Decision I
(if Statement)
Outline
 What is Decision
 if statement (section 4.3)
 Relational & equality operators
 Logical operators
 Multiple Decision - Nested if statements
(section 4.7)
 if statement with compound alternatives
(section 4.4)
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Decision
Or selection is the
choice of alternate
paths (branches)
depending on a
condition that may
arise in the logical
flow of the
algorithm.
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if Statement
if (condition)
statementT;
else
optional
statementF;
two alternatives
 if allows a question to be asked
 There are two basic forms for an if
statement:


a dependent (conditional) statement
a choice between two alternatives
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Table 4.1
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Rational and Equality Operators
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Relational & Equality Operators
(cont’d)
 Relational expressions (conditions) are
used to perform tests for selecting among
alternative statements to execute.
 Typical forms:
variable
variable
variable
variable
relational-operator variable
relational-operator constant
equality-operator variable
equality-operator constant
 Evaluate to Boolean (bool) value of true or
false
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Examples
x
y
-5
-5
x <= 0
true
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-5
7
7
x >= y
false
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Logical Operators
 What if you have more complex conditions, like checking
ranges (0<x<y)?
 The normal way you represent ranges is not valid in
programming.
 You use logical operators to form more complex
conditions
&& (and)
|| (or)
! (not)
 E.g.
(10000 < salary) && (salary <= 20000)
(salary < minSalary) || (dependents > 5)
(temperature > 90.0) && (humidity > 0.90)
winningRecord && (!probation)
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Table 4.3
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&& Operator
9
Table 4.4
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|| Operator
10
Table 4.5
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! Operator
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Exercise
Please analyse, design and implement an
algorithm to calculate and output the tax due
according to the annual income which is
input by the user. The tax rate should be
calculated as follows:
Tax rate = 10% for annual income < 20,000 LE
= 20% for annual income >= 20,000
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Solution Steps
 Problem statement/Requirements
phase.
 Analysis




Input: income
Output: taxDue
Additional program variables.
Processing formulas.
 Design phase.
 Implementation phase.
 Testing phase.
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START
 Design
INPUT
income
taxDue = income * 0.1
True
income <
20000 ?
False
taxDue = income * 0.2
if (income < 20000)
taxDue = income *
0.1;
else
taxDue = income *
0.2;
OUTPUT
taxDue
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STOP
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Multiple Selection
 Problem Modification
Tax rate = 10% for annual income < 20,000 LE
= 20% for 20,000<=annualincome<30000
= 30% for annual income >= 30,000
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 Design Modification
taxDue = income * 0.1
True
if (income < 20000)
taxDue = income *
0.1;
else
income <
20000 ?
False
taxDue = income * 0.2
True
income <
30000 ?
if (income < 30000)
False
taxDue=income*0.2;
else
taxDue = income * 0.3
taxDue=income*0.3;
Nested
if Statements
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Nested if Statements
 Nested logic, in general, is one control
structure containing another similar
control structure
 E.g. one if statement inside another
 Makes it possible to code decisions
with several alternatives
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General Form
if (condition1)
statement1;
else if (condition2)
statement2;
.
.
.
else if (conditionn)
statementn;
else
statemente;
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Example
if (x > 0)
numPos = numPos + 1;
else
if (x < 0)
numNeg = numNeg + 1;
else // x must equal 0
numZero = numZero + 1;
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if Statements with Compound
Alternatives
 Uses { } to group multiple statements.
 Any statement type (e.g. assignment,
function call, if) can be placed within {
}.
 Entire group of statements within { }
are either all executed or all skipped
when part of an if statement.
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Example
if (transactionType == ‘c’)
{ // process check
cout << “Check for $” << transactionAmount << endl;
balance = balance - transactionAmount;
}
else
{ // process deposit
cout << “Deposit of $” << transactionAmount << endl;
balance = balance + transactionAmount;
}
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Next lecture we will continue
decision construct in C++
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