1. SE1H421 Procedural Programming LECTURE 4 Operators & Conditionals (1)

2. Lecture Slides are unsuitable for learning how to program!
You just get an illusion of learning!
Programming is a craft. You have to do it to learn it.

3. 'switch'-'case' statements
Conditionals
'if' statements
'switch'-'case' statements 3

4. if ( (x > 5) && (y < 7) ) { // do stuff }
We Need 3 Things:
2. Logical Operators
1. Comparison Operators
3. 'if' statement structure
if ( (x > 5) && (y < 7) ) {
// do stuff } 4

5. Not used for Conditionals (still, let’s have a quick look)
Operators
Operators come in 4 different 'flavours':
arithmetic * / % ( )
Not used for Conditionals (still, let’s have a quick look)
incremental
comparison < > == != >= <=
logical && || ! 5

6. Using Arithmetic Operators
incremental
comparison
logical
Using Arithmetic Operators
Arithmetic operators are used as we know it from maths at school.
Only difference: there may be variables or constants in the equation, too:
What gets printed ?
int a=5, b=6, c;
const int TAX = 20;
c = a * b TAX;
cout << " c is: " << c; 20 6

7. arithmetic
incremental
comparison
logical
Operator Precedence
The usual rules for "what gets calculated first" apply:
1. first everything in braces ( )
2. then * and / and %
3. finally + and -
int a=5, b=6, c;
const int TAX = 20; 20
Therefore: c = a * b TAX;
is different from: c = a * (b TAX);
-20 7

8. Quick Reminder When you see an equation sign in a line:
arithmetic
incremental
comparison
logical
Quick Reminder
When you see an equation sign in a line:
….things on the left are calculated first
c = a * b TAX;
… and then copied into the variable to the left. 1
destination source 2
This is called an ASSIGNMENT 8

9. Incremental Operators
arithmetic
incremental
comparison
logical
Incremental Operators
Programmers are lazy.
In order to add 1 to an int you could write:
int myIntegerVariable = 100; // declare and initialise
myIntegerVariable = myIntegerVariable + 1; // add one
The second line can be written in "shorthand":
++ myIntegerVariable; 9

10. Incremental Operators (cont.)
arithmetic
incremental
comparison
logical
Incremental Operators (cont.)
Similarly in order to subtract 1 you could write
-- myIntegerVariable;
NOTE:
++ and – – work with integer type variables (char, short, int and long) and on float types (double, float).
(But don't even try that on strings)
Could you subtract 2 by writing
-- -- myIntegerVariable ? 10

11. Example int x = 5; // declare and initialise
arithmetic
incremental
comparison
logical
Example
int x = 5; // declare and initialise
cout << "x is " << x; // prints: x is 5
-- x; // decrement
cout << " x is now " << x; // prints: x is now 4
++ x; // increment
cout << "x back to " << x; // x back to 5 11

12. Check your knowledge now:
arithmetic
incremental
comparison
logical
Check your knowledge now:
Spot the mistakes in the code:
const float TAX = 0.78;
float income = £50.00;
float netIncome;
netIncome = income * TAX;
++netIncome;
TAX should be uppercase
should be just 50.0
correct
correct 12

13.  Operators arithmetic + - * / % ( ) incremental ++ --
comparison
logical
Operators 
arithmetic * / % ( )
incremental
comparison < > == != >= <=
logical && || ! 13

14. if ( (x > 5) && (y < 7) ) { // do stuff }
We Need 3 Things:
2. Logical Operators
1. Comparison Operators
3. 'if' statement structure
if ( (x > 5) && (y < 7) ) {
// do stuff } 14

15. Comparison Operators Example (using two ints x and y):
arithmetic
incremental
comparison
logical
Comparison Operators
Used in loops and conditionals
Compares two expressions
Outcome is either true or false
Example (using two ints x and y):
if ( x > y ) {
// yes, x is greater than y, do stuff here }
else
// no, it isn't, do things here 15

16. Comparison Operators (cont.)
arithmetic
incremental
comparison
logical
Comparison Operators (cont.)
An expression with these operators is called a boolean expression
List of 6 operators:
> greater than
< smaller than
>= greater or equal than
<= smaller or equal than
== equal to
!= not equal to
George Boole, 16

