1. Introduction to Computing Lecture 04: Booleans & Selection
Assist.Prof.Dr. Nükhet ÖZBEK
Ege University
Department of Electrical & Electronics Engineering

2. Topics
Booleans
if and else statements

3. Boolean A special “type” with only two values: false and true
Used to implement the conditions for selection and looping in an algorithm

4. Boolean Expressions
...are expressions that can be evaluated to either strictly true or strictly false
A Boolean expression which evaluates to true has integer value 1; otherwise, 0

5. Boolean Operators ... are used in forming Boolean expressions
... are also known as “logical” or “relational” operators
Not (!)
Equality (==)
Inequality (!=)
Comparison (<, >, <=, >=)
And (&&)
Or (||)

6. Not True only if the single Boolean argument is false Examples: ! 1
! 0
! happy

7. Equality True only if both arguments have identical values Examples:
1 == 2
1 == 0
42 == 42
truth == beauty

8. not to be confused with assignment (=)
Equality
True only if both arguments have identical values
Examples:
1 == 2
1 == 0
42 == 42
truth == beauty
not to be confused with assignment (=)

9. Inequality True only if arguments have different values Examples:
1 != 2
1 != 0
42 != 42
num != 54

10. Comparison True only if the specified relationship holds Examples:
1 < 2
0 > 1
42 <= 42
age >= 18

11. Comparison True only if the specified relationship holds Examples:
1 < 2
0 > 1
42 <= 42
age >= 18
Careful!

12. And True only if both Boolean arguments are true Examples: 1 && 2
0 <= n && n <= 100
1 && 0 && -1

13. not to be confused with “bitwise AND” (&)
True only if both Boolean arguments are true.
Examples:
1 && 2
0 <= n && n <= 100
1 && 0 && -1
not to be confused with “bitwise AND” (&)

14. Or True only if either Boolean argument is true (or both are true)
Examples:
1 || 2
11 || 0 || 1
good || bad || ugly

15. not to be confused with “bitwise OR” (|)
True only if either Boolean argument is true (or both are true).
not to be confused with “bitwise OR” (|)
Examples:
1 || 2
11 || 0 || 1
good || bad || ugly

16. Truth Table P Q !P
P && Q
P || Q
false
true

17. Precedence Highest to lowest: Brackets Not (!)
Comparison (<, >, <=, >=)
Equality (==)
Inequality (!=)
And (&&)
Or (||)
Note: The assignment operator (=) is lower in order of precedence than Boolean / Logical operators

18. Example: bool.c #include <stdio.h> int main() { int age = 18;
int haveMoney = 0;
int haveCard = 1;
float thirst = 0.31;
int afterHours = 1;
int result;
result = age >= 18 && (haveMoney || haveCard)
&& thirst > 0.3 && ! afterHours;
printf("%d\n", result);
return 0; }
bool.c

19. Example: bool.c #include <stdio.h> int main() { int age = 18;
int haveMoney = 0;
int haveCard = 1;
float thirst = 0.31;
int afterHours = 1;
int result;
result = age >= 18 && (haveMoney || haveCard)
&& thirst > 0.3 && ! afterHours;
printf("%d\n", result);
return 0; }
bool.c

20. The if statement Determines whether a block is executed
Implements the selection instructions within an algorithm
Decides what to do by evaluating a Boolean expression
If the expression is true (non-zero), the block is executed

21. Example: oddnum.c Read in a number, and print it if it is odd.
output “Enter an integer”
input number
if (number is odd)
then {
output the number }

22. Example: oddnum.c #include <stdio.h>
/* Read in a number, and echo it
if it is odd. */
int main() {
return 0; }
Read in a number, and print it if it is odd.
output “Enter an integer”
input number
if (number is odd)
then {
output the number }

23. Example: oddnum.c #include <stdio.h>
/* Read in a number, and echo it
if it is odd. */
int main() {
int number;
printf("Enter an integer: ");
scanf("%d", &number);
return 0; }
Read in a number, and print it if it is odd.
output “Enter an integer”
input number
if (number is odd)
then {
output the number }

