LOOPS

  Control Structures

  Types of Loops

  The while loop 

  The do/while loop 

  The for loop 

  Nested Loops

  The break and continue options 


 Control Structures

  Sequence -- one after another 

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

  Iteration -- loop
 while
 do...while
 for


 Types of Loops

  Pretest -- test a condition each time BEFORE the loop is executed
  (while)

  Posttest -- test a condition each time AFTER the loop is executed
  (do...while)

  Fixed repetition -- execute the loop a pre-determined number of times
  (for)


 The while loop 

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

  if expression != 0 (true), execute block and return to while
statement 

  if expression == 0 (false), skip block

  If expression false first time encountered, statement block not
executed even once
 while (melt < final_score)
   {
    cin >> plenum;
    melt += plenum;
    discrep = mass * accel;
   }

 while (number != 0) 
   {...}
 same as...
 while (number)
   {...}


  Be careful of un-ending loops...
 while (melt < final_score)
   {...}

  Either melt must increase or final_score must decrease...


 The do/while loop 

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

  If expression false first time encountered, statement block still
executed once


  if expression != 0 (true), execute block again and return to
while statement

  if expression == 0 (false), do not execute block again

  Note the important semi-colon after (expression)
 do
   {
    pay = base - deduct + overtime;
    hours -= BASE_HOURS;
   }
    while (hours > 0);
 do
   {...}
 while (number != 0);
 same as...
 do
   {...}
 while (number);


 The for loop 

 for (initialization; expression; increment)
   {
    statement-1;
    statement-2;
    ...
    statement-n;
   }

 for (count = 1; count <= 12; ++count)
   {
    cin >> temp;
    cout << "The temperature at " << count;
    cout << ":00 is " << temp << endl;
   }

  Initialization -- give initial value to variable

  Expression -- test variable against some limit 

  Increment -- Add (or subtract) something (usually 1) to (from)
variable

  Loop variable (count) can be used in loop, but probably should not be
altered...
 // Caution - this does not do what you want...
 for (count = 1; count <= 12; ++count)
   {
    cin >> temp;
    cout << "The temperature at " << count;
    cout << ":00 is " << temp << endl;
    count = count +1;
   }

 for (count = 0; count <= TABLE_SIZE; 
      count = count + 5)
   {...}

  Suppose TABLE_SIZE is -9.  Loop statements will not be executed even
once (pretest loop).
 for (count = 25; count > 0; --count)
   {...}
 for (count = 25; count; --count)
   {...}
 for (letter = 'a'; letter <= 'z'; ++letter)


 Nested Loops

 for (hours = 0; hours < 24; ++hours)
   {
    for (mins = 0; mins < 60; ++mins)
      {
       for (secs = 0; secs < 60; ++secs)
         {
          ...
         }
      }
   }

  While and do...while loops can be nested as well


 The break and continue options 

  break statement -- immediately terminates loop

  Usually used with if statement within loop to terminate if some
condition develops
 while (radiation > LOWEST)
   {
    ...
    if (radiation > HIGHEST)
      break;
    ...
   }

  Some loops are un-ending on purpose... 
 char code;
 const char STOP = 'q';
 while (1)
   {
    cin >> code;
    if (code == STOP)
      break;
    ...
   }


  continue statement -- skips this iteration

  Usually used with if statement within loop to avoid some problems but
to continue the looping mechanism 
 while (input_flag == MORE_DATA)
   {
    ...
    if (group_size == 0)
      continue;
    average = sum / group_size;
    ...
   }