Matlab Concise Notes

Transcription

1 Gang Wang Oct. 3, 9. Introduction Matlab stands for MATrix LABoratory and the objects of basic operations are vectors and matrices (user should be good at linear algebra). It is written in C. Matlab is an interactive/interpretative high-level language for scientific computing. () interactive: line command, easy to debug () interpretative: not pre-compiled, most time, must run within Matlab environment (3) scientific computing: high level build-in algorithms for scientific computing Algorithms + Data Structures = Programs Algorithms: Let the best mathematician work for you. Data structures: Everything is linear algebra (ODE, PDE ) Programs: made it easy Ax=B Best tutorial: Matlab help files () Use help to >> help plot () Search help files (3) Some online tutorial: google search Matlab tutorial (4) Practice!. Matlab Demonstration >> demos () data visualization Demos Matlab: More Examples Graphics: 3d plot, volume visualization 3d drawing Movie clips () toolbox demos Matlab toolbox spline construct a spline image processing edge detection (blood) statistics robust regression (3) symbolic operation x=sym('x') % create a symbolic variable, x f=/[5+4*cos(x)] ezplot(f) % plot function f(x) f=diff(f) % differentiate the function once

2 ezplot(f) f=diff(f,) % differentiate the function twice ezplot(f) g=int(int(f)) % integrate nd derivative twice ezplot(g) 3. Matlab Environment Path Current working directory, know where your programs are Workspace script echo or not echo ( ; ) Comments (%) 4. Basic Operations () Basic Mathematics Operators Example: Matlab as a calculator: 5*sin(.5^(3-pi))+/ * / ^ (power, eg. ^3=8).34e3 (=.34*^3) abs(x) sqrt(x) log(x) -- natural log log(x) (base log) exp(x) triangular functions: sin cos tan cot asin acos atan acot () Some Constants pi inf eps NaN -- not a number is an important Example >>X=[,,5,NaN, 8, 9]; >>plot(x,'ro-'); (3) Logic operators = = eq(,) ~= ne(,) < > =< >= max(a,b) min(a,b) Example: >> a=[,,3,4] >> a = = 5. Basic variables () Real Numbers

3 >>pi >>format long format short format long e format short e () Strings/Characters string= hello world string=['hello ','world'] string3=numstr(3) % convert from num to string [string,' + = ',string3] A=[,,4,8] disp(a) disp(string) sprintf('this is %d + %d = %d',,,+) %Write formatted data to string. sprintf('this is %d + %d = %.4f',,,+) sprintf('this is %d + %d \n= %.4f',,,+) disp(sprintf('this is %d + %d = %.4f',,,+)) (3) load/save variables >> clear all >> mymatrix=[,3, 5, 7, 9] >> save('savedmymatrix.mat','mymatrix') % save as.mat file >> load('savedmymatrix.mat') % load variables stored in.mat file >> load savedmymatrix % the same >> who % check variable in workspace 6. Basic Data Structurs () Vectors and matrices x=[ ] x=: x=:: x=linspace(,,3) A=[ 3 ; ; ] A(3,) Matrix operation + - * / A*B (m,n) (n*p) (m*p) Example: Solve AX=B A=[,;3,4] B=[5;6] X=inv(A)*B X=A\B % partial pivoting size(a) inv(a) 3

4 A*A trace(a) det(a) eye(3) zeros(,3) [i,j]=find(a==) eig(a) see matlab demos: matrices, basic operation () string is also stored in matrix structure (3) Structural Array. Construct the structural array >>GMXEmployee().ID=34 >>GMXEmployee().Name='Gang Wang' >>GMXEmployee().ID=567 >>GMXEmployee().Name='Miao Zhang' >>GMXEmployee.ID(). Make the query >>GMXEmployee().Name ans = Demos: matlab-> language-> structures (4) Cells Enclose the cell subscripts in parentheses using standard array notation. Enclose the cell contents on the right side of the assignment statement in curly braces, "{}." For example, create a -by- cell array A. >>A(,) = {[ 4 3; 5 8; 7 9]}; >>A(,) = {'Anne Smith'}; >>A(,) = {3+7i}; >>A(,) = {-pi:pi/:pi}; Try to calculate A(,)+, oops, function '+' not defined for variables of class 'cell'. What should we do? Convert cell to matrix first, then do calculation >>cellmat(a(,))+ ans = i >> cellmat(a(,)) Advantage of using cells:. assemble different data types. assemble data of different length, especially string variables B()={'this is a long sentence'} 4