24. Example: oddnum.c #include <stdio.h>
/* Read in a number, and echo it
if it is odd. */
int main() {
int number;
printf("Enter an integer: ");
scanf("%d", &number);
if (number % 2 != 0)
printf("%d\n", number); }
return 0;
Read in a number, and print it if it is odd.
output “Enter an integer”
input number
if (number is odd)
then {
output the number }

25. Example: oddnum.c Do not put “then” here! #include <stdio.h>
/* Read in a number, and echo it
if it is odd. */
int main() {
int number;
printf("Enter an integer: ");
scanf("%d", &number);
if (number % 2 != 0)
printf("%d\n", number); }
return 0;
Read in a number, and print it if it is odd.
output “Enter an integer”
input number
if (number is odd)
then {
output the number }
Do not put “then” here!

26. Do not put semicolon here!
Example: oddnum.c
#include <stdio.h>
/* Read in a number, and echo it
if it is odd. */
int main() {
int number;
printf("Enter an integer: ");
scanf("%d", &number);
if (number % 2 != 0)
printf("%d\n", number); }
return 0;
Read in a number, and print it if it is odd.
output “Enter an integer”
input number
if (number is odd)
then {
output the number }
Do not put semicolon here!

27. Example: oddnum.c #include <stdio.h>
/* Read in a number, and echo it
if it is odd. */
int main() {
int number;
printf("Enter an integer: ");
scanf("%d", &number);
if (number % 2 != 0)
printf("%d\n", number); }
return 0;
Read in a number, and print it if it is odd.
output “Enter an integer”
input number
if (number is odd)
then {
output the number }

28. Syntax for if statement
if (condition)
statement; OR {
statement1; …
statementn; }

29. Notes on if
Which of the following code fragments are equivalent? A
if (number % 2 != 0) {
printf("%d", number); }
printf(” is odd\n"); B
if (number % 2 != 0)
printf("%d", number);
printf(” is odd\n"); C
if (number % 2 != 0) {
printf("%d", number);
printf(” is odd\n"); }

30. Notes on if Common mistakes if (number % 2 != 0); {
printf("%d is an odd ", number); }
printf("number\n");

31. Notes on if Common mistakes if (number % 2 != 0); {
printf("%d is an odd ", number); }
printf("number\n");
No semi-colon here!

32. Notes on if Common mistakes if (number = 0) { printf("%d\n", number);}

33. Notes on if Common mistakes if (number = 0) { printf("%d\n", number);}
Should be ==

34. The else statement Can only occur after an if statement
Is only executed when the if’s block does not execute

35. Example: oddeven.c #include <stdio.h>
/* Determine whether an input number
is odd or even. */
main() {
int number;
printf("Enter an integer: ");
scanf("%d", &number);
if (number % 2 != 0)
printf("%d is an odd number\n",
number); }
Read in a number, and determine if it’s odd or even.
output “Enter an integer”
input number
if (number is odd)
then {
output: number “ is an odd number” }
else
output: number “ is an even number”

36. Example: oddeven.c #include <stdio.h>
/* Determine whether an input number
is odd or even. */
main() {
int number;
printf("Enter an integer: ");
scanf("%d", &number);
if (number % 2 != 0)
printf("%d is an odd number\n",
number); }
else
printf("%d is an even number\n",
Read in a number, and determine if it’s odd or even.
output “Enter an integer”
input number
if (number is odd)
then {
output: number “ is an odd number” }
else
output: number “ is an even number”

37. Example: oddeven.c No semicolons here! No semicolons here!
#include <stdio.h>
/* Determine whether an input number
is odd or even. */
main() {
int number;
printf("Enter an integer: ");
scanf("%d", &number);
if (number % 2 != 0)
printf("%d is an odd number\n",
number); }
else
printf("%d is an even number\n",
Read in a number, and determine if it’s odd or even.
output “Enter an integer”
input number
if (number is odd)
then {
output: number “ is an odd number” }
else
output: number “ is an even number”
No semicolons here!
No semicolons here!

