AnnouncementsHW1 deadline extended to Saturday.New (2nd) homework will be assigned next week

will be due following week 22nd July Wednesday 19:00About strings and loops

Common Submission Mistakes In HW1, some students submitted empty/wrong files

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 homeworksUse of global variables (variables defined outside of functions) prohibitedNo abrupt program termination in the middle of the program.Modularity and code duplication are important

Code duplication must be avoidedPlease write comments to your programs.

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Summary of what we learned last weekFunctions – Divide and conquerFunctions with ParametersLocal variables vs Parameters

Scope

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Textbook Sections 4.2, 4.3, 4.4 and 4.7

Conditional Statements(if–else)

Conditional StatementsSo 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

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Syntaxif (<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 ( )

5

Another Syntax (without { })

if (<condition>)<statement_true>;

else<statement_false>;

if (<condition>) <statement_true>;

• Can be used when there is only one statement• Not suggested (we will see why)

6

Flow diagram of if-else

test condition

truestatements

true

nextstatement

false

falsestatements

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Example (not in the book)Write a program that inputs two integer numbers and

displays the maximum one.Two solutions

using if and else togetherusing only if (no else)

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Solution 1 – with_if_else.cppint 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;

}

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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;

}

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Boolean type and expressions The condition in an if statement must be a Boolean expression

(named for George Boole) Values are true or falsebool 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 valueif (3 * 4 – 8)

cout << "hello";else

cout << "bye" ; prints hello

boolean output yields 0 (for false) or 1 (true) cout << (4 < 5);

prints 111

< 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

Relational Operators Relational operators are used to compare values:

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)

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ExamplesLet’s look at some examples with literals to better see

the boolean results.

23 >= 45 false 49.0 == 7*7 true34-3 != 30+1 false

string s1= "elma", s2= "armut", s3= "Elma";s1 < s2 falses3 < s1 true

Why s3 < s2 is true? ‘E’ has a smaller code than ‘a’ Uppercase letters have smaller codes than lowercase letters

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Logical operatorsBoolean expressions can be combined using

logical operators: AND, OR, NOTIn C++ we use && || ! respectively

A B A || B A && B

true true true true

true false true false

false true true false

false false false false

A ! A

true false

false true

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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 operatorif (num >= 0 && num <= 100)cout << "number in the range";

elsecout << "number is out of range";

• Solution 2: using logical AND and NOT operators

if ( ! (num >= 0 && num <= 100) )cout << "number is out of range";

elsecout << "number is in the range";

• Solution 3: using logical OR operatorif (num < 0 || num > 100)cout << "number is out of range";

elsecout << "number is in the range";

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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

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Operator Precedence - RevisitedUpper 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 left-to-right

<< >> stream insertion and extraction left-to-right

< <= > >= inequality comparison operators left-to-right

== != equal, not equal comparison left-to-right

&& logical and left-to-right

|| logical or left-to-right

= += -= *= /= %=

assignment operators right-to-left

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Operator Precedence Examplescout << num1 < year;

syntax error (very cryptic)the problem is that << has precedence over <

does not compile as intendedSolution: cout << (num1 < year);Advice: use parenthesized expressions in cout

What about (0 <= num <= 100) for range check?not a syntax errorbut 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

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Nested if statementsif/else statements are inside other if/else statementsMethod to select from multiple choicesExample: input a numeric grade and display messages

according to its value0 .. 50 low51 .. 70 average71 .. 100 goodotherwise invalid grade

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Nested if ExampleExample: input a numeric grade and display messages

according to its value0 .. 50 low51 .. 70 average71 .. 100 goodotherwise invalid grade

Several solutions exist (not in the book)First solution: if’s are after if’s

see if_after_if.cppSecond solution: if’s are after else’s

see if_after_else.cpp or if_after_else2.cpp

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if (<condition_1>)

{

if (<condition_2>)

{

if (<condition_3>)

{

<statements_alltrue>;

}

else

{

< statements_true_1and2>;

}

}

else

{

<statements_true_1_only>;

}

}

else

{

<statements_1_false>;

}

Nested if-else Syntax 1if condition_1 is TRUE then check

condition_2

if condition_2 is TRUE then check

condition_3if condition_3 is TRUE

statements_alltrue are executed,

if condition_3 is FALSEstatements_true_1and2 are executed,

if condition_2 is FALSEstatements_true_1_only are executed,

if condition_1 is FALSEstatements_1_false are executed,

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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>;

}

Nested if-else Syntax 2 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 thenstatement_2true_1 … statement_2true_N are executed,

if condition_2 is FALSE then check condition_3 and if condition_3 is TRUE thenStatement_3true_1 … statement_3true_N are executed,

… if condition_(N-1) is FALSE then

statement_allfalse_1 … statement_allfalse_N are executed.22

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; } }

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Dangling Else Problemif ( 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) ifelse belongs to the most recent if

Solution: use braces (see next slide)24

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 ifif – else matching rule

Each else belongs to the nearest if for which there is no else and in the same compound block25