1. Announcements Homework 1 will be assigned this week,
will be due on July 14th, next Friday, at 19:00
Common Submission Mistakes
Make sure that you send the cpp file; otherwise we cannot grade
Please submit the required files only, not the entire project folder
7z, rar and tar format is not allowed for compression; please use zip
Do not use blanks, Turkish characters, special symbols in the filenames
Only English alphabet letters, digits and underscore are allowed
General rules about homeworks
Add the following at the end of your code to get a prompt at the end:
cin.ignore();
cin.get();
Write comments in your program
Use of global variables (variables defined outside of functions) prohibited
Modularity and code duplication are important
Code duplication must be avoided

2. Summary of what we learned yesterday
Functions – Divide and conquer
Functions with Parameters
Local variables vs Parameters
Scope

3. Conditional Statements (if–else)
Textbook Sections 4.2, 4.3, 4.4 and 4.7

4. Conditional Statements
So far statements of our programs execute sequentially one after another.
What happens when
we want to execute a statement depending on a condition?
e.g. If it snows, announce that the schools are on vacation
e.g. If there is enough money in the bank account, give the money
we want to execute one statement when a condition holds and another statement when a condition does not hold?
e.g. If dollar is high, sell dollar. Otherwise, buy dollar.
we want to select from many statements according to one or more criteria (selection).
e.g. If dollar is high and euro is low, sell dollar and buy euro. If dollar is low and euro is high, sell euro and buy dollar. If both of them are high, sell both and buy TL.
You achieve conditional execution with if-else statements

5. Syntax <condition> must be in parantheses ( )
if (<condition>) {
<statement_true_1>;
...
<statement_true_N>; }
else
<statement_false_1>;
<statement_false_N>;
If condition is TRUE then
statement_true_1 …
statement_true_N
are executed,
if condition is FALSE
statement_false_1 …
statement_false_N
are executed.
if (<condition>) {
<statement_true_1>;
...
<statement_true_N>; }
else and statement_false’s are optional
if condition is FALSE then nothing will be executed and execution continues with the next statement in the program
<condition> must be in parantheses ( )

6. Another Syntax (without { })
if (<condition>)
<statement_true>;
if (<condition>) <statement_true>; else <statement_false>;
Can be used when there is only one statement
Not suggested (we will see why)

7. Flow diagram of if-else
test condition
true
false
Loops start with testing a condition first: if the condition is false, the loop exits and moves to the next statement. If the condition is true, the loop statements execute and move to the beginning of the loop. This continues until the condition becomes false. Hence this enables the repetition of the loop statements.
true
statements
false
statements
next
statement

8. Example (not in the book)
Write a program that inputs two integer numbers and displays the maximum one.
Two solutions
using if and else together
using only if (no else)

9. Solution 1 – with_if_else.cpp
int main () { int num1, num2, max; cout << "Enter two numbers: "; cin >> num1 >> num2; if (num1 > num2) // check if first number is larger than the second one max = num1; // if so maximum is the first number } else max = num2; // otherwise maximum is the second number cout << "maximum of these two numbers is: " << max << endl; return 0;

10. Solution 2 – with_if.cpp (no else)
int main () { int num1, num2, max; cout << "Enter two numbers: "; cin >> num1 >> num2; max = num1; // default assignment - maximum is the first number if (num2 > max) // check if second number is larger than the first one max = num2; // if so update the maximum, if not do nothing } cout << "maximum of these two numbers is: " << max << endl; return 0;

11. Boolean type and expressions
The condition in an if statement must be a Boolean expression (named for George Boole)
Values are true or false
bool is a built-in type like int, double
int degrees;
bool isHot = false;
cout << "enter temperature: “;
cin >> degrees;
if (degrees > 35) {
isHot = true; }
Boolean values have numeric equivalents
false is 0, true is any nonzero value
if (3 * 4 – 8)
cout << "hello";
else
cout << "bye" ;
prints hello
boolean output yields 0 (for false) or 1 (true)
cout << (4 < 5);
prints

12. Relational Operators Relational operators are used to compare values:
<
less than
number < 5
<=
less than or equal
number <= 0
>
greater than
num1 > num2
>=
greater than or equal
num1 >= num2 ==
equality check
num1 == 0 !=
inequality check
num1 != num2
They take two operands
operands can be literals, variables or expressions
Used for many types
numeric comparisons
string comparisons (lexicographical, i.e. alphabetical)
boolean comparisons (false is less than true)

13. Examples
Let’s look at some examples with literals to better see the boolean results.
23 >= 45 false
49.0 == 7* true
34-3 != 30+1 false
string s1= "elma", s2= "armut", s3= "Elma";
s1 < s2 false
s3 < s1 true
Why s3 < s2 is true?
‘E’ has a smaller code than ‘a’
Uppercase letters have smaller codes than lowercase letters

14. Logical operators
Boolean expressions can be combined using logical operators: AND, OR, NOT
In C++ we use && || ! respectively A B
A || B
A && B
true
false A
! A
true
false

