Indexing Subarrays

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• Questions?
  • Pattern for drawing one frame to avoid multiple images accumulating
    

    loop
        ...
        plot 1st part (e.g., cell)
        hold on
        plot 2nd part (e.g., ball)
        plot 3rd part (if any) ...
        hold off
        ...

• Subarrays
  • Sections 9.6 - 9.8
    • Indexing
      • Indexing arrays are arrays of indices
      • Indexed array is array in which indexing accesses elements
      • Row-column indexing generates rectangular arrays with restrictions on selectable elements
      • Linear indexing uses 1 indexing array, returns 1D vector, allows selection of any element(s)
      • Multi-element indexing uses 2 index arrays, generates 2D result, unless indexes refer to just 1 row or column
      • Logical indexing uses true/false indexing array
        • Selects items corresponding to true indices
  • Examples
    

    >> M = [ 10, 20, 30, 40; 100, 200, 300, 400; 1000, 2000, 3000, 4000; -1, -2, -3 -4 ]
    M =
              10          20          30          40
             100         200         300         400
            1000        2000        3000        4000
              -1          -2          -3          -4
    >> % Pick out central 2-x-2 submatrix
    >> M( [6,7,10,11] )
    ans =
             200        2000         300        3000
    >> M( [2:3], [2:3] )
    ans =
             200         300
            2000        3000
    >> M2 = M
    M2 =
              10          20          30          40
             100         200         300         400
            1000        2000        3000        4000
              -1          -2          -3          -4
    >> % swap 2nd and 3rd rows in M2
    >> M2( [2,3], : ) = M2( [3,2], : )
    M2 =
              10          20          30          40
            1000        2000        3000        4000
             100         200         300         400
              -1          -2          -3          -4
    >> R = rand( 4 )
    R =
        0.1270    0.2785    0.1576    0.8003
        0.9134    0.5469    0.9706    0.1419
        0.6324    0.9575    0.9572    0.4218
        0.0975    0.9649    0.4854    0.9157
    >> % all elements from R < 0.3
    >> R < 0.3
    ans =
         1     1     1     0
         0     0     0     1
         0     0     0     0
         1     0     0     0
    >> R( R < 0.3 )
    ans =
        0.1270
        0.0975
        0.2785
        0.1576
        0.1419
    >> mask = R < 0.3
    mask =
         1     1     1     0
         0     0     0     1
         0     0     0     0
         1     0     0     0
    >> R .* mask
    ans =
        0.1270    0.2785    0.1576         0
             0         0         0    0.1419
             0         0         0         0
        0.0975         0         0         0
    >> % Make all elements of R < 0.3 be -1
    >> R( R < 0.3 ) = -1
    R =
       -1.0000   -1.0000   -1.0000    0.8003
        0.9134    0.5469    0.9706   -1.0000
        0.6324    0.9575    0.9572    0.4218
       -1.0000    0.9649    0.4854    0.9157
    >> M
    M =
              10          20          30          40
             100         200         300         400
            1000        2000        3000        4000
              -1          -2          -3          -4
    >> R = rand( 4 )
    R =
        0.7922    0.8491    0.7431    0.7060
        0.9595    0.9340    0.3922    0.0318
        0.6557    0.6787    0.6555    0.2769
        0.0357    0.7577    0.1712    0.0462
    >> R( R < 0.3 ) = M( R < 0.3 )
    R =
       1.0e+03 *
        0.0008    0.0008    0.0007    0.0007
        0.0010    0.0009    0.0004    0.4000
        0.0007    0.0007    0.0007    4.0000
       -0.0010    0.0008   -0.0030   -0.0040

  • Subarrays and images
    • Grayscale images are just 2D matrices of color intensities for each spot of gray, or pixel, in image
    • Bird example
      

      >> load 'birdImage.mat'
      >> imshow( bird )
      >> bird( 1:10, 1:10 )
      ans =
        255  255  255  255  255  255  255  255  255  255
        255  255  255  255  255  255  255  255  255  255
        255  255  255  255  255  255  255  255  255  255
        255  255  255  255  255  255  255  255  255  255
        255  255  255  255  255  255  255  255  255  255
        255  255  255  255  255  255  255  255  255  255
        255  255  255  255  255  255  255  255  255  255
        255  255  255  255  255  255  255  255  255  255
        255  255  255  255  255  255  255  255  255  255
        255  255  255  255  255  255  255  255  255  255
      >> bird( 121:131, 121:131 )
      ans =
        254  255  255  255  255  255  254  254  255  251  255
        255  255  255  255  254  253  251  250  255  245  248
        255  255  255  254  253  251  249  248  255  253  249
        255  255  255  255  253  252  250  249  252  248  249
        255  255  255  255  255  255  254  253  251  243  249
        254  255  255  255  255  255  255  254  243  255  250
        255  255  255  255  255  254  253  252  241  254  189
        255  255  255  255  253  251  249  248  250  191   87
        248  255  255  239  255  255  248  255  192   90   17
        255  239  248  255  253  253  241  204   94   29    0
        250  248  255  252  251  255  209  111   27    0    0

• Next
  • Color images and 3D arrays
  • Read sections 11.1, 11.2, and 11.5 in textbook

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