38. Example: oddeven.c #include <stdio.h>
/* Determine whether an input number
is odd or even. */
main() {
int number;
printf("Enter an integer: ");
scanf("%d", &number);
if (number % 2 != 0)
printf("%d is an odd number\n",
number); }
else
printf("%d is an even number\n",
Read in a number, and determine if it’s odd or even.
output “Enter an integer”
input number
if (number is odd)
then {
output: number “ is an odd number” }
else
output: number “ is an even number”

39. Syntax for if & else statement
if (condition)
statementT;
else
statementF; OR {
statementT1; …
statementTn; }
statementF1;
statementFn;

40. Cascaded if statement
Multiple alternative blocks each with a Boolean expression
First expression which evaluates to true causes execution of the associated block
Only at most one block will be executed

41. Example: months.c Determine the number of days in a given month:
30 days hath September,
April, June and November.
All the rest hath 31,
Excepting February alone,
Which hath 28 days clear,
And 29 in each leap year.
output “Enter an integer”
input month
if (month is September,
or April,
or June,
or November)
then {
output “30 days” }
else if (month is February)
output “28 or 29 days”
else
output “31 days”

42. Example: months.c int main() { return 0; } #include <stdio.h>
/*************************\
Determine the number of days in a given month:
30 days hath September,
April, June and November;
All the rest have 31,
Excepting February alone,
And that has 28 days clear
And 29 in each leap year.
\*************************/
const int September = 9;
const int April = 4;
const int June = 6;
const int November = 11;
const int February = 2;

43. Example: months.c int main() { int month;
printf("Enter number of month: ");
scanf("%d", &month);
return 0; }
#include <stdio.h>
/*************************\
Determine the number of days in a given month:
30 days hath September,
April, June and November;
All the rest have 31,
Excepting February alone,
And that has 28 days clear
And 29 in each leap year.
\*************************/
const int September = 9;
const int April = 4;
const int June = 6;
const int November = 11;
const int February = 2;

44. Example: months.c int main() { int month;
printf("Enter number of month: ");
scanf("%d", &month);
if (month==September ||
month==April ||
month==June ||
month==November )
printf("30 days\n"); }
return 0;
#include <stdio.h>
/*************************\
Determine the number of days in a given month:
30 days hath September,
April, June and November;
All the rest have 31,
Excepting February alone,
And that has 28 days clear
And 29 in each leap year.
\*************************/
const int September = 9;
const int April = 4;
const int June = 6;
const int November = 11;
const int February = 2;

45. if (month==September || April || June || November )
Example: months.c
int main() {
int month;
printf("Enter number of month: ");
scanf("%d", &month);
if (month==September ||
month==April ||
month==June ||
month==November )
printf("30 days\n"); }
return 0;
#include <stdio.h>
/*************************\
Determine the number of days in a given month:
30 days hath September,
April, June and November;
All the rest have 31,
Excepting February alone,
And that has 28 days clear
And 29 in each leap year.
\*************************/
const int September = 9;
const int April = 4;
const int June = 6;
const int November = 11;
const int February = 2;
Common mistake:
if (month==September || April || June || November )

46. Example: months.c int main() { int month;
printf("Enter number of month: ");
scanf("%d", &month);
if (month==September ||
month==April ||
month==June ||
month==November )
printf("30 days\n"); }
else if (month==February)
printf("28 or 29 days\n");
return 0;
#include <stdio.h>
/*************************\
Determine the number of days in a given month:
30 days hath September,
April, June and November;
All the rest have 31,
Excepting February alone,
And that has 28 days clear
And 29 in each leap year.
\*************************/
const int September = 9;
const int April = 4;
const int June = 6;
const int November = 11;
const int February = 2;

