Thursday, December 14, 2006

“Would you tell me, please, whichway I ought to go from here?”

“That depends a good deal on where you want to get to,” said the Cat.

- LEWIS CARROL, Alice in Wonderland.

Concept of true and falsetrue is any value other than zerofalse is zero

bool b=true;cout<<b<<endl;b=false;cout<<b<<endl;//zero is false and non-zero is trueb=35;cout<<b<<endl;b=-45;cout<<b<<endl;b=0;cout<<b<<endl;

bool b=true;cout<<b<<endl; //prints 1b=false;cout<<b<<endl; //prints 0//zero is false and non-zero is trueb=35;cout<<b<<endl; //prints 1b=-45;cout<<b<<endl; //prints 1b=0;cout<<b<<endl; //prints 0

Relational Operators

>

>=

<

<=

= =

!=

Logical Operators

&&

| |

! --

Relational expressions

int p=5, q=2;

bool bl;

bl = true;

cout<<bl<<endl;

bl = (p<=q);

cout<<bl<<endl;

bl=!bl;

cout<<bl<<endl;

Relational expressions

int p=5, q=2;

bool bl;

bl = true;

cout<<bl<<endl; //prints 1

bl = (p<=q);

cout<<bl<<endl; //prints 0

bl=!bl;

cout<<bl<<endl; //prints 1

Boolean expressions

Truth tables

Simple Flow of Control

Flow of control The order in which statements are executed

Branch Lets program choose between two alternatives

If: selective executionThe "if" statement allows us to execute a piece

of code or not, based on the value of the expression we pass to it.

Pseudocode:

if(<condition>) { <statements> } If <condition> is TRUE (non-zero), then <statements>

are executed.

If <condition> is FALSE (zero), then <statements> are not executed.

If: selective executionint main () {

int age;

cout << "Please enter your age: ";cin >> age;if(age <= 0) { cout << "Age must be > 0\n"; return 0;}cout << "Your age is " << age << endl;return 0;

}

If: selective executionint p=5;

if (p)

cout<<“done”;

Boolean expressions

Difference between == and =

Boolean expressions

Programming help: Instead of x == 3 use 3 == x. Then, the compiler will catch the error if you write only one =

If: selective executionIf: choosing between two alternatives

Pseudocode: if(<condition>) { <true_statements> } else { <false_statements> } If <condition> is TRUE (non-zero), then

<true_statements> are executed.

If <condition> is FALSE (zero), then <false_statements> are executed.

Example

gross_pay = rate * 40 + 1.5 * rate * (hours - 40);

Branch Example

To calculate hourly wages there are two choices Regular time ( up to 40 hours)

• gross_pay = rate * hours;

Overtime ( over 40 hours)• gross_pay = rate * 40 + 1.5 * rate * (hours - 40);

The program must choose which of these expressions to use

Designing the Branch

Decide if (hours >40) is true If it is true, then use

gross_pay = rate * 40 + 1.5 * rate * (hours - 40);

If it is not true, then use gross_pay = rate * hours;

Implementing the Branch

if-else statement is used in C++ to perform a branch

if (hours > 40) gross_pay = rate * 40 + 1.5 * rate * (hours - 40);

else

gross_pay = rate * hours;

Boolean expressions

Logical AND is represented by && (score >= 0) && (score <= 10)

Logical OR is represented by || (score < 0) || (score > 10)

Boolean expressions

Logical AND is represented by && (score >= 0) && (score <= 10)is true if score is between 0 and 10 (including 0 and 10).

Logical OR is represented by || (score < 0) || (score > 10)

Boolean expressions

Logical AND is represented by && (score >= 0) && (score <= 10)is true if score is between 0 and 10 (including 0 and 10).

Logical OR is represented by || (score < 0) || (score > 10)is true exactly in the cases where the previous condition would be false.

Boolean expressions Logical NOT is represented by an exclamation

mark: ! (score < 0) || (score > 10)

is equivalent to ! ( (score >= 0) && (score <= 10) )

Pitfall: & and | are also operators in C++. However, they do something different!

Boolean expressions

Pitfall: comparison operator for equality is = =

x == yis true if x has the same value as y.

x = yon the other hand, assigns the value of y to the

variable x.

int x=2, y=9;

cout<< (x==y) <<endl;

cout<< (x=y) <<endl;

int x=2, y=9;

cout<< (x==y) <<endl; // prints 0

cout<< (x=y) <<endl; // prints 9

Another example

Boolean expressions

Pitfall: Be careful with precedences.

When in doubt, use parentheses.

Relational and logical operators have lower precedence than arithmetic operators

15<100-90 is evaluated as 15<(100-90)

cout << (a && b); //parenthesis are necessary

Boolean expressions

Pitfall: Be careful with precedences.

When in doubt, use parentheses.

int a=3, c=4;

cout<<(a && c)<<endl;//parenthesis are necessary