Advanced Vector Operations

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• Questions?
  • “Concatenate” = to join 2 things together end-to-end
• Advanced Vector Operations
  • Sections 5.7 - 5.9
    • Indexing with array of indices accesses multiple elements at once
      • array1( array2 )
      • Array being accessed = indexed array
      • Array of indices = indexing array
    • Concatenating arrays
      • e.g., array1 = [ array2, array3, … ]
    • Extend array by assigning to position beyond end
    • Delete element(s) by assigning [] to their positions
    • end keyword means last position in an array
  • Examples / practice
    • Notice that, by and large, programming features work in any context where they make sense, not just in some
      • For instance, you can concatenate vectors anywhere joining two vectors end-to-end is a meaningful calculation, not just as part of an assignment statement
        

        >> % Create some vectors
        >> v = 1:5
        v =
             1     2     3     4     5
        >> w = 10:10:100
        w =
            10    20    30    40    50    60    70    80    90   100
        >> % First 3 elements in w
        >> w(1:3)
        ans =
            10    20    30
        >> w([1:3])
        ans =
            10    20    30
        >> % Last 3 elements of w
        >> w(end-3:end)
        ans =
            70    80    90   100
        >> % Oops, that was the last 4 elements
        >> w(end-2:end)
        ans =
            80    90   100
        >> % Reverse w
        >> wReverse = w( end:-1:1 )
        wReverse =
           100    90    80    70    60    50    40    30    20    10
        >> % Delete 1st element of v
        >> v
        v =
             1     2     3     4     5
        >> v(1) = []
        v =
             2     3     4     5
        >> % Delete 2nd and 3rd elements of v
        >> v(2,3) = []
        {Subscripted assignment dimension mismatch.}
        % Oops, forgot the square brackets that make the list of indices a vector 
        >> v([2,3]) = []
        v =
             2     5
        >> % Concatenate v to itself
        >> [ v, v ]
        ans =
             2     5     2     5
        >> [ v, v, v, v ]
        ans =
             2     5     2     5     2     5     2     5
        >> v4 = [ v, v, v, v ]
        v4 =
             2     5     2     5     2     5     2     5
        >> v12 = [ v4, v4, v4 ]
        v12 =
          Columns 1 through 11
             2     5     2     5     2     5     2     5     2     5     2
          Columns 12 through 22
             5     2     5     2     5     2     5     2     5     2     5
          Columns 23 through 24
             2     5
        >> v50 = [ v12, v12, v12, v12, v, v ]
        v50 =
          Columns 1 through 11
             2     5     2     5     2     5     2     5     2     5     2
          Columns 12 through 22
             5     2     5     2     5     2     5     2     5     2     5
          Columns 23 through 33
             2     5     2     5     2     5     2     5     2     5     2
          Columns 34 through 44
             5     2     5     2     5     2     5     2     5     2     5
          Columns 45 through 55
             2     5     2     5     2     5     2     5     2     5     2
          Columns 56 through 66
             5     2     5     2     5     2     5     2     5     2     5
          Columns 67 through 77
             2     5     2     5     2     5     2     5     2     5     2
          Columns 78 through 88
             5     2     5     2     5     2     5     2     5     2     5
          Columns 89 through 99
             2     5     2     5     2     5     2     5     2     5     2
          Column 100
             5
        >> % Concatenation and deletion are particularly helpful for working with strings
        >> % (which are just vectors of characters)
        >> first = 'Jill'
        first =
        Jill
        >> last = 'Smith'
        last =
        Smith
        >> full = [first,last]
        full =
        JillSmith
        >> full = [first,' ',last]
        full =
        Jill Smith
        >> vowels = full([2,8])
        vowels =
        ii

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  • Logical indexing
  • Read sections 5.10 - 5.13

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