“For” Loops

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• Questions?
• For Loops
  • Sections 5.1 and 5.2
    • What does a for loop do vs if-then?
      • Do statement(s) repeatedly for each element in vector
      • (As opposed to choosing to do statement or not)
  • For loop idioms
    • Do something to all numbers in a range, e.g., print each one
      

      % print every number between 10 and 20
      >> for i = 10:20
      fprintf( 'The current number is %d\n', i )
      end
      The current number is 10
      The current number is 11
      The current number is 12
      The current number is 13
      The current number is 14
      The current number is 15
      The current number is 16
      The current number is 17
      The current number is 18
      The current number is 19
      The current number is 20

      • Or in generic pseudocode form
        

        for i = first : last
            ...
            calculate with i
            ...
        end

    • Do something to all elements of a vector
      Warning: Undefined variable $allELts in /Web/baldwin/public_html/math230/spring2015/0309for.php on line 144
      
      
      • Or generically
        

        for i = 1:length(v)
            ...
            calculate with v(iv)
            ...
        end
        or
        for i = v
            ...
            calculate with i
            ...
        end

    • Do something to all elements of a matrix
      

      % Print all elements of M
      >> M = [ 12 48; 72 17 ]
      M =
          12    48
          72    17
      >> for i =1:2
      for j = 1:2
      fprintf('The current value = %f\n', M(i,j) );
      end
      end
      The current value = 12.000000
      The current value = 48.000000
      The current value = 72.000000
      The current value = 17.000000

      • Or generically
        

        for i = 1:rows
            for j = 1:columns
                ...
                calculate with M(i,j)
                ...
            end
        end

    • Compute some summary of all elements in a vector
      

      % Add all elements in v
      >> sum = 0;
      >> for i = 1:length(v)
      sum = sum + v(i);
      end
      >> v
      v =
          17.0000    9.0000   -4.0000    3.1416    9.2000
      >> sum
      sum =
         34.3416

      • Generically this looks like
        

        initialize total
        for i = v
            ...
            total = f( total, i )
            ...
        end

      • Beware that initialization is outside loop and update inside
    • Map and reduce?
      • Computing a summary is classical reduce
      • Other examples are map
• Next
  • Function handles
    • • Read sections 10.1 and 10.2

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