1. Basic Of Computer Science
Dr. Mohamed Khafagy

2. Conditional Statements
A conditional statement allows us to control whether a program segment is executed or not.
Two constructs
if statement if
if-else
if-else-if
switch statement

3. The Basic if Statement Syntax if(condition)
action
if the condition is true then execute the action.
action is either a single statement or a group of statements within braces.
condition
false
true
action

4. Choice (if) Put multiple action statements within braces
if (it's raining){
<take umbrella>
<wear raincoat> }

5. Absolute Value // program to read number & print its absolute value
#include <iostream>
using namespace std;
int main(){
int value;
cout << "Enter integer: ";
cin >> value;
if(value < 0)
value = -value;
cout << "The absolute value is " << value << endl;
return 0; }

6. Relational Operators
Relational operators are used to compare two values to form a condition.
Math C++ Plain English
= == equals [example: if(a==b) ]
[ (a=b) means put the value of b into a ]
< < less than
 <= less than or equal to
> > greater than
 >= greater than or equal to
 != not equal to

7. Operator Precedence Answer: Which comes first? * / % + -
* / %
+ -
< <= >= >
== != =
Answer:

8. The Boolean Type C++ contains a type named bool for conditions.
A condition can have one of two values:
true (corresponds to a non-zero value)
false (corresponds to zero value)
Boolean operators can be used to form more complex conditional expressions.
The and operator is &&
The or operator is ||
The not operator is !

9. The Boolean Type Truth table for "&&" (AND): Operand1 Operand2
true
false

10. The Boolean Type Truth table for “||" (OR): Operand1 Operand2
true
false

11. The Boolean Type Truth table for "!" (NOT): Operand !Operand true
false

12. A Boolean Type Assignments to bool type variables
bool P = true; bool Q = false; bool R = true; bool S = P && Q; bool T = !Q || R; bool U = !(R && !Q);

13. Sorting Two Numbers int value1; int value2; int temp;
cout << "Enter two integers: ";
cin >> value1 >> value2;
if(value1 > value2){
temp = value1;
value1 = value2;
value2 = temp; }
cout << "The input in sorted order: "
<< value1 << " " << value2 << endl;

14. The if-else Statement Syntax if (condition) Action_A else Action_B
if the condition is true then
execute Action_A else
execute Action_B
Example:
if(value == 0)
cout << "value is 0";
else
cout << "value is not 0";
condition
false
true
Action_A
Action_B

15. Choice (if and else) <go to beach> <take umbrella>
if <it's sunny>{
<go to beach> }
else{
<take umbrella>

16. 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;

17. Area of the circle 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;

18. 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);

19. 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 S T

20. 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";}

21. if-else-if Example if (score >= 90) 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;

22. 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>

23. 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>

24. Nested if Statements if the customer is a member, then
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.

25. 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.

26. Nested if Statements If (member) { if (age < 18) fee = fee * 0.5; }

27. “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;

28. “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;
(A) is the correct interpretation. To enforce (B), braces have to be explicitly used, as above.

29. 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.

30. Multiple Selection: The switch Statement
multiway
expression
value1
action 1
value2
action 2
value3
action 3
value4
action 4

31. Multiple Selection: The switch Statement
Syntax:
switch (<selector expression>) {
case <label1> : <sequence of statements>;
break;
case <label2> : <sequence of statements>;
case <labeln> : <sequence of statements>;
default : <sequence of statements>; }

32. Multiple Selection: The switch Statement
Meaning:
Evaluate selector expression.
The selector expression can only be: a bool, an integer, an enum constant, or a char.
Match case label.
Execute sequence of statements of matching label.
If break encountered,
go to end of the switch statement.
Otherwise continue execution.

33. Multiple Selection: The switch Statement
case 1
action
case 2
action
case 3
action
default
action

34. switch Statement: Example 1
If you have a 95, what grade will you get?
switch(int(score)/10){
case 10:
case 9: cout << "Grade = A" << endl;
case 8: cout << "Grade = B" << endl;
case 7: cout << "Grade = C" << endl;
case 6: cout << "Grade = D" << endl;
default:cout << "Grade = F" << endl; }

35. switch Statement: Example 2
switch(int(score)/10){
case 10:
case 9: cout << "Grade = A" << endl;
break;
case 8: cout << "Grade = B" << endl;
case 7: cout << "Grade = C" << endl;
case 6: cout << "Grade = D" << endl;
default:cout << "Grade = F" << endl; }

36. switch Statement: Example 2
is equivalent to:
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 // score < 59
cout << "Grade = F" << endl;

37. switch Statement: Example 2
#include <iostream>
using namespace std;
int main()
{ char answer;
cout << "Is comp102 an easy course? (y/n): ";
cin >> answer;
switch (answer){
case 'Y':
case 'y': cout << "I think so too!" << endl;
break;
case 'N':
case 'n': cout << "Are you kidding?" << endl;
default:
cout << "Is that a yes or no?" << endl; }
return 0;

38. switch Statement with Multiple Labels: Example 3
switch (watts) {
case 25 : lifespan = 2500;
break;
case 40 :
case 60 : lifespan = 1000;
case 75 : lifespan = 750;
default :
lifespan = 0;
} // end switch

39. Points to Remember
The expression followed by each case label must be a constant expression.
No two case labels may have the same value.
Two case labels may be associated with the same statements.
The default label is not required.
There can be only one default label, and it is usually last.

