MATLAB. Input/Output. CS101 lec

Transcription

1 MATLAB CS101 lec24 Input/Output

2 MATLAB Review MATLAB Review

3 Question ( ) How do we access 6 in this array? A A(2,1) B A(1,2) C A(3,2) D A(2,3) MATLAB Review

4 Question ( ) How do we access 6 in this array? A A(2,1) B A(1,2) C A(3,2) D A(2,3) MATLAB Review

5 Question ( How can we produce this array? A [ [ ] [ ] ] B [ [ 1 2 ] [ 3 4 ] [ 5 6 ] ] C [ [ ] ; [ ] ] D [ [ 1 2 ] ; [ 3 4 ] ; [ 5 6 ] ] ) MATLAB Review

6 Question ( How can we produce this array? A [ [ ] [ ] ] B [ [ 1 2 ] [ 3 4 ] [ 5 6 ] ] C [ [ ] ; [ ] ] D [ [ 1 2 ] ; [ 3 4 ] ; [ 5 6 ] ] ) MATLAB Review

7 Question ( Which of the following will produce this array? A 3*ones( 2,2 ) - 2*eye( 2,2 ) B 2*ones( 2,2 ) + eye( 2,2 ) C 3*ones( 2,2 ) - eye( 2,2 ) D ones( 2,2 ) + eye( 2,2 ) ) MATLAB Review

8 Question ( Which of the following will produce this array? A 3*ones( 2,2 ) - 2*eye( 2,2 ) B 2*ones( 2,2 ) + eye( 2,2 ) C 3*ones( 2,2 ) - eye( 2,2 ) D ones( 2,2 ) + eye( 2,2 ) ) MATLAB Review

9 Question ( ) How can we produce this array? A ones(3,3) - 2*eye(3,3) B ones(3,3) + 2*eye(3,3) C 2*ones(3,3) + eye(3,3) D 2*ones(3,3) - eye(3,3) MATLAB Review

10 Question ( ) How can we produce this array? A ones(3,3) - 2*eye(3,3) B ones(3,3) + 2*eye(3,3) C 2*ones(3,3) + eye(3,3) D 2*ones(3,3) - eye(3,3) MATLAB Review

11 Arrays Redux Arrays Redux

12 Basics a = [ ]; %row vector b = [ ] ; %column vector A = [ ; ]; %matrix B = [ a ; b ]; % matrix composition Arrays Redux

13 Array operations Matrix v. elementwise operations: Matrix operations are matrix vector operations: ( ) ( ) ( ) = 23 Arrays Redux

14 Array operations Matrix v. elementwise operations: Matrix operations are matrix vector operations: ( ) ( ) ( ) = 23 [ 1 0 ; 0 1 ] * [ 2 3 ] Arrays Redux

15 Array operations Matrix v. elementwise operations: Matrix operations are matrix vector operations: ( ) ( ) ( ) = 23 [ 1 0 ; 0 1 ] * [ 2 3 ] ( ) ( ) ( = ) ( ) = 85 Arrays Redux

16 Array operations Matrix v. elementwise operations: Matrix operations are matrix vector operations: ( ) ( ) ( ) = 23 [ 1 0 ; 0 1 ] * [ 2 3 ] ( ) ( ) ( = [ 1 2 ; 1 1 ] * [ 2 3 ] ) ( ) = 85 Arrays Redux

17 Array operations Matrix v. elementwise operations: Elementwise operations are spreadsheet-like operations: ( ) ( ) = ( ) Arrays Redux

18 Array operations Matrix v. elementwise operations: Elementwise operations are spreadsheet-like operations: ( ) ( ) = [ 1 0 ; 0 1 ].* [ 2 4 ; 3 5 ] ( ) Arrays Redux

19 Indexing arrays We can index arrays with arrays. A = 0:10:100; B = A( [ 5,9,2,2 ] ); Arrays Redux

20 Indexing arrays We can index arrays with arrays. A = 0:10:100; B = A( [ 5,9,2,2 ] ); This permits slicing. A = 0:10:100; B = A( 4:7 ); Arrays Redux

