1. Decision I (if Statement)
Control structures regulate the flow of execution.
So far, you know of three types of control structure: sequential, selection, repetition
if statement is used for selection/decision in C++

2. 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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3. Decision
Or selection is the choice of alternate paths (branches) depending on a condition that may arise in the logical flow of the algorithm.
Decision/Selection construct is a question with two outcomes.
In flow charts the notation used for decision is the diamond shape with the condition/question written inside.
The diamond shape has one arrow going in and two arrows marked true or false coming out.
Notice how the two selection diagrams differ. One with two alternatives, and one with only a dependant statement.
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4. if Statement if allows a question to be asked
if (condition)
statementT;
else
statementF;
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
two alternatives
optional
else is optional in the structure of an if statement.
if with Dependent Statement
When condition evaluates to true, the statement is executed; when false, it is skipped
E.g.: if (x != 0)
product = product * x;
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5. Table 4.1 Rational and Equality Operators
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6. Relational & Equality Operators (cont’d)
Relational expressions (conditions) are used to perform tests for selecting among alternative statements to execute.
Typical forms:
variable relational-operator variable
variable relational-operator constant
variable equality-operator variable
variable equality-operator constant
Evaluate to Boolean (bool) value of true or false
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7. Examples x
x <= 0 y
x >= y -5 7 -5 -5 7
true
false
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8. 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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9. Table 4.3 && Operator
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10. Table 4.4 || Operator
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11. Table 4.5 ! Operator
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12. 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
The selection construct has to be used to solve this exercise.
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13. Solution Steps Problem statement/Requirements phase. Analysis
Input: income
Output: taxDue
Additional program variables.
Processing formulas.
Design phase.
Implementation phase.
Testing phase.
Let’s first start with analysing the problem.
What is the input, output, and processing?
Then design by drawing the flow chart.
Please draw the flow chart in your notes.
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14. Design if (income < 20000) taxDue = income * 0.1; else
START
Design
INPUT
income
True
income < ?
if (income < 20000)
taxDue = income * 0.1;
else
taxDue = income * 0.2;
taxDue = income * 0.1
False
taxDue = income * 0.2
OUTPUT
taxDue
STOP
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15. 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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16. Design Modification Nested if Statements if (income < 20000)
taxDue = income * 0.1;
else
if (income < 30000)
taxDue=income*0.2;
taxDue=income*0.3;
True
income < ?
taxDue = income * 0.1
False
True
income < ?
taxDue = income * 0.2
False
taxDue = income * 0.3
Nested if Statements
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17. 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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18. General Form if (condition1) statement1; else if (condition2).
else if (conditionn)
statementn;
else
statemente;
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19. 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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20. 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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21. 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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22. Next lecture we will continue decision construct in C++
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