1. The Ohio State University
Selection Structures
CSE1222: Lecture 5
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2. The Ohio State University
Flow of Control (1)
Consider the following code:
int main() {
int x;
x = 46;
x++;
cout << x << endl;
return 0; }
Its control flow
CSE1222: Lecture 5
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3. The Ohio State University
Flow of Control (2)
Notice in the previous slide, that the flow of control is completely sequential. In other words, every statement is executed in order.
This makes for powerless and probably insignificant programs. Can we do better?
Can the flow of control not be as deterministic?
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4. The Ohio State University
Selection Structures
We want to add some branching to the flow of control, which allow certain statements to be executed out of sequential order, or skipped entirely.
C++ allows for this with selection structures:
- if-then-else statements
- switch statements
CSE1222: Lecture 5
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5. if–then Statement Syntax
The if-then statement is the first selection structure we examine. Basically, it says:
“IF some condition is true, THEN run a sequence of statements.”
The if-then syntax is shown below:
if (conditional expression) {
statements executed if condition is true }
CSE1222: Lecture 5
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6. if–else Statement Syntax (1)
We can optionally extend the if-then statement to an if-then-else statement, meaning: “IF some condition is true, THEN run a sequence of statements. ELSE, run a sequence of other statements.”
The ELSE portion is optional because it is not always the case that the program should take action if some condition is false.
CSE1222: Lecture 5
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7. if–else Statement Syntax (2)
The if-then-else syntax is shown below:
if (conditional expression) {
statements executed if condition is true }
else
statements executed if condition is false
CSE1222: Lecture 5
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8. Example of an if-then-else Statement
. . .
int main() {
double x(0.0);
cout << “Enter a number: “;
cin >> x;
// Handle divide by zero!
if (x == 0)
cout << “ERROR: Input must be not be 0!” << endl; }
else
cout << “The fraction is: “ << 1/x << endl;
return 0;
CSE1222: Lecture 5
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9. The Ohio State University
Notice how of the flow of control is able to branch and select which statements to execute, and which ones to avoid from running!
Also notice that after the selection structure execution, the code returns back to sequential operation.
CSE1222: Lecture 5
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10. The Ohio State University
slope.cpp
#include <iostream>
using namespace std;
int main() {
double x1, y1, x2, y2, xdiff, ydiff, slope;
cout << "Enter x and y coordinates of first point : ";
cin >> x1 >> y1;
cout << "Enter x and y coordinates of second point : ";
cin >> x2 >> y2;
...
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11. The Ohio State University
slope.cpp (cont.)
...
xdiff = x1 - x2;
ydiff = y1 - y2;
if (xdiff != 0.0) {
slope = ydiff / xdiff;
cout << "The slope is: " << slope << endl; }
else
cout << "The slope is infinite." << endl;
return 0;
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12. The Ohio State University
...
xdiff = x1 - x2;
ydiff = y1 - y2;
if (xdiff != 0.0) {
slope = ydiff / xdiff;
cout << "The slope is: " << slope << endl; }
else
cout << "The slope is infinite." << endl;
> slope.exe
Enter x and y coordinates of first point : 0 3
Enter x and y coordinates of second point : 4 1
The slope is: -0.5
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13. The Ohio State University
...
xdiff = x1 - x2;
ydiff = y1 - y2;
if (xdiff != 0.0) {
slope = ydiff / xdiff;
cout << "The slope is: " << slope << endl; }
else
cout << "The slope is infinite." << endl;
> slope.exe
Enter x and y coordinates of first point : 4 3
Enter x and y coordinates of second point : 4 1
The slope is infinite.
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14. The Ohio State University
...
xdiff = x1 - x2;
ydiff = y1 - y2;
if (xdiff != 0.0) {
slope = ydiff / xdiff;
cout << "The slope is: " << slope << endl; }
else
cout << "The slope is infinite." << endl;
> slope.exe
Enter x and y coordinates of first point : 4 3
Enter x and y coordinates of second point : 4 3
The slope is infinite.
What is wrong with this answer?
CSE1222: Lecture 5
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15. The Ohio State University
slope2.cpp: Nested ifs
...
xdiff = x1 - x2;
ydiff = y1 - y2;
if (xdiff != 0.0) {
slope = ydiff / xdiff;
cout << "The slope is: " << slope << endl; }
else
if (ydiff != 0.0) {
cout << "The slope is infinite." << endl;
else {
cout << "Input Error: First point equals second point."