40. Problem 1
Write a program that reports the contents of a compressed-gas cylinder based on the first letter of the cylinder’s color. The program input is a character representing the observed color of the cylinder: ‘Y’ or ‘y’ for yellow, ‘O’ or ‘o’ for orange, and so on. Cylinder colors and associated contents are as follows: orange -> ammonia brown -> carbon monoxide yellow -> hydrogen green -> oxygen Your program should respond to input of a letter other than the first letters of the given colors with the message “Contents unknown”.

41. Input(s)
color
char

42. Output(s)
content

43. brown -> carbon monoxide yellow -> hydrogen green -> oxygen
Relevant Formula
orange -> ammonia
brown -> carbon monoxide
yellow -> hydrogen
green -> oxygen

44. ALTERNATIVE SOLUTION #include <iostream.h> int main() {
char color;
cout<<"Please enter the first letter of the cylinder's color : “;
cin>>color ;
if (color == 'O' || color == 'o')
cout<<“The contents of the compressed-gas cylinder is Ammonia”<<endl;
else if (color == 'B' || color == 'b')
cout<<"The contents of the compressed-gas cylinder is Carbon monoxide\n“;
else if (color == 'Y' || color == 'y')
cout<<"The contents of the compressed-gas cylinder is Hydrogen\n“;
else if (color == 'G' || color == 'g')
cout<<"The contents of the compressed-gas cylinder is Oxygen\n“;
else
cout<<"Contents unknown\n";
return 0; }

45. Exercise: Instead of using both uppercase and lowercase letters of the given character in every if-condition or in switch-cases, convert the character given by the user into uppercase or lowercase and use only this character in if-conditions and switch-cases.

46. Problem 2
The National Earthquake Information Center has asked you to write a program implementing the following decision table to characterize an earthquake based on its Richter scale number. Richter Scale Number (n) Characterization n < 5.0 Little or no damage 5.0 <= n < 5.5 Some damage 5.5 <= n < 6.5 Serious damage 6.5 <= n < 7.5 Disaster higher Catastrophe

47. Input(s) n
double

48. Output(s)
characterization

49. Richter Scale Number (n) Characterization
Relevant Formula
Richter Scale Number (n) Characterization
n < Little or no damage
5.0 <= n < Some damage
5.5 <= n < Serious damage
6.5 <= n < Disaster
higher Catastrophe

50. #include <iostram.h>
int main() {
double n;
cout<<"Please enter Richter Scale Number : “;
cin>>n;
if (n < 5.0)
cout<<"\nLittle or no damage\n”;
else if (5.0 <= n && n < 5.5) /* else if (n < 5.5) */
cout<<"\nSome damage\n”;
else if (5.5 <= n && n < 6.5) /* else if (n < 6.5) */
cout<<"\nSerious damage\n”;
else if (6.5 <= n && n < 7.5) /* else if (n < 7.5) */
cout"\nDisaster\n”;
else
cout<<"\nCatastrophe\n”;
return 0; }

51. Problem 1
Write a program that reports the contents of a compressed-gas cylinder based on the first letter of the cylinder’s color. The program input is a character representing the observed color of the cylinder: ‘Y’ or ‘y’ for yellow, ‘O’ or ‘o’ for orange, and so on. Cylinder colors and associated contents are as follows: orange -> ammonia brown -> carbon monoxide yellow -> hydrogen green -> oxygen Your program should respond to input of a letter other than the first letters of the given colors with the message “Contents unknown”.

52. Input(s)
color
char

53. Output(s)
content

54. brown -> carbon monoxide yellow -> hydrogen green -> oxygen
Relevant Formula
orange -> ammonia
brown -> carbon monoxide
yellow -> hydrogen
green -> oxygen

55. default: cout<<"\nContents unknown\n“; }
#include <iostream.h>
int main() {
char color;
cout<<"Please enter the first letter of the cylinder's color : “;
cin>>color ;
switch (color) {
case 'O':
case 'o':cout<<"\nThe contents of the compressed-gas cylinder is Ammonia\n“;
break;
case 'B':
case 'b':cout<<"\nThe contents of the compressed-gas cylinder is Carbon monoxide\n";
case 'Y':
case 'y':cout<<"\nThe contents of the compressed-gas cylinder is Hydrogen\n";
case 'G':
case 'g':cout<<"\nThe contents of the compressed-gas cylinder is Oxygen\n“;
default: cout<<"\nContents unknown\n“; }
return 0; }