5 B()={'this is short'} Colon notation Generate a list with start:step: a=:: %(step, from to ) a=: %(step as default) Retrieve sub-block of a matrix A=[,,3; 4,5,6; 7,8,9] A(:,) %the first column of matrix A A(,:) %the first row of matrix A A(:3, [,3])=3 SUM operation >> A = [:3; 4:6; 7:9] A = >> s = sum(a) s = 5 8 >> ss = sum(sum(a)) ss = 45 Matrix entry-to-entry operation A=[ 3]; B=[3 4 5];. Dot product: A.*B ans = Compare it with A *B = ( 3 4 5) ans = and A*B = ( 3 4 5) =6 3. Dot division: A./B ans =

6 3. Dot power : A.^ ans = Building Blocks of Structured Programming () IF statement if expression statements else statements The statements are executed if the real part of the expression has all non-zero elements. The ELSE and ELSEIF parts are optional. Zero or more ELSEIF parts can be used as well as nested IF's. The expression is usually of the form expr rop expr where rop is ==, <, >, <=, >=, or ~=. () FOR Loops Repeat statements a specific number of times. The general form of a FOR statement is: for variable = expression statement statement example: for i=['a','b', 'c'] disp(i) The BREAK statement can be used to terminate the loop prematurely. Example: for i=: disp(i) if i ==5 break (3) WHILE Loops: Repeat statements an indefinite number of times. The general form of a WHILE statement is: while expression statements 6

7 The statements are executed while the real part of the expression has all non-zero elements. The expression is usually the result of expr rop expr where rop is ==, <, >, <=, >=, or ~=. Example: eps = ; while (+eps) > eps = eps/ eps = eps* (4) SWITCH CASE: Switch among several cases based on expression. Example: METHOD='LINEAR'; switch lower(method) case {'linear','bilinear'} disp('method is linear') case 'cubic' disp('method is cubic') case 'nearest' disp('method is nearest') otherwise disp('unknown method.') 8. Matlab Scripts A script is an m-file without the function declaration at the top. Be aware that all the variables in a scripts can be altered. hit : () always use clear all to start a new script () make sure the path saved 9. Function Example: function [A] = myarea(a,b,c) % Compute the area of a triangle whose % sides have length a, b and c. % Inputs: % a,b,c: Lengths of sides % Output: % A: area of triangle % Usage: % Area = area(,3,4); 7

8 % Written by dfg, Oct 4, 996. s = (a+b+c)/; A = sqrt(s*(s-a)*(s-b)*(s-c)); >> help area >> Area = area(,5,) Change function declaration to function [A,s] = area(a,b,c) then, the returned variables are A and s >> Area = area(,5,) % by default, the st returned variable is assigned >> [Area, hlen] = area(,5,). File Operation READ/WRITE a file () fopen, fclose >> fid = fopen('sound.dat','r'); >> fclose(fid) () fgetl: Read line from file, discard newline character Example fid=fopen('fgetl.m'); while tline = fgetl(fid); if ~ischar(tline), break, disp(tline) fclose(fid); (3) fprintf: Write formatted data to file Example: x = :.:; y = [x; exp(x)]; fid = fopen('exp.txt','w'); fprintf(fid,'%6.f %.8f\n',y); fclose(fid) (4) fwrite: Write binary data to a file (5) fread: Read binary data from file 8

9 Matlab Graphics. Matlab is a powerful tool to visualize data.. Two Dimensional (-D) Plot Y=f(X). Three Dimensional (3-D) Plot Z=f(X, Y) 3. Volume Data (4-D) Visualization X, Y, Z, V(X, Y, Z). Two Dimensional (-D) Plot () Linear -D plot command Syntax Syntax plot(y) plot(x,y,...) plot(x,y,linespec,...) plot(...,'propertyname',propertyvalue,...) h = plot(...) Example: x = -pi:pi/:pi; y = tan(sin(x)) - sin(tan(x)); plot(x,y) Use File-> Save; Edit->Copy Figure in the control bar to save/copy the figure Use arrow and double click the graphic to open Graphic Property panel. () Use short symbols to specify COLOR, MARKER, LINE TYPE plot(x,y,'ro-'); % r red; o circle, - solid line plot(x,y,'g*:'); % g green; * - star, : dot line 9

10 (3) Specification of other graphic properties You can also specify other line characteristics using graphics properties. The most commonly used graphic properties are: Color -- Three ways to assign color in Matlab LineWidth - specifies the width (in points) of the line. MarkerEdgeColor - specifies the color of the marker or the edge color for filled markers (circle, square, diamond, pentagram, hexagram, and the four triangles). MarkerFaceColor - specifies the color of the face of filled markers. MarkerSize - specifies the size of the marker in units of points. Examples: h=plot(x,y,'--rs','linewidth',, 'MarkerEdgeColor','k', 'MarkerFaceColor','g','MarkerSize',)