<< endl;
cout << "Slope undefined." << endl;
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16. The Ohio State University
...
if (xdiff != 0.0) {
slope = ydiff / xdiff;
cout << "The slope is: " << slope << endl; }
else
if (ydiff != 0.0) {
cout << "The slope is infinite." << endl;
else {
cout << "Input Error: First point equals second point."
<< endl;
cout << "Slope undefined." << endl;
> slope.exe
Enter x and y coordinates of first point : 4 3
Enter x and y coordinates of second point : 4 3
Input Error: First point equals second point.
Slope undefined.
CSE1222: Lecture 5
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17. Nested Conditional Statements
...
int main() {
int age(0);
cout << "Enter your age: ";
cin >> age;
if (age >= 15)
if (age == 15)
{ cout << "You can get a learners permit." << endl; }
else
{ cout << "You can get a license." << endl; } }
{ cout << "You are too young to drive." << endl; }
return 0;
What is the output on input: 10 15 20
CSE1222: Lecture 5
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18. Nested Conditional Statements
...
int main() {
int age(0);
cout << "Enter your age: ";
cin >> age;
if (age >= 18)
cout << "You can vote." << endl;
if (age >= 35)
{ cout << "You can become President." << endl; } }
else
{ cout << "You are too young to vote." << endl; }
return 0;
What is the output on input: 10 20 40
Does the else clause execute on input 20?
Why or why not?
CSE1222: Lecture 5
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19. The Ohio State University
Selection Criteria
The if- conditions are common relational expressions.
The outcomes of the expression is Boolean; it can only have two outcomes: true or false (or 1 or 0 respectively).
In the previous example, age >= 16 is a relational expression that evaluates to either true or false depending on what the user inputs!
CSE1222: Lecture 5
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20. Relational Operators (1)
Meaning
Example
<
less than
x < 0
>
greater than
speed > 65
<=
less than or equal to
age <=17
>=
greater than or equal to
gpa >= 3.5 ==
equal to
initial == ‘a’ !=
not equal to
divisor != 0
Relational operators return false (0) and true (1).
CSE1222: Lecture 5
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21. Relational Operators (2)
Relational operators can compare character data, for example ‘a’ < ‘b’ evaluates to true (why is this true?)
‘G’ > ‘M’ evaluates to false
‘E’ != ‘e’ evaluates to true
2 < 0 evaluates to false
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22. The Ohio State University
Ascii Table
CSE1222: Lecture 5
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23. The Ohio State University
booleanExpr.cpp
// Examples of Boolean expressions
#include <iostream>
using namespace std;
int main() {
double x(10);
cout << "(4 != 4) evaluates to " << (4 != 4) << endl;
cout << "('a' <= 'b') evaluates to " << ('a' <= 'b') << endl;
cout << "('B' < 'b') evaluates to " << ('B' < 'b') << endl;
cout << "(( ) == 7) evaluates to " << (( ) == 7)
<< endl;
cout << "((5/3) > 1.0) evaluates to " << ((5/3) > 1.0) << endl;
cout << "(x == 10) evaluates to " << (x == 10) << endl;
cout << "(x = 8) evaluates to " << (x = 8) << endl;
return 0; }
CSE1222: Lecture 5
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24. The Ohio State University
Logical Operators (1)
Every one of these expressions only take 2 operands, so it is NOT possible to use the familiar expression 18 < x < 60 in order to test if x is a number in between 18 and 60.
Instead, to build more complicated expressions, we can use the logical operators used in Boolean algebra:
AND (in C++, &&)
OR (in C++, ||)
NOT (in C++, !)
CSE1222: Lecture 5
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25. The Ohio State University
Logical Operators (2)
Going back to the previous example, if you wanted to verify that a variable x is between 18 and 60 inclusive, you could use the following expression:
(age >= 18 && age <= 60)
AND, OR are both binary operators, NOT is a unary operator.
If an expression has an && operator, then both operands must evaluate to true for the entire expression to be true.
The || operator returns true if either operand evaluates to true.
The unary ! Operator returns true if its operand is false, and false if its operand is true.
CSE1222: Lecture 5
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26. The Ohio State University
The NOT Operator
NOT is a unary operator (it only takes one operand), so how does it work?
Let’s say that now we want to check if x is not between 18 and 60. We simply use:
!(age >= 18 && age <= 60)
The NOT operator only takes one operand, evaluates it to true or false, then flips the result.
