Lecture 7. MATLAB and Numerical Analysis (4)

Transcription

1 Lecture 7 MATLAB and Numerical Analysis (4)

2 Topics for the last 2 weeks (Based on your feedback) PDEs How to results (after FFT Analysis (1D/2D) Advanced Read/Write Solve more problems Plotting3Dscatter points Symbolic computation Nonlinear DEs Advanced Graphics Fitting data to a distribution finishing an analysis)? Neural networks Iterative methods (for solution of nonlinear equations) More curve fitting More optimization

3 Today Advanced d Read and Wit Write More about the plot function Plotting in 3D Irregularly spaced data

4 Read & Write Remember that there is no one magic function that will work for all types of files and data. You need to know how data is organized in your file(s) and use the appropriate functions Reading & Writing data can be frustrating: You need to read MATLAB documentation carefully

5 Read and Write We learned how to use the load and save commands These work well for both text and binary data Use the flag ASCII to save output in text format Examples: >> m = magic(4) >> save data.txt m ascii Note that you can operate on matrices using commands such as reshape and horzcat to change your output Example (Learn to use matrix concatenation): How can we add one row and one column to the above matrix m?

6 Binary Files Use the command save to save binary files in.mat format How to open the.mat files again? Use the load command (do not use the open command) Reading binary files can be a non trivial task, especially if you are trying to read a file that was not written by you (why?) Due to differences in the way numbers are stored internally in computers (IEEE floating point with big endian or small endian byte ordering, for example) Endianness in computer science refers to byte order Use fread to read binary data. Before reading, open the file using fid = fopen(filename)

7 Opening Files for Read/Write >> fid = fopen( filename, permissions) fid = 11 if open fails fil

8 Read / Write Functions fscanf Read text data fread Read binary data until eof or a text field is encountered (vector) textread Name says it all. Useful if you have mixed data in a row (text, numbers) fprintf Does the opposite of fscanff strread Read formatted data from a string textscan Read formatted data from text file or string better for large files compared to textread; can read from any point in the file sscanf Read formatted data from a string sprintf The opposite of sscanf xlsread Read data from the first worksheet in a Microsoft Excel file dlmread Read ASCII delimited file

9 fscanf: example Use this function to read complex data files Reads numbers until it encounters characters You can also read element by element >> fid=fopen('test.txt','rt') >> [a,count]=fscanf(fid,'%e') >> a1 =reshape(a,3,3) >> fclose(fid) >> [b,count]=fscanf(fid,'%c',31) >> [c,count]=fscanf(fid,'%e'), >> fid=fopen('test.txt','rt'); >> for i = 1:9 >> a(i)=fscanf(fid,'%e,1); % Notice the 1 number of elements to read >> end >> fclose(fid)

10 The format string %12e field width is optional %*e * ki di * means skip reading this field

11 sscanf: example You can use fscanffwith the %c option to read the entire file; Then, you can use sscanf to separate the numeric and character values as you like >> fid=fopen('test.txt','rt'); >> [a,count]=fscanf(fid,'%c') This is a string of characters This is yet another character string >> a1=sscanf(a,'%f')

12 textscan: example Sally Level e10 inf NaN Yes Joe Level e19 -inf No Bill Level e No fid = fopen('scan1.dat'); C = textscan(fid, '%s %s %f32 %d8 %u %f %f %s'); fclose(fid); textscan returns a 1-by-8 cell array C with the following cells: C{1} = {'Sally'; 'Joe'; 'Bill'} class cell C{2} = {'Level1'; 'Level2'; 'Level3'} class cell C{3} = [12.34; 23.54; 34.9] class single C{4} = [45; 60; 12] class int8 C{5} = [ ; ; ] class uint32 C{6} = [Inf; -Inf; 10] class double C{7} = [NaN; 0.001; 100] class double C{8} = {'Yes'; 'No'; 'No'} class cell

13 Useful Functions num2str Floating point number to string conversion str2num does the opposite int2str Integer to string Example: >> a=2.35; >> plot(peaks); >> title(strcat('the value of variable a is:',num2str(a)));

14 Plotting 3D data >> x=1:10; >> y = x; >> z=sin(x+y)./(x+y); y); >> plot3(x,y,z, ro- ); >> grid on; axis square;

15 More about the plot function: How to change the axis limits? gca Current axes handle >> plot(peaks); >> set(gca,'xlim',[20 40]); >> get(gca) Returns all properties of the axes

16 More about the plot function: How to use custom colors? >> x=1:0.01:10; >> y1=cos(x)./x; >> y2=sin(x)./x; >> huntergreen = [ ]; >> msugreen = [ ]; 1 >> plot(x,y1,huntergreen); >> hold on; 0.8 >> plot(x,y2,msugreen);

17 More about the plot function: How to fill symbols with ihcustom colors? >> x=1:0.1:10; >> y1=cos(x)./x; >> y2=sin(x)./x; >> huntergreen = [ ]; >> msugreen = [ ]; >> plot(x,y1,'o','color',huntergreen,'markerfacecolor',huntergreen),,,,, g >> hold on; >> plot(x,y2,'o','color',msugreen,'markerfacecolor',msugreen);

18 More about the plot function: The set and get functions >> x=1:0.01:10; >> y1=cos(x)./x; >> h=plot(x,y1); >> get(h) >>

19 More about the plot function: The set and get functions 1 >> x=1:0.01:10; 0.8 >> y1=cos(x)./x; >> y2=sin(x)./x; 0.6 >> h=plot(x,y1,x,y2); 0.4 >> get(h) x1 struct array with fields: DisplayName Annotation Color EraseMode LineStyle LineWidth >> get(gca)

20 How to save plots as MATLAB plot files? (Useful if you haven t decided which format to save your output to) File Save As *fig *.fig

21 Irregularly spaced data Use the griddata function to interpolate data onto a regular grid