17. That's an assignment, remember ?
arithmetic
incremental
comparison
logical
a boolean expression
Don't confuse with single '=' e.g. x=10;
That's an assignment, remember ?
Notes on '=='
It is a double '==' e.g. if (x == 10)
Never use '==' on floats or doubles:
The float x=3.0 may internally be stored as or , therefore the outcome of
if( x == 3.0 ) { could be false! 17

18. Comparison Operators (cont.)
arithmetic
incremental
comparison
logical
Examples:
int a = 5, b = 10, c = 5;
Code
English
a > b a greater than b
a < b a less than b
a >= b a greater or equal to b
a <= b a less or equal to b
a == c a equal to c
a != b a not equal to b
False
True
False
True
True
True 18

19. Three Steps to Conditional Heaven
arithmetic
incremental
comparison
logical
Three Steps to Conditional Heaven
Step 1: Comparison Operators
Step 2: Logical Operators ( &&, ||, ! )
Step 3: 'if' statement structure  19

20. if ( (x > 5) && (y < 7) ) { // do stuff }
We Need 3 Things:
2. Logical Operators
1. Comparison Operators
3. 'if' statement structure
if ( (x > 5) && (y < 7) ) {
// do stuff } 20

21. Logical Operators With comparison operators alone we can say:
arithmetic
incremental
comparison
logical
Logical Operators
With comparison operators alone we can say:
" if a is greater than b, then do …."
But how can we say:
" if a is greater than b AND c is greater than d, then do …. "
Here is how :
if ( ( a>b) && (c>d) ) {
// then do stuff here } 21

22. Logical Operators (cont.)
arithmetic
incremental
comparison
logical
Logical Operators (cont.)
&& and
|| or
! not
List of 3 operators:
|| is two of these 22

23. Logical Operators (cont.)
arithmetic
incremental
comparison
logical
Logical Operators (cont.)
&& is evaluated before ||
Example:
if ( (a>b) || (c>d) && (x==y) ) {
// then do stuff here }
1 tab indentation 23

24. Logical Operators (cont.)
arithmetic
incremental
comparison
logical
Logical Operators (cont.)
Another Example:
if ( !(a>b) && !(x==y) ) {
// then do stuff here }
This is equivalent to:
if ( (a<=b) && (x!=y) ) {
// then do stuff here } 24

25. Logical Operators (cont.)
arithmetic
incremental
comparison
logical
Logical Operators (cont.)
Logical Truth Tables:
True && True = True
False && True = False
True && False = False
False && False = False
AND
True || True = True
False II True = True
True II False = True
False || False = False OR
! Adding a NOT will invert the result.
XOR study this in your own time 25

26. if ( (x > 5) && (y < 7) ) { // do stuff }
We Need 3 Things:
2. Logical Operators
1. Comparison Operators
3. 'if' statement structure
if ( (x > 5) && (y < 7) ) {
// do stuff } 26

27. Conditions Happen in our day to day lives They are logical arguments
Have true or false answers
If your age is 17 or older then
You can drive a car.
If your licence points are more than 12 then
You cannot drive a car.
If thirsty then
Take a drink

28. Conditions If age 18 or older then Allow nightclub entry.

29. Conditions If age 18 or older then { Allow nightclub entry. If thirsty
Get a drink }
Allow nightclub entry
thirsty?
Get a drink

30. 'if' Statements, version 1 of 4
simple ‘if’
'if' statements can come in different forms
The simplest form is this:
condition
go here only if true
continue here
// program comes from here
if (condition true ) {
// go here if only true
// execute lines coded here }
// continue here 30

31. 'if' Statements, version 2 of 4
‘if’ ’else’
The second version looks like this:
// program comes from here
if (condition true ) {
// go here if true
// execute code written here }
else
// go here if false
// continue here
condition
go here if false
go here if true
continue here 31

32. 'if' Statements, version 3 of 4
‘else if’
’else’
The third version looks like this:
condition1
condition2
go here if cond1 true
else go here if cond2 true
else go here
// program comes from here
if (condition true ) {
// go here if true }
else if ( another condition true )
// go here if 1st condition false and 2nd true
else
// go here if none of the above expressions true
// continue here 32

33. 'if' Statements, version 4 of 4
‘else if’ :
’else’
'if' Statements, version 4 of 4
1st Condition
2nd Condition
3rd Condition
If 1st cond true do this
If 2nd cond true do this
If 3rd cond true do this
else to this
if (condition here ) {
// Go here if first condition is true }
else if (2nd condition here ) {
// only if 1st condition is false
// and the 2nd condition one is true
else if (3rd condition here ) {
// only if 1st and 2nd conditions are false
// and the 3rd one true
else {
// if all the above conditions are false do this.
//Continue here 33

