Slicing and Assembling Matrices

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• Misc
  • First hour exam
    • Next Friday, Oct. 23
    • Material from start of semester through matrices (e.g., arithmetic, scripts & functions, vectors, plotting, matrices, etc.)
    • 3 - 5 short-answer questions
    • Precise Matlab syntax will only matter at the “what I expect” vs “perfect” level
    • Full class period
    • Open book, notes, computer; closed person
• Questions?
• Slicing and Recombining Matrices
  • Sections 9.5 through 9.8
    • Concatenation joins arrays to make larger ones
      • Comma for row arrays, semicolon for columns
      • Number of rows/columns has to match
      • Making 1D from 2D
      • Equivalent functions (vertcat, horzcat)
    • Indexing is much like for 1D arrays
      • Colon shortcut for indexing whole row/column
      • Can flatten arrays with colon
  • Examples
    • Extract first half of a sound
      

      [ tune, rate ] = audioread( 'Vogelfaenger.wav' );
      >> size( tune )
      ans =
           1123353           2
      >> sound( tune, rate )
      >> length(tune)
      ans =
           1123353
      >> halfLength = floor(length(tune)/2)
      halfLength =
            561676
      >> halfTune = tune( 1:halfLength, : );
      >> sound( halfTune, rate )
      >> rate
      rate =
             44100

    • Rseample at lower rates and listen for aliasing per Nyquist
      

      >> tune8 = tune( 1:8:end, : )
      >> sound( tune8, rate/8 );
      >> tune12 = tune( 1:12:end, : );
      >> sound( tune12, rate/12 );

    • Make 2 copies of the first half
      

      >> doubleTune = [ halfTune, halfTune ];
      >> size( doubleTune )    % Oops, wrong concatenation
      ans =
          561676           4
      >> doubleTune = [ halfTune; halfTune ];
      >> size( doubleTune )
      ans =
          1123352           2
      >> sound( doubleTune, rate );

• Matrices Lab
• Next
  • Control structure
  • Specifically, “while” loops
    • (And sort of a preview of “if” statements)
  • Read sections 12.1 and 12.2

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