While Loops

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• Misc
  • New Matlab (r2014b) available from software.geneseo.edu
  • Math colloquium
    • Thursday, Oct. 30, 2:30 PM
    • Newton 202
    • Paul Wenger, RIT
    • “Unique Saturation and Eigenvalue Methods for Graphs”
    • Extra credit for going and writing summary
• Questions?
• While Loops
  • Section 5.3
    • While vs for: for repeats set number of times, while repeats until condition becomes false
    • Initialization and infinite loops
      

      ... initialization ...
      while C
          % C (and body) have to be evaluable on 1st iter.
          % Body has to eventually make C false
          ...
      end

      • Example
        • Estimate base-10 logarithm of x
        • Divide x by 10 until results <= 1
        • Count how many divisions you did
          

          >> while x > 1
          x = x/10;
          % increment count
          end
          {Undefined function or variable 'x'.
          }
          % Oops, needed to initialize x (and the counter) 
          >> x = 7829;
          >> c = 0;
          >> while x > 1
          x = x/10;
          c = c + 1;
          end
          >> x = 7829;
          >> c = 0;
          >> while x ~= 1
          x = x / 10;
          c = c + 1;
          end
          % Program runs and runs and runs because x is never exactly 1. Interrupt with control-C
          >> c
          c =
             1.7941e+10

    • Can use to error-check input
  • Example: read input until user enters a divisor of n (an arbitrary pre-assigned value)
    • i.e., loop until input not 0 and n/d is an integer
    • How about...
      

      >> n = 8935;
      >> d = input( 'Guess a divisor of n ' );
      Guess a divisor of n 0
      >> while d == 0 && n/d ~= int32( n/d )
      d = input( 'Guess a divisor of n ' );
      end
      Guess a divisor of n 3
      >> n/d
      ans =
         2.9783e+03

    • So-called “divisor” wasn’t. Why not?
      • While not( input not 0 and n/d is an integer )
      • = input == 0 or n/d is not integer
      • Not( a and b) = (not a) or (not b): de Morgan’s Law
      • Also not( a or b ) = (not a) and (not b)
• Start Coding Probability Lab
• Next
  • Finish probability lab

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