15. Example Range check: between 0 and 100 (includes 0 and 100), or not?
If so, display a message saying that the number is in the range.
If not, the message should say “out of the range”.
Solution 1: using logical AND operator
if (num >= 0 && num <= 100)
cout << "number in the range";
else
cout << "number is out of range";
Solution 2: using logical AND and NOT operators
if ( ! (num >= 0 && num <= 100) )
cout << "number is in the range";
Solution 3: using logical OR operator
if (num < 0 || num > 100)

16. De Morgan’s Rules (Section 4.7)
Compare solution 2 and 3
two conditions are equivalent
( ! (num >= 0 && num <= 100) )
( num < 0 || num > 100 )
De Morgan’s Rules (assume a and b are two boolean expressions)
! (a && b) = !a || !b
! (a || b) = !a && !b
De Morgan’a Rules can be generalized to several expressions (e.g. 4 boolean expressions case)
! (a && b && c && d) = !a || !b || !c || !d
! (a || b || c || d) = !a && !b && !c && !d

17. Operator Precedence - Revisited
Upper operator groups have precedence
Operator
Explanation
Associativity !
plus and minus signs, logical NOT
right-to-left
* / %
multiplication, division and modulus
left-to-right
+ -
addition, subtraction
<< >>
stream insertion and extraction
< <= > >=
inequality comparison operators
== !=
equal, not equal comparison
&&
logical and ||
logical or
= += -= *= /= %=
assignment operators

18. Operator Precedence Examples
cout << num1 < year;
syntax error (very cryptic)
the problem is that << has precedence over <
does not compile as intended
Solution: cout << (num1 < year);
Advice: use parenthesized expressions in cout
What about (0 <= num <= 100) for range check?
not a syntax error
but that expression does not make a range check. It is always true. Why?
What is the value of !12+5&&32/35 ?
result is 0

19. Nested if statements
if/else statements are inside other if/else statements
Method to select from multiple choices
Example: input a numeric grade and display messages according to its value
low
average
good
otherwise invalid grade

20. Nested if Example
Example: input a numeric grade and display messages according to its value
low
average
good
otherwise invalid grade
Several solutions exist (not in the book)
First solution: if’s are after if’s
see if_after_if.cpp
Second solution: if’s are after else’s
see if_after_else.cpp
or if_after_else2.cpp

21. Nested if-else Syntax 1 if condition_1 is TRUE then check condition_2
if (<condition_1>) { if (<condition_2>) if (<condition_3>) <statements_alltrue>; } else < statements_true_1and2>; <statements_true_1_only>; <statements_1_false>;
if condition_1 is TRUE then check
condition_2
if condition_2 is TRUE then check
condition_3
if condition_3 is TRUE
statements_alltrue are executed,
if condition_3 is FALSE
statements_true_1and2
are executed,
if condition_2 is FALSE
statements_true_1_only
if condition_1 is FALSE
statements_1_false are executed,

22. Nested if-else Syntax 2 if condition_1 is TRUE then
if (<condition_1>) { <statement_1true_1>; ... <statement_1true_N>; } else if (<condition_2>) <statement_2true_1>; <statement_2true_N>; else if (<condition_3>) <statement_3true_1>; <statement_3true_N>; else <statement_allfalse_1>; <statement_allfalse_N>;
if condition_1 is TRUE then
statement_1true_1 …
statement_1true_N
are executed,
if condition_1 is FALSE then check condition_2 and if condition_2 is TRUE then
statement_2true_1 …
statement_2true_N
if condition_2 is FALSE then check condition_3 and if condition_3 is TRUE then
Statement_3true_1 …
statement_3true_N …
if condition_(N-1) is FALSE then statement_allfalse_1 …
statement_allfalse_N
are executed.

23. Short-circuit Evaluation
Some subexpressions in Boolean expressions are not evaluated if the entire expression’s value is already known using the subexpression evaluated so far.
Rule: Evaluate the first (leftmost) boolean subexpression. If its value is enough to judge about the value of the entire expression, then stop there. Otherwise continue evaluation towards right.
if (count != 0 && scores/count < 60) {
cout << "low average" << endl; }
In this example, if the value of count is zero, then first subexpression becomes false and the second one is not evaluated.
In this way, we avoid “division by zero” error (that would cause to crash the execution of the program)
Alternative method to avoid division by zero without using short-circuit evaluation:
if (count != 0)
if (scores/count < 60)
cout << "low average warning" << endl;

24. Dangling Else Problem What does it display for x=4?
if ( x % 2 == 0)
if ( x < 0 )
cout << x << " is an even, negative number" << endl;
else
cout << x << " is an odd number" << endl;
What does it display for x=4?
The problem is that it displays “odd number” message for positive even numbers and zero.
Reason is that, although indentation says the reverse, else belongs to second (inner) if
else belongs to the most recent if
Solution: use braces (see next slide)

25. Solution to Dangling Else Problem
if ( x % 2 == 0) {
if ( x < 0 )
cout << x << " is an even, negative number"<< endl; }
else
cout << x << " is an odd number" << endl;
Now else belongs to the first if
if – else matching rule
Each else belongs to the nearest if for which there is no else and in the same compound block