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If Statements & Relational Operators,
Part 2
Programming
COMP102 Prog Fundamentals I: If Statements & Relational Operators /Slide 2
The if-else Statement
Syntax
if (condition) Action_Aelse Action_B
if the condition is true then execute Action_A elseexecute Action_B
Example:if(value == 0)
cout << "value is 0";else
cout << "value is not 0";
condition
Action_A Action_B
true false
COMP102 Prog Fundamentals I: If Statements & Relational Operators /Slide 3
Choice (if and else)
if <it's sunny>{
<go to beach>
}
else{
<take umbrella>
}
COMP102 Prog Fundamentals I: If Statements & Relational Operators /Slide 4
Finding the Big One
int value1;
int value2;
int larger;
cout << "Enter two integers: ";
cin >> value1 >> value2;
if(value1 > value2)
larger = value1;
else
larger = value2;
cout << "Larger of inputs is: " << larger << endl;
COMP102 Prog Fundamentals I: If Statements & Relational Operators /Slide 5
Finding the Big One
const double PI = 3.1415926;
int radius;
double area;
cout << "Enter the radius of the circle: ";
cin >> radius;
if(radius > 0){
area = radius * radius * PI;
cout << "The area of the circle is: " << area;
}
else
cout << "The radius has to be positive " << endl;
COMP102 Prog Fundamentals I: If Statements & Relational Operators /Slide 6
Even or Odd
int value1;
bool even;
cout << "Enter a integer : ";
cin >> value;
if(value%2 == 0)
even = true;
else
even = false;
// even = ！（ value%2);
COMP102 Prog Fundamentals I: If Statements & Relational Operators /Slide 7
if-else-if Statements
if <condition 1 exists>{
<do Q>
}
else if <condition 2 exists>{
<do R>
}
else if <condition 3 exists>{
<do S>
}
else{
<do T>
}
Q
R
TS
COMP102 Prog Fundamentals I: If Statements & Relational Operators /Slide 8
if-else-if Statements
if <1PM or 7PM>{
<eat>
}
else if <Mon, Wed or Fri>{
<goto COMP 102>
}
else if <Tues or Thurs AM>{
<goto MATH 113>
}
else{
<sleep>
}
COMP102 Prog Fundamentals I: If Statements & Relational Operators /Slide 9
if-else-if Statement int people, apples, difference;
cout << "How many people do you have?\n";cin >> people;cout << "How many apples do you have?\n";cin >> apples;
if(apples == people) cout << "Everybody gets one apple.\n"; else if(apples > people){ difference = apples - people; cout << "Everybody gets one apple, & there are " << difference << " extra apples.\n";}
else{ difference = people - apples;
cout << "Buy " << difference << " more apples so that everyone gets one apple.\n";}
COMP102 Prog Fundamentals I: If Statements & Relational Operators /Slide 10
if-else-if Example int score; cout << "Please enter a score: "; cin >> score;
if (score >= 90) cout << "Grade = A" << endl;
else if (score >= 80) cout << "Grade = B" << endl; else if (score >= 70) cout << "Grade = C" << endl; else if (score >= 60) cout << "Grade = D" << endl; else // totalscore < 59 cout << "Grade = F" << endl;
COMP102 Prog Fundamentals I: If Statements & Relational Operators /Slide 11
Nested if Statements Nested means that one complete statement is inside
another
if <condition 1 exists>{
if <condition 2 exists>{
if <condition 3 exists>{
<do A>
}
<do B>
}
<do C: sleep>
}
COMP102 Prog Fundamentals I: If Statements & Relational Operators /Slide 12
Nested if Statements Example:
if <it's Monday>{
<go to HKUST>
if <it's time for class>{
if <it's raining>{
<bring umbrella>
}
<go to COMP 102>
}
}
COMP102 Prog Fundamentals I: If Statements & Relational Operators /Slide 13
Nested if Statements
Consider the following example: if the customer is a member, then { If the customer is under 18, then the entrance fee is half the full fee. If the customer is 18 or older, then the entrance fee is 80% of the full fee. }
The if statements deciding whether to charge half fee to someone under 18 or whether to charge 80% to someone over 18 are only executed if the outer if statement is true, i.e. the customer is a member. Non-members, no matter what their age, are charged full fee.
COMP102 Prog Fundamentals I: If Statements & Relational Operators /Slide 14
Nested if Statements
Consider a variant of the previous example: if the customer is a member, then { If the customer is under 18, then the entrance fee is half the full fee. } If the customer is 18 or older, then the entrance fee is 80% of the full fee.
Here, member customers under 18 will be charged half fee and all other customers over 18 will be charged 80% of the full fee.
COMP102 Prog Fundamentals I: If Statements & Relational Operators /Slide 15
Nested if Statements
If (member)
{
if (age < 18)
{
fee = fee * 0.5;
}
if (age >=18)
fee = fee * 0.8;
}
If (member)
{
if (age < 18)
{
fee = fee * 0.5;
}
}
if (age >=18)
fee = fee * 0.8;
COMP102 Prog Fundamentals I: If Statements & Relational Operators /Slide 16
“Dangling Else” Problem
Always pair an else with the most recent unpaired if in the current block. Use extra brackets { } to clarify the intended meaning, even if not necessary. For example, what is the value of c in the following code?
int a = -1, b = 1, c = 1;if( a > 0 ) if( b > 0 ) c = 2;else c = 3;
COMP102 Prog Fundamentals I: If Statements & Relational Operators /Slide 17
“Dangling Else” Problem(A) int a = -1, b = 1, c = 1; if (a > 0) { if (b > 0) c = 2; else c = 3; }(B) int a = -1, b = 1, c = 1; if (a > 0) { if (b > 0) c = 2; } else c = 3;
(A) is the correct interpretation. To enforce (B), braces have to be explicitly used, as above.
COMP102 Prog Fundamentals I: If Statements & Relational Operators /Slide 18
Short-circuit Evaluation If the first operand of a logical and expression is false,
the second operand is not evaluated because the result must be false.
If the first operand of a logical or expression is true, the second operand is not evaluated because the result must be true.
