char

Selection (if)

Programs so far are sequential and always operate the same way.
What happens when you get up in the morning?

   Wake up. 
   Put on favorite t-shirt. 
   Eat breakfast.

What if it is cold?
"if" allows you to decide among alternatives.

   Wake up. 
   If it is warm  
       put on favorite t-shirt. 
   Else put on sweater. 
   Eat breakfast.

One way selection:

if (condition) 
   action

example

Relational operators:
(condition) is a logical expression that is true or false.
e.g. (age > 62)
Values can be compared using relational operators > < = = >= <= !=

   temp = = 70 	
   temp != 70 
   (2+5 ) < 3  //false but correct syntax

Possible pitfall:

   if (age = = 21) 
       cout << "Voter!" << endl; 
  
   if (age = 21)   //this assigns 21 to age and becomes true 
       cout << "Voter!" << endl;

Two way selection:

   if (condition) 
       statement1 
   else 
       statement2;

examples: 1, 2

Logical operators && (AND) || (OR) ! (NOT):
Temperature between 70 and 80:

   (temp >= 70) && (temp <= 80)

How to express below 70 or over 80:

   (temp < 70) || (temp > 80)

   !((temp >= 70) && (temp <= 80))

How would you express:

   older than 25 and between 63 and 67 inches 
   older than 25 but not taller than 67 inches 
   between 25 and 27 or between 63 and 65 inches

What about this:

   (age > 62 || < 21)

Boolean variables:
Assigned a value true or false

   bool pleasant; 
   pleasant=true;
   ...

Boolean variables can make a program more readable:

   bool pleasant; 
   pleasant = (temp > 70) && (temp < 80); 
   if (pleasant) 
   { ...

if2.cpp

Relational operators with floating point:

   float x; 
   float y; 
   x = 1.0/3.0; 
   y = 1.0; 

   (3.0*x == y)                //false! 

   (fabs( 3.0*x - y) < 0.0001) //true

float.cpp

Nested if:
nest0.cpp.
Getting dressed again:

              WARM        COLD
   WEEKDAY    t shirt     sweater

   WEEKEND    black dress black dress

nest1.cpp.
Using booleans: nest2.cpp.

Checking for valid data:
We want temp > 0 and < 100.

   if (temp >0 & < 100) ...?

nest3.cpp.
Sometimes used for multiway branches: nest4.cpp.
There is a better way in Ch12.

Dangling else (a reason to use braces):
What does this mean?

   if (temp > 70)               sunny\temp    50	80
   if (sunny)                                                            
   cout << "shorts";               true      shorts      ?
   else                            false        ?        ?
   cout << "slacks";

Which matches?

   if (temp > 70)  
   { 
       if (sunny) 
           cout << "shorts"; 
       else   
           cout << "slacks"; 
   }

   if (temp > 70) 
   { 
       if (sunny) 
           cout << "shorts"; 
   } 
   else   
       cout << "slacks";

Note indenting.