3.6 loop and exit Statements

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A loop statement is a compound statement that controls repetitious execution of a sequence of statements. There are three kinds of iteration scheme:

Every loop uses the reserved words or combinations: loop and end loop. The reserved words for or while may appear as part of the "iteration scheme," and a "loop name" may optionally be used.  The reserved words exit and when may appear in the statement sequence.

General Form

 
Loop_Name:              -- optional
  iteration-scheme loop
    -- statement_sequence;
  end loop Loop_Name;

If a Loop_Name is given at the beginning, it must appear again after end loop. The statement_sequence may include one or more exit statements, as illustrated in the first of three examples below. An exit statement can be simply "exit;" or can be "exit when <condition>" where the <condition> is a Boolean expression that evaluates to true or false.

Im3-6.gif (3941 bytes)

 This diagram depicts a program that illustrates two kinds of  loop statement.

Two packages from the predefined environment are withed in by the Assign_Grades procedure.

A separate code fragment depicts an alternative to the first loop statement. The alternative is a "while loop."

 

The first loop statement in the following code contains an exit statement within it. The  "while loop" statement provides an alternative way to achieve the same functionality without an exit statement.

The two arrays defined in the declaration part are initialized using aggregate expressions.

Source Code Listing

-------------------------------------------------------------------
--  This program asks the user to enter a series of numerical 
--  (integer) grades, computes the class average, and then computes 
--  and displays letter grades.
-------------------------------------------------------------------
with Ada.Text_IO, Ada.Integer_Text_IO;
use  Ada.Text_IO, Ada.Integer_Text_IO;
procedure Assign_Grades is
                             
  Number_Grade : array(1..25) of Integer   -- 25 is max class size
               := (1..25 => 0);            -- array aggregate

  Letter_Grade : array(1..25) of Character;
               := (1..25 => 'U');          -- array aggregate
                                           -- U = Unassigned
  Total              : Integer := 0;
  Number_Of_Students : Integer := 0;
  I                  : Integer := 0;
  Class_Average      : Integer;

  Finished           : Boolean := False;
  Response           : Character;

begin

  -- basic loop with exit            -- enter number grades and
                                     -- determine Number_Of_Students
  loop
    I := I + 1;
    Number_Of_Students := I;
    Put("Enter next grade (0 to 100)  ");
    Get(Number_Grade(I));               -- if I>25, raises exception
    Total := Total + Number_Grade(I);
    Put("Are you finished? (Y or N)  ");
    Get(Response);
    New_Line
    if (Response = 'Y') or (Response = 'y') then
      Finished := True;
    end if;
    exit when Finished;
  end loop;

  Class_Average := Total/Number_Of_Students;
  Put("Class Average = ");
  Put(Class_Average);
  New_Line;
  
  -- for (1..N) loop                  -- determine and display 
                                      -- letter grades

  for J in 1..Number_Of_Students loop
    if (Number_Grade(J)> 90) or 
       (Number_Grade(J) - Class_Average > 20) then
      Letter_Grade(J) := 'A';

    elsif (Number_Grade(J)> 80) or 
          (Number_Grade(J) - Class_Average > 10) then
      Letter_Grade(J) := 'B';

    elsif (Number_Grade(J)> 70) or 
          (Class_Average - Number_Grade(J) < 11) then
      Letter_Grade(J) := 'C';

    elsif (Number_Grade(J)> 60) or 
          (Class_Average - Number_Grade(J) < 21) then
      Letter_Grade(J) := 'D';

    else
      Letter_Grade(J) := 'F';
    end if;

    Put("Student ");
    Put(J);
    Put(" gets ");
    Put(Letter_Grade(J));
    Put(" based on score of ");
    Put(Number_Grade(J));
    End_Line;

  end loop;

end Assign_Grades;
------------------------------------------------------------------
-- while <condition> loop               -- alternative first loop
                                        -- (exit not required)
------------------------------------------------------------------
  while not Finished loop
    I := I + 1;
    Number_Of_Students := I;
    Put("Enter next grade (0 to 100)  ");
    Get(Number_Grade(I));  -- if I>25, raises exception
    Total := Total + Number_Grade(I);
    Put("Are you finished? (Y or N)  ");
    Get(Response);
    New_Line;
    if (Response = 'Y') or (Response = 'y') then
      Finished := True;
    end if;
  end loop;
------------------------------------------------------------------

Here is a sample output from the above program: 

  Enter next grade (0 to 100)  88
  Are you finished? (Y or N)  n

  Enter next grade (0 to 100)  92
  Are you finished? (Y or N)  n

  Enter next grade (0 to 100)  71
  Are you finished? (Y or N)  y

  Class Average =      83
  Student          1 gets B based on score of          88
  Student          2 gets A based on score of          92
  Student          3 gets C based on score of          71

Related Topics

4.6 Array Types A.2 Simple and Compound Statements

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