CS99, Fall 2002
Mon 10/21
Lecture 8
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(0) Announcements:
    + reading: finish Chapter 4
    + future reading: reread 2.1-2.8, 6.2, 6.3, 7.3
                      last month will be Chapter 5
    + HW8 due Nov 4

    Topics:
    + summary
    + $for$
    + designing loops
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(1) Summary:
    + repetition statements
    + using $while$ to repeat instructions
    + syntax: while (c), s, end
    + pseudocode: if c is true, do statements, repeat; otherwise, stop
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(2) for-loop

    FOR variable = expr, statement, ..., statement END

    ex) for ii=1:4
           ii
        end

    Output?

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(3) Types of loops

    + how to choose between FOR and WHILE?

    + DEFINITE:


    + INDEFINITE:
  
 
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(4) Definite loops

    ex)  Print out all integers between 1 and 4.

    sol) Use FOR or WHILE, though ________ is better:

           for ii=1:4,ii,end

           ii=1; while ii < 5, ii, ii=ii+1, end
 










    ex)  The user enters a positive integer.
         Print out all integers from 1 to that value.

    sol) Use FOR or WHILE, though ________ is better:


           % for-loop with break:

             flag=input('Enter a positive integer [1,100]: ');
             for ii=1:100
	        disp(['Current value: ',num2str(ii)]);
                if ii==flag
                   break
                end
             end

           % while-loop:

             flag=input('Enter a positive integer [1,100]: ');
             ii=0;
             while flag <= ii
                ii=ii+1;    
                disp(['Current value: ',num2str(ii)]);
             end
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(5) Indefinite loops

    ex)  Simple guessing game:

         % Initialize variables:
   
           target = floor( ___________ ) + ____ ;  % computer picked-number

           guess = input('Enter a value: ');       % user guesses

         % Prompt user to guess correct number:

           while _____________________________ 

              disp('Wrong!');

              __________________________________ ; % user guesses again

           end
 
         % Congratulate user:


           disp('Right!');













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(6) "Mixed loops"

    ex)  Simple guessing game, but don't allow the user to guess
         too many times!

    % Initialize variables:
      max = 2;                              % max guess
      min = 1;                              % min guess
      target = floor(rand*(max-min+1)) + 1; % computer picked-number
      guess = input('Enter a value: ');     % user guess
      count = 1;                            % number of guesses so far
      maxcount = 2;                         % max allowed guesses

    % Prompt user to guess correct number:

      while _______________________________________

         disp('Wrong!');

         guess = input('Enter another value: ');   % user guesses again

         count = ______________________________ ;

      end

    % Congratulate user:

      if __________________________

         disp('Right!');
         disp(['It took ',num2str( ___________ ),' guesses.']);
      else
         disp('You took too many guesses!');
      end
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(7) Designing a loop
 
    + General programming:
      - Brainstorm and research phase
      - Write algorithm
      - Determine classification of loop

    + Pattern to follow:

      - initialize variables (from research/brainstorm phases)
        + boundary conditions?
        + what are LIMITS to the data you need to work with?

      - get values to test
      - if test conditions are true
        + account for the values you obtained
        + update new values
        + get next value(s) to test
      - repeat

      - aftermath:
        + account for never entering the loop or other "boundary conditions"
        + account for data that did get processed in loop  



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(8) Examples of common patterns for indefinite loops

    % REPETITION:
      v = input('enter value: ')
      while v ~= stop
         disp(v);
         g = input('enter value: ');
      end      






    % SUMMATION:
      sum = 0;
      v = input('enter value: ');
      while v ~= stop
         sum = sum+v;
         v = input('enter value: ');
      end
      if sum==0
         disp('no values');
      else
         sum
      end







    % CONDITIONAL UPDATE:
      max=-1;
      v = input('enter value: ');
      while v ~= stop
         if v > max
           max=v;
         end
         v = input('enter value: ');
      end
      if max == -1
         disp('no values'),
      else
         max
      end














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