1. Introduction to MATLAB
Session 3
File I/O
Graphics
Mihaela Duta
MSc in Mathematical and Computational Finance Introduction to MATLAB
Session 3

2. Today’s topics File I/O Saving and loading the workspace Low-level I/O
Interfaces with various file types
Graphics
2D/3D line plots
3D surfaces
Figure properties
MSc in Mathematical and Computational Finance Introduction to MATLAB
Session 3

3. Saving/loading the workspace
When closing a MATLAB session the content of the workspace is lost unless specifilacally saved
save/load – saves/loads workspace in/from the file matlab.mat in binary format
save/load(<filename>, <var1>,…,<var2>) – saves/loads in/from a specified file <filename> a selection of variables <var1>,…,<var2>
Example 1
>> a = 2; b = 3;
>> save
Saving to: matlab.mat
>> clear
>> load
Loading from: matlab.mat
Example 2
>> save('file.mat')
>> load('file.mat')
Example 3
>> save('file.mat','a')
MSc in Mathematical and Computational Finance Introduction to MATLAB
Session 3

4. File I/O To read/write information from/in a file we need to
Open the file – establish a data channel
Perform data transfer
Close the file
ASCII files - consist only of alphanumeric chars
MSc in Mathematical and Computational Finance Introduction to MATLAB
Session 3

5. Opening/closing files
Open files with fopen - returns a file identifier
>> fid = fopen(<filename>, <mode>);
<mode> - access type ( e.g. 'r', 'w', 'a', etc)
Close files with fclose
>> fclose(fid);
fid = fopen('file.txt')
fid = -1
>> fid = fopen('file.txt','w') 3
Check the directory
>> fclose(fid);
Now we have the file – empty -, we can open it for reading
>> fid = fopen('file.txt')
MSc in Mathematical and Computational Finance Introduction to MATLAB
Session 3

6. Data transfer on ASCII files
Write data in an opened file with fprintf
>> count = fprintf(fid,<format_string>,<var>,...)
<format>: %f, %d, %e, %s etc
general structure
Read data from an opened file with fscanf
>> [A,count] = fscanf(fid,<format_string>,<size>);
Read a line from an opened file with/without newline character with fgets/fgetl
>> tline = fgets(fid);
>> tline = fgetl(fid);
fprintf example
a = [ ];
fid = fopen('first_file.dat', 'w');
fprintf(fid, '%e ', a); / fprintf(fid, '%5.2f ', a);
fclose(fid)
fscanf example
fid = fopen('file_to_read.dat', 'r');
[m c] = fscanf(fid, '%f %f %f', [3 Inf]);
m = m'; m c
fgetl example
fid = fopen('file_with_text.txt', 'r');
tline = fgetl(fid); tline
while tline ~= -1
end
fclose(fid);
MSc in Mathematical and Computational Finance Introduction to MATLAB
Session 3

7. Various file types csv files
csv files – csvread, csvwrite
dlm files - dlmread, dlmwrite
textread
xls files – xlsfinfo, xlsread, xlswrite
MSc in Mathematical and Computational Finance Introduction to MATLAB
Session 3

8. Graphical objects Basic elements for displaying graphs
user interface (UI) components
Hierarchical structure
MSc in Mathematical and Computational Finance Introduction to MATLAB
Session 3

9. 2D/3D line plots plot(x, y, <line_spec_string>)
<line_spec>: 'r-*', 'b:^', etc (full specs here)
plot3(x, y, z, <line_spec>)
Logarithmic plots: semilogx/semilogy, loglog 2D
>> t = [0:2*pi/20:2*pi];
>> plot(t, sin(t), 'r:*'); 3D
>> plot3(t, sin(t), cos(t), 'r:*')
Combine plots
>> plot(t, sin(t), 'r:*', t, cos(t), 'b-d')
MSc in Mathematical and Computational Finance Introduction to MATLAB
Session 3

10. Customizing graphs Grids: grid on/off
Control axis: axis(<array_with_axis_limits>)
Legend: legend(<legend_matrix>)
Label axis:
xlabel(<label>), ylabel(<label>), ylabel(<label>)
Add title: title(<title_string>)
Add text: text(<x_pos>, <y_pos>, <text_str>)
Subplots: sublot(<n_rows>, <n_cols>, <card_no>)
>> plot(t, sin(t), 'r:*', t, cos(t), 'b-d'); grid on; axis tight;
>> xlabel('t'); ylabel('Function');
>> legend(['sin';'cos']);
>> title('Plot');
MSc in Mathematical and Computational Finance Introduction to MATLAB
Session 3

11. Examples of properties:
Figure properties
the handle: gives access to the object’s properties:
get(<handle_var>)
set(<handle_var>)
Examples of properties:
children: Children: [0x1 double]
colour: Color: [ ]
menu bar: MenuBar: 'figure'
name: Name: ''
number title: NumberTitle: 'on'
type: Type: 'figure'
MSc in Mathematical and Computational Finance Introduction to MATLAB
Session 3

12. 3D surfaces
For producin a 3D surface we specify the z coordinates above the (x,y) grid
Starting from data in arrays(x,y) we create the 2D grid with [X Y] = meshgrid(x, y)
3D graphs:
mesh(x,y,z) – wireframe plot
surf(z,y,z) – shaded surface
pcolor(x,y,z)
Utility functions
colormap
shading
colorbar
caxis
MSc in Mathematical and Computational Finance Introduction to MATLAB
Session 3