CSE1222: Lecture 5
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27. The Ohio State University
logicalOperators1.cpp
// Examples of logical operators
#include <iostream>
using namespace std;
int main() {
int x(5);
int y(25);
if (x < 7 && y < 12) { cout << “true” << endl; }
else { cout << “false” << endl; };
if (x < 7 || y < 12) { cout << “true” << endl; }
if (x < 7 && !(y < 12)) { cout << “true” << endl; }
return 0; }
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28. The Ohio State University
logicalOperators2.cpp
// Examples of logical operators
#include <iostream>
using namespace std;
int main() {
int x(5);
int y(25);
if ((x < 7 && y < 12) || (x > 7 && y > 12))
{ cout << “true” << endl; }
else { cout << “false” << endl; };
if ((x < 7 || y < 12) && (x > 7 || y > 12))
return 0; }
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29. The Ohio State University
Exercises
Give a boolean expression which is true if x is a non-negative number strictly less than 50;
Give a boolean expression which is true if x is not in the range [25-30].
(Multiple solutions.)
CSE1222: Lecture 5
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30. Example of Boolean variables
...
int main() {
int age(0);
bool flag_discount(false);
cout << "Enter your age: ";
cin >> age;
if (age < 18)
{ flag_discount = true; }
if (age >= 65)
if (flag_discount)
{ cout << "You receive a discount." << endl; }
else
{ cout << "No discount." << endl; }
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31. The Ohio State University
Note on Truth
Any integer that is not 0 is considered to be a Boolean true in C++.
Relational and logical operators only ever return false (0) and true (1).
CSE1222: Lecture 5
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32. Order of (more) Operations
Operator
Associativity
! unary
right to left
* / %
left to right
+ -
< <= > >=
== !=
&& ||
= += -= *= /=
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33. The Ohio State University
comparisonExample.cpp
// comparison example
#include <iostream>
using namespace std;
int main() {
int year(0);
cout << "Enter year: ";
cin >> year;
int k = year%4;
if (k == 0)
cout << year << " is a US presidential election year." << endl; }
else
cout << year << " is NOT a US presidential election year." << endl;
return 0;
CSE1222: Lecture 5
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34. The Ohio State University…
int k = year%4;
if (k == 0) // if k is equal to 0, ... {
cout << year << " is a US presidential election year." << endl; }
else
cout << year << " is NOT a US presidential election year." << endl;
> comparisonExample
Enter year: 2007
2007 is NOT a US presidential election year.
Enter year: 2008
2008 is a US presidential election year.
CSE1222: Lecture 5
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35. The Ohio State University
comparisonError.cpp
// comparison error
#include <iostream>
using namespace std;
int main() {
int year(0);
cout << "Enter year: ";
cin >> year;
int k = year%4;
if (k = 0)
cout << year << “ is a US pres election year.” << endl; }
else
cout << year << “ is NOT a US presidential election year.” << endl;
return 0;
CSE1222: Lecture 5
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36. The Ohio State University…
int k = year%4;
if (k = 0) {
cout << year << " is a US presidential election year." << endl; }
else
cout << year << " is NOT a US presidential election year." << endl;
> comparisonError
Enter year: 2007
2007 is NOT a US presidential election year.
Enter year: 2008
2008 is NOT a US presidential election year.
CSE1222: Lecture 5
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37. The Ohio State University
comparisonError2.cpp
// comparison example
#include <iostream>
using namespace std;
int main() {
int year(0);
cout << "Enter year: ";
cin >> year;
int k = year%4;
if (k == 0) // if k is equal to 0, ...
cout << year << " is a US presidential election year." << endl; };
else
cout << year << " is NOT a US presidential election year." << endl;
return 0; }
CSE1222: Lecture 5
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38. The Ohio State University
comparisonError2.cpp …
int k = year%4;
if (k == 0) // if k is equal to 0, ... {
cout << year << " is a US presidential election year." << endl; };
else
cout << year << " is NOT a US presidential election year." << endl;
What’s the error? (Try compiling this program.)
CSE1222: Lecture 5
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39. The Ohio State University
Warning
Use ‘==’ (equal to), not ‘=’ (assignment).
Use ‘&&’ not ‘&’.
Use ‘||’, not ‘|’.
Do not put a semi-colon before an “else” clause.
CSE1222: Lecture 5
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