47. Example: months.c int main() { int month;
printf("Enter number of month: ");
scanf("%d", &month);
if (month==September ||
month==April ||
month==June ||
month==November )
printf("30 days\n"); }
else if (month==February)
printf("28 or 29 days\n");
else
printf("31 days\n");
return 0;
#include <stdio.h>
/*************************\
Determine the number of days in a given month:
30 days hath September,
April, June and November;
All the rest have 31,
Excepting February alone,
And that has 28 days clear
And 29 in each leap year.
\*************************/
const int September = 9;
const int April = 4;
const int June = 6;
const int November = 11;
const int February = 2;

48. “Default” block. Example: months.c int main() { int month;
printf("Enter number of month: ");
scanf("%d", &month);
if (month==September ||
month==April ||
month==June ||
month==November )
printf("30 days\n"); }
else if (month==February)
printf("28 or 29 days\n");
else
printf("31 days\n");
return 0;
#include <stdio.h>
/*************************\
Determine the number of days in a given month:
30 days hath September,
April, June and November;
All the rest have 31,
Excepting February alone,
And that has 28 days clear
And 29 in each leap year.
\*************************/
const int September = 9;
const int April = 4;
const int June = 6;
const int November = 11;
const int February = 2;
“Default” block.

49. Example: months.c int main() { int month;
printf("Enter number of month: ");
scanf("%d", &month);
if (month==September ||
month==April ||
month==June ||
month==November )
printf("30 days\n"); }
else if (month==February)
printf("28 or 29 days\n");
else
printf("31 days\n");
return 0;
#include <stdio.h>
/*************************\
Determine the number of days in a given month:
30 days hath September,
April, June and November;
All the rest have 31,
Excepting February alone,
And that has 28 days clear
And 29 in each leap year.
\*************************/
const int September = 9;
const int April = 4;
const int June = 6;
const int November = 11;
const int February = 2;

50. Syntax for cascaded if statements
if (condition1)
statement1;
else if (condition2)
statement2; .
else if (conditionn)
statementn;
else
statemente;

51. Q: Notes on Cascaded if What is the output if: number is equal to 5{
printf(“S1\n”); }
else if (number <= 40)
printf(“S2\n”);
else if (number >= 10)
printf(“S3\n”);
else if (number <= 20)
printf(“S4\n”); Q:
What is the output if:
number is equal to 5
number is equal to 20
number is equal to 30
number is equal to 35

52. Common Mistakes
Using = instead of ==

53. Example: #include <stdio.h>
/* Probably the most common C error. */
int main() {
int score;
scanf("%d", &score);
if (score = 48 || score = 49)
printf("Almost!\n"); }
return 0;

54. Usual error message (if any): “LValue required...”
Example:
#include <stdio.h>
/* Probably the most common C error. */
int main() {
int score;
scanf("%d", &score);
if (score = 48 || score = 49)
printf("Almost!\n"); }
return 0;
Usual error message (if any): “LValue required...”

55. Example: #include <stdio.h>
/* Probably the most common C error. */
int main() {
int score;
scanf("%d", &score);
if (score == 48 || score == 49)
printf("Almost!\n"); }
return 0;

56. Common Mistakes
Using = instead of ==
Multiple comparisons

57. Example: #include <stdio.h> /* Another common C error. */
int main() {
int score;
scanf("%d", &score);
if ( 0 < score < 48 )
printf("Fail\n"); }
return 0;

58. 0 or 1 Example: #include <stdio.h> /* Another common C error. */
int main() {
int score;
scanf("%d", &score);
if ( 0 < score < 48 )
printf("Fail\n"); }
return 0;
0 or 1

59. always 1 0 or 1 Example: #include <stdio.h>
/* Another common C error. */
int main() {
int score;
scanf("%d", &score);
if ( 0 < score < 48 )
printf("Fail\n"); }
return 0;
always 1
0 or 1

60. Example: #include <stdio.h> /* Another common C error. */
int main() {
int score;
scanf("%d", &score);
if ( 0 < score && score < 60 )
printf("Fail\n"); }
return 0;

61. Summary Booleans Boolean expressions Boolean Operators Precedence
if and else statements
Common mistakes