11 Important Concept: Use Object Handle to Operate gca -- get current axes handle gcf -- get current figure handle Examples: set(gca,'color','k') % gca get current axes set(gcf,'color','y') % gcf get current figure set(gcf,'color','yellow') set(gcf,'color',[,,]) set(gcf,'color',[,.5,.5]) % another fancy color h=plot(x,y,'g*:'); % set h as handle of the line object set(h, 'Color', 'b'); % set the handle object (line) to blue get(h,'color'); % use get command to get color property of the handle object (i.e. line) get(h,'linewidth') set(h,'markersize',5) (4) More Operations on the Figure h=plot(x,y,'g*:'); xlabel('x value') ylabel('y value') title('my first plot') 3 my first plot y value x value grid on grid off h=plot(x,-y,'bo-'); % first figure was erased!! What should we do?

12 HOLD ON to plot multiple lines in the same figure, until you HOLD OFF h=plot(x,y,'g*:'); hold on; h=plot(x,-y,'bo-'); Now put in label, and change the font size of the lable hh=ylabel('y value') set(hh,'fontsize',4) Put a text on the figure text(,5,'my comments') Put on leg leg('st Line','nd Line',3) % note that the last entry is the position; try,, 3, 4 Save current figure to windows bitmap file saveas(gcf, 'output', 'bmp') You can save to other formats: tiff, jpeg, eps, pdf. (5) subplot clf % clear figure income = [ ]; outgo = [ ]; subplot(,,); plot(income) subplot(,,); plot(outgo) You can also make x subplots like this (6) Other D plot commands: loglog( ) semilogx( ) semilogy( ). Three Dimensional (3D) Plot

13 () Linear 3-D plot command: plot3 Syntax plot3(x,y,z,...) plot3(x,y,z,linespec,...) plot3(...,'propertyname',propertyvalue,...) h = plot3(...) Example: t = :pi/5:*pi; h=plot3(sin(t),cos(t),t) grid on axis square set(h,'color','r','linewidth',3,'linestyle',':') change view angle: [,,] by default, you can manually rotate the view view([,,]) () D surface plot SURF : 3-D shaded surface plot SURFC: 3-D shaded surface plot with contour SURFL: 3-D shaded surface with lighting. Display a surface and contour plot of the peaks surface. [X,Y,Z] = peaks(3); % peaks is just a 3D function surf(x,y,z) % can use surf(x,y,z), then the plot is without contour colormap hsv % choose colormap scheme axis([ ]) % specify the range of axis 3

14 Try surf(x,y,z) Add on color bar colorbar % show colorbar % change colormap scheme to jet colormap jet Change shading to interpolated scheme shading interp; colormap(pink); Other important commands: meshgrid; mesh; [X,Y] = meshgrid(-:.5:, -:.5:); % construct X,Y grids Z = X.* exp(-x.^ - Y.^); mesh(x,y,z) surf(x,y,z) 4

15 Note: NaN data is NOT visualized! % assign some z values as NaN in the middle rectangular area Z(7:3, 7:3)=NaN; surf(z) This example shows a hole in the figure! Demostration >> demos Choose Matlab -> Graphics 3D plot Transparency 5

16 3. Volume Data (4-D) Visualization Example : isosurface, isonormal data = cat(3, [. ;.3 ; ], [.. ; ;..7 ], [.4.;..4 ;.. ]); data = interp3(data,3,'cubic'); subplot(,,) p = patch(isosurface(data,.5), 'FaceColor','red','EdgeColor','none'); view(3); daspect([,,]); axis tight camlight; camlight(-8,-); lighting phong; title('triangle Normals') subplot(,,) p = patch(isosurface(data,.5), 'FaceColor','red','EdgeColor','none'); isonormals(data,p) view(3); daspect([ ]); axis tight camlight; camlight(-8,-); lighting phong; title('data Normals') Example Slices of volumetric data [x,y,z] = meshgrid(-:.:,-:.5:,-:.6:); v = x.*exp(-x.^-y.^-z.^); xslice = [-.,.8,]; yslice = ; zslice = [-,]; slice(x,y,z,v,xslice,yslice,zslice) colormap hsv 6

17 Example 3 Isosurface, isocaps, coneplot, and streamlines of wind data load wind % load wind data, it contains u v w x y z variables spd = sqrt(u.*u + v.*v + w.*w); p = patch(isosurface(x,y,z,spd, 4)); isonormals(x,y,z,spd, p) set(p, 'FaceColor', 'red', 'EdgeColor', 'none'); p = patch(isocaps(x,y,z,spd, 4)); 7