21 Indexing arrays In more dimensions: A = [ 1,2,3 ; 4,5,6 ; 7,8,9 ]; B = A( 1:2,1:2 ); C = A( :,1:2 ); Arrays Redux

22 Indexing arrays What is the difference in these? Why? A = [ 1,2,3 ; 4,5,6 ; 7,8,9 ]; A( 2 ) A( 2,: ) A( :,2 ) Arrays Redux

23 Summary Like NumPy, but no imports (anywhere). Remember: parentheses, indexing from 1. Hard to do dict-like things, easy to do numpy-like things. Arrays Redux

24 Data Processing Data Processing

25 Modeling Data Processing

26 File Input File Input

27 File I/O MATLAB encourages the storage of complicated variables, such as the results of numerical calculations, as mat files. Saving data uses save: A = [ ; ]; save( test, A ); File Input

28 File I/O MATLAB encourages the storage of complicated variables, such as the results of numerical calculations, as mat files. Saving data uses save: A = [ ; ]; save( test, A ); Note that the string version of the variable name is required! load also useful: A = load( test, A ); File Input

29 File I/O A more advanced tool: importdata data = importdata( rainfall.txt ); File Input

30 File I/O A more advanced tool: importdata data = importdata( rainfall.txt ); Can be used to process CSVs. File Input

31 File I/O A more advanced tool: importdata data = importdata( rainfall.txt ); Can be used to process CSVs. Old process using fopen, fscanf, fclose, fprintf also common. Often returns an array, but sometimes a cell (MATLAB s list). File Input

32 Web Input Web Input

33 Web I/O webread processes data gracefully. url = data = webdata( url ); image( data ); Web Input

34 Plotting Plotting

35 Plotting plot works identically to plt.plot. figure creates a new figure (window for plots). x = 0:.1:2*pi y = sin( x ) figure plot( x,y, o ) title( sin(x) ) xlabel( x values ) ylabel( y values ) Plotting

36 Plotting plot works identically to plt.plot. figure creates a new figure (window for plots). x = 0:.1:2*pi y = sin( x ) figure plot( x,y, o ) title( sin(x) ) xlabel( x values ) ylabel( y values ) MATLAB also supplies an excellent plot editor. Plotting

37 Plotting Other plots to use: fplot plot3 fcontour subplot Plotting

38 Aside on functions You can define a single-line function locally using the syntax: f cos( 3*t ); Plotting

39 Plotting x cos( 3*t ); y sin( 2*t ); fplot( x,y ) t = 0:pi/50:10*pi; st = sin(t); ct = cos(t); plot3(st,ct,t) Plotting

40 Plotting f sin( x ) + cos( y ); fcontour( f ) subplot(2,1,1); x = linspace(0,10); y1 = sin(x); plot(x,y1) subplot(2,1,2); y2 = sin(5*x); plot(x,y2) Plotting

41 Images Images

42 Images Images can also be opened as files. A = importdata( rabbit-bw.jpg ); image( A ); Images

43 Images Images can also be opened as files. A = importdata( rabbit-bw.jpg ); image( A ); Black and white images are arrays of 0s and 1s. Greyscale images are values from 0 and 1. Color images are three-dimensional arrays. (Why?) Variations exist depending on the underlying data. Images

44 Odds & Ends Odds & Ends

45 Multiple returns Functions can return several values. Odds & Ends

46 Multiple returns Functions can return several values. function [ a,b ] = nonsense( x,y ) a = x ^ 2; b = y ^ 3; end [ q r ] = nonsense( 3,4 ); Odds & Ends

47 Character arrays Indicated with single quotes (only!). s = XFEM ; Print formatted character arrays with fprintf: fprintf( %f %f, sin(pi/3), cos(pi/4) ); Odds & Ends

48 Character arrays Indicated with single quotes (only!). s = XFEM ; Print formatted character arrays with fprintf: fprintf( %f %f, sin(pi/3), cos(pi/4) ); Recently MATLAB has introduced a string data type as well. Odds & Ends

49 Multiple returns But be careful sizes cause surprises. A = [ HELLO ; WORLD ]; A( 2,3 ) C = [ HELLO ; WORLD! ]; Odds & Ends

50 Multiple returns But be careful sizes cause surprises. A = [ HELLO ; WORLD ]; A( 2,3 ) C = [ HELLO ; WORLD! ]; How could this affect function return values? Odds & Ends