IF (DECISION) STATEMENTS

  Control Structures

  The if statement

  The if/else statement

  Nested ifs 

  The switch statement 


 Control Structures

  Sequence -- one after another 

  Selection -- take one path or another 
 if
 if...else
 switch

  Iteration -- loop
 while
 do...while
 for


 The if statement

 if (expression)
   {
    statement-1;
    statement-2;
    ...
    statement-n;
   }

  if expression == 0 (false), skip block

  if expression != 0 (true), execute block

 if (sum == SIZE)
   {
    result = total / SIZE;
    cout << result << endl;
   }


  What if only 1 statement in block?

  OK ---
 if (sum == SIZE)
    result = total / SIZE;

  Preferred ---
 if (sum == SIZE)
   {
    result = total / SIZE;
   }


  Be very careful!
 if (sum = SIZE)
   {
    ...
   }


 The if/else statement

 if (expression)
   {
    statement-1;
    ...
    statement-m;
   }
 else
   {
    statement-1;
    ...
    statement-n;
   }

  if expression != 0 (true), execute 1st block, skip 2nd

  if expression == 0 (false), skip 1st block, execute 2nd
 if (sum == SIZE)
   {
    result = total / SIZE;
    cout << result << endl;
   }
 else
   {
    total += voltage;
    sum++;
   }

  Again, still use {...} if only one statement in either block


 Nested ifs 

 if (temp > 0)
   {
    if (temp >= 100)
      {
       cout << "Steam";
      }
    else
      {
       cout << "Water";
      }
   }
 else
   {
    cout << "Ice";
   }



 The switch statement 

  If statement has one branch

  If/else statement has two branches

  Switch statement has n branches

  Case statement 

  Selector variable

  Compared to set of possible matches


 switch (selector variable)
 {
  case value-1:
       statement-1;
       ...
       break;
  case value-2:
       statement-1;
       ...
       break;
  ...
  case value-n:
       statement-1;
       ...
       break;
 }



  Selector variable should be int or char -- not float

  Values must be constants

 switch (size)
 {
  case SMALL:
       x = s - z;
       break;
  case MEDIUM:
       x = s * z;
       --y;
       break;
  case LARGE:
       x = s + z;
       break;
  }

  Suppose SMALL is 5, MEDIUM is 10, LARGE is 15

  If size is 10, MEDIUM case of statements
       x = s * z;
       --y;
       break;
 are executed


  What if leave out break statement?
 switch (size)
 {
  case SMALL:
       x = s - z;
       break;
  case MEDIUM:
       x = s * z;
       --y;
  case LARGE:
       x = s + z;
       break;
  }

  If size is 10, MEDIUM and LARGE case of statements
       x = s * z;
       --y;
       x = s + z;
       break;
 are executed

  Do you need the last break statement?  Not really.  But what if you
add a new case???


 switch (size)
 {
  case TINY:
  case SMALL:
       x = s - z;
       break;
  case REGULAR:
  case MEDIUM:
  case NORMAL:
       x = s * z;
       --y;
  case LARGE:
       x = s + z;
       break;
  }



  What if selector variable matches two cases?  Normally goes to first
match.

  What if selector variable does not match any cases?  Skips all
statements.

  default used if no other matches


 switch (size)
 {
  case SMALL:
       x = s - z;
       break;
  case MEDIUM:
       x = s * z;
       --y;
  case LARGE:
       x = s + z;
       break;
  default:
       cout << "Incorrect size." << endl;
       break;
  }