While Loops

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• Questions?
• While Loops
  • Sections 13.1 - 13.3
    • Executes statements more than once
      

      while expression
          statements
      end

    • If expression is true, execute statements, continue until expression becomes false
    • Can use loop variable to count iterations
      

      i = …
      while i < …
          …
          i = i + 1
      end

      • Or can subtract, add different number, etc.
  • input and fprintf statements?
    • fprintf gives programmer fairly tight control over format of displayed information
    • input reads numbers (for now) from user, evaluating expressions if it has to
      

      >> fprintf( 'Hello World' );
      Hello World>> fprintf( 'Hello World\n' );
      Hello World
      >> fprintf( 'Hello\nWorld\n\n' );
      Hello
      World
      
      >> x = 17.6;
      >> fprintf( 'x is %f\n', x );
      x is 17.600000
      >> fprintf( 'x is %d\n', x );      % I expected this to print as a base-10 integer
      x is 1.760000e+01
      >> y = 3;
      >> fprintf( 'y is %d\n', y );
      y is 3
      >> fprintf( 'y is %f\n', y );
      y is 3.000000
      >> fprintf( 'x is %5.2f\n', x );
      x is 17.60
      >> fprintf( 'x is %5.2f and y is %d\n', x, y );
      x is 17.60 and y is 3
      >> x = input( 'Enter a number ' );
      Enter a number 4
      >> x
      x =
           4
      >> x = input( 'Enter a number ' );
      >> Enter a number sin( pi + 17 )
      x
      x =
          0.9614
      >> fprintf( 'x = %5.2f\n', x );
      x =  0.96
      >> x = 0.966;
      >> fprintf( 'x = %5.2f\n', x );
      x =  0.97

  • Examples / discussion
    • Text’s add-positive-numbers example (using debugger for “animation”)
      

      function totalSum = addPositive()
          totalSum = 0;
          usrNum = input('Enter positive number (<0 to quit): ');
          while (usrNum >= 0) 
              totalSum = totalSum + usrNum
              usrNum = input('Enter positive number (<0 to quit): ');
          end
      end

    • Validate user input (age, i.e., non-negative number below some upper bound)
      

      function [ age ] = getAge()
      
          max_possible_age = 120;
          age = input( 'Please enter your age ' );
      
          % while age is invalid
          % input new age
          while ~(age >= 0 & age <= max_possible_age)
              age = input( 'Please enter your age ' );
          end
      
      end

      • De Morgan’s Laws
        • not( a and b ) = (not a) or (not b)
        • not( a or b ) = (not a) and (not b)
• Newton Lab
  • Handout
• Next
  • Newton’s method
  • Watch video at https://www.youtube.com/watch?v=WO0GMNUCMNw

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