34. For Conditionals We Need
Logical Operators
Comparison Operators
'if' statement structure
if ( (x > 5) && (y < 7) ) {
// do stuff }
Done ! 34

35. Check your knowledge now:
What is wrong here ?
float premium, carValue=500.0;
int driverAge=30;
if ( (driverAge>=18) & (driverAge <30 ) ) {
premium = 0.20; }
else if (driverAge>30) && (driverAge <40) )
premium = 0.10;
else if
premium = 0.05;
cout << "Premium is " << premium*carValue;
correct
& should be &&
'(' missing. Also:
what if driver is precisely 30?
'else if' but no boolean expr. 35

36. Revision This was tricky ! You need to: revise the material
study more examples
practice
Let’s do a few quick ones now...

37. Examples.. if (7 == 5) { cout << “true”; } else
cout << “false“;

38. Examples.. if (5 > 4 ) { cout << “true”; } else
cout << “false“;

39. Examples.. if ( 3 != 2) { cout << “true”; } else
cout << “false“;

40. Examples.. if (6 >= 9) { cout << “true”; } else
cout << “false“;

41. Examples.. if ( 5 < 12 ) { cout << “true”; } else
cout << “false“;

42. A bit more..
Instead of using only numeric constants, we can use any valid expression, including variables. Suppose that a=2, b=3 and c=6,
if ( a == 5) {
cout << “true”; }
else
cout << “false“;

43. Examples.. if ( a * b >= c) { cout << “true”; } else
cout << “false“;

44. Examples.. if ( b + 4 >= a * c) { cout << “true”; } else
cout << “false“;

45. Examples.. if ( a * b >= c) { cout << “true”; } else
cout << “false“;

46. Examples.. if ((b=2) == a ) { cout << “true”; } else
cout << “false“;

47. Logical operators (!, && ||)
! Not operator
&& And operator
|| Or operator

48. Not operator !
The Operator ! is the C++ operator to perform the Boolean operation NOT, it has only one operand, located at its right, and the only thing that it does is to inverse the value of it, producing false if its operand is true and true if its operand is false. Basically, it returns the opposite Boolean value of evaluating its operand.
Lets look at some examples:

49. Examples.. if ( ! (5 ==5) ) { cout << “true”; } else
cout << “false“;
OUTPUT ….. false

50. Examples.. if ( ! (6<=4) ) { cout << “true”; } else
cout << “false“;
OUTPUT ….. true

51. Examples.. if ( ! true ) { cout << “true”; } else
cout << “false“;
OUTPUT ….. false

52. Examples.. if ( ! false ) { cout << “true”; } else
cout << “false“;
OUTPUT ….. true

53. && || operators
The logical operators && and || are used when evaluating two expressions to obtain a single relational result.
The operator && corresponds with Boolean logical operation AND. This operation results true if both its two operands are true, and false otherwise. The following panel shows the result of operator && evaluating the expression “a” && “b”:

54. && operator
The operator && corresponds with Boolean logical operation AND. This operation results true if both its two operands are true, and false otherwise. The following panel shows the result of operator && evaluating the expression “a” && “b”:

55. && operator if ((6 > 2) && (7 > 3)) { cout << “true”; }
else
cout << “false“;

56. && operator int c = 7; int d = 12 if ((d==12) && (c < 3)) {
cout << “true”; }
else
cout << “false“;

57. || operator
The operator || corresponds with Boolean logical operation OR. This operation results true if either one of its two operands is true, thus being false only when both operands are false themselves. Here are the possible results of a || b:

58. || operator if ((6==(2+4)) || (7 > 3)) { cout << “true”; }
else
cout << “false“;

59. Demo Session Testing In-Class Test #1 Several ‘if’ statements
Simple if
if - else
if - else if – else
Testing
Exploring boundary conditions
In-Class Test #1
Resit next week if needed

60. Summary Today we have learned how to...
use arithmetic operators (+, -, %, *, / )
use incremental operators (++, --)
build boolean expressions in conditional statements ('if') by
using comparison operators (<,>, ==,…)
using logical operators (&&, ||, ! )
apply the structure of the 'if' statement

61. END OF LECTURE 4