CS99, Fall 2002
Lecture 7, Mon 10/7
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(0) Announcements
    + reading in chapter 4 continues
    + future reading: Chapter 5
    + need to know about prelim conflicts (see Syllabus)
    + Prelim 1 coming up! (see Exams)
    + advice for studying... (see Exams)
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(1) Topics:
    + repetition statements
    + while
    + for
    + tracing
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(2) Summary To Date:
    + problems -> automate solution process
    + convert human solution to computer solution
    + words-> tokens, sentences-> statements
    + kinds of statements: expression, assignment, selection
    + what's next?
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(3) Weakness of selection
 
    Use example:

    + problem:   find the biggest value in an array
    + algorithm: pick the first number, assume it is the max, 
                 compare it with the next number,swap if necessary, repeat
                 the comparison until done processing
    + solution:

       x = [1 3 2 4 5]
       max = x(1)
       % use $if$ now...:






    

    Pretty bad, huh? Excessively redundant code is generally BAD

    + more problems
      - how to reprompt the user?
      - how to find a max of millions of numbers?
      - how to run the simulation from H1?

    + solution:
      - need another statement!
      - this statement will control flow by repeating other statements
      - called a REPETITION STATEMENT or LOOP
        (sometimes also ITERATION)






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(4) Repetition Statements: WHILE

    Syntax of $while$

    while condition
       statements
    end

    + condition: boolean expression (see selection statement notes)
    + while statement is compound!
    + shortcut syntax: while condition,statements,end
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(5) How WHILE works

    + encounter the WHILE statement

    +--> + if the condition is true
    |      - do each statement top-down, left-to-right
    |      - stop when reaching the end statement
    |      - go back to the "top" of while statement
    +<--     and repeat
     
    + if the condition is false
      - do not evaluate the body of the while statement
      - go to the statement just after the ENTIRE while statement

    + continue with rest of statements
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(6) Examples of WHILE

    (A) problem:   write program that prints Hi! 10 times.

    algorithm: start count
               if count < max # of times
                  print Hi!
               repeat

    code:      count = _____________     % initialize counter
 
               while _______________     % start loop

                     _______________     % statement(s) to repeat

               _______                   % when to repeat loop
 

    (B) Write program that beeps "forever"

    while _____     % while true

       ________ ;   %   beep

    end             % repeat








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(7) Tracing

    + output your variables!
      - don't use semicolon
      - maybe use disp

    + example:

      x = 0
      while x < 4
         x
         x = x + 1
      end
      x

          +---------+--------+--------+--------+--------+--------+
          | initial | loop 1 | loop 2 | loop 3 | loop 4 | final  |
 +--------+---------+--------+--------+--------+--------+--------+
 | old x  |    0    |   0    |        |        |        |        |
 +--------+---------+--------+--------+--------+--------+--------+
 | new x  |    -    |   1    |        |        |        |   -    |
 +--------+---------+--------+--------+--------+--------+--------+

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(8) Commenting and style

    + place comments about repetition statement
    + indent block underneath the while
    + don't forget the end statement!
    + use semicolon to suppress needless output
    + avoid using break to quit loop

    BAD:
    while x < 3
    x=x+1
    end

    BETTER:
    while x < 3
      x = x + 1 ; 
    end

    BEST?


















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(9) Longer example
   
    Write a program that prompts a user to enter a grade. The program
    quits if the user enters an out-of-bounds value. Assume that the 
    user only enters numbers. If a grade is better than 90, alert the
    user. Count the number of VALID grades entered by the user.

    % Initialize data:   

      highGrade = ____________ ;  % highest possible grade

      lowGrade =  ____________ ;  % lowest possible grade

      count =     _____ ;         % count of VALID grades so far

    % Prompt for first grade:

      _____________  = input('Enter a grade: ');
      
    % Continue processing grades if necessary

      while _____________________________
         
	 % Increment count of valid grades:
	   
	   _______________________________ ;

	 % Check if grade is good:

	   if _________________________

	       disp('That is a good grade!');
	   end

	 % Get next grade:

	   ______________________________________________ ;

      end

    % Report the number of valid grades entered:

      if ___________________
      
          disp('No valid grades entered!');

      elseif _________________
      
	  disp(['You entered ',_________________,' grade.']);

      else

	  disp(['You entered ',_________________,' grades.']);

      end






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