the prior cases apply
Grouping Cases
switch (number)
{
case 1:
case 3:
case 5:
case 7:
case 9:
cout << number << " is an even number." << endl;
break;
case 2:
case 4:
case 6:
case 8:
cout << number << " is an odd number. " << endl;
break;
default:
cout << number << " is not a value between or including 1 and 9." <<
endl;
break; }
Conditional Operator
 General
form:
conditional expression ? exp-1:exp-2 ;

The conditional expression is evaluated first. If
the result is non-zero, exp-1 is evaluated and is
returned as the value of the conditional
expression. Otherwise, exp-2 is evaluated and
its value is returned
f=(c = = 'y') ? 1: 0;
GOTO Statement
 Unconditional
branching
GOTO label;
…
label: statement;
 Not recommended but may be used
occasionally
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CPS120: Introduction to Computer Science