MATLAB Minicourse:
Lecture 3


David S. Touretzky


March 2001
Multidimensional Arrays

MATLAB supports arrays with more than two dimensions.

m = rand(4,3,2)

whos m
size(m)

Matrix multiplication does not apply to higher dimensional arrays.

Array concatenation can't be done with [ ] notation for more than 2 dimensions, so use cat(dim,A,B) instead.

cat(1,m,m)
cat(2,m,m)
cat(3,m,m)
Sparse Matrices
Sparse matrices provide an efficent means to store matrices that are mostly empty.


   s = sparse(1000,1000)

   s(2,11) = 3.5
   s(875,992) = 47
   s(992,875) = 2

   whos s


All arithmetic operations work on sparse as well as full matrices.

   s + 10

   s'*s
Cell Arrays
Ordinary arrays contain elements of a single type, such as doubles or chars.

Cell arrays contain objects of heterogeneous types.  

     b = {1 'two' [3 3 3]}

Cell arrays offer greater flexibility.

Drawback:  cell arrays require much more storage, because each element requires a separate pointer and type information.

     ra = [1 2 3 4];
     ca = {1 2 3 4};
     whos ra ca

     ra(2:3)
     ca{2:3}
Structure Arrays

MATLAB supports "structure arrays" with named fields.

    a.name = 'John Smith'
    a.age = 35
    a.department = 'Accounting'
    whos a

The array a is a scalar (1x1) structure array.

    f = fieldnames(a)
    f{1}

The what and get functions return structures.

    w = what
    w.m

    p = get(gcf)
Structure Arrays (cont.)

Structure arrays can contain more than one element.

All elements have the same set of field names; some fields may be empty.


   a(2).name = 'Mary Brown'
   a(2).seniority = 8


   whos a

   a(1)

   a(2)
Call-by-Value Semantics
MATLAB uses call-by-value semantics, making it impossible for functions to modify their arguments.

In C, integers and reals are passed by value, but arrays are passed by reference, making them modifiable.

The following doesn't work:

birthday(a(1))

a(1)
Return Modified Structures
Modified arrays or structures must be returned, and assigned back to the original variable.

Otherwise the modifications are lost.


   a(1)

   a(1) = birthday(a(1));

   a(1)

Efficiency Considerations

When an array or structure is passed as an argument, MATLAB doesn't necessarily copy it.

Objects are passed to functions by reference, but are copied if the function modifies the argument.

The modify-and-return approach is not an efficient way to maintain large objects, due to excessive copying.

Alternative solution: store values in a global variable and let functions modify that
variable directly.
Property Arguments
The plot function and many other graphics
functions accept property/value pairs as extra arguments.


clf, hold on

plot(rand(5,1), 'Color', [0 0.5 0.8])

plot(rand(5,1),'r','LineWidth',8, ...
         'LineStyle','--')

plot(rand(5,1),'Color', [0.7 0.9 0.2], ...
        'LineWidth', 6, 'LineStyle','^')
GUI Facility

UIcontrol objects include pusbuttons, sliders, pop-up menus, and radio buttons.


clf

hb = uicontrol('Style','pushbutton')

set(hb, 'String', 'Foo')

set(hb, 'BackgroundColor', [0.2 0.6 1])

set(hb, 'Callback', 'datestr(now)')

set(hb, 'Style', 'radiobutton')
Units Property
The Units property controls whether certain subsequent properties are interpreted as 
pixels, points, or percentage of screen size ("normalized" units).


set(hb,'Units','pixels', ...     
      'Position',[100 100 80 25])

set(hb,'Units','normalized', ...
      'Position',[0.5 0.5  0.25 0.25])

Pop-up Menus and List Boxes

hp = uicontrol('Style','popup', ...
  'String',{'eeny','meeny','miney','moe'}, ...
  'Units','normalized', ...
  'Position',[0.2 0.2 0.3 0.1], ...
  'BackgroundColor', [0.8 0.8 0.5], ...
  'ForegroundColor',[0.5 0.5 0.2])

set(hp,'Callback','get(gcbo,''Value'')')


The gcbo function returns the object whose callback function is currently executing.


set(hp,'Position',[0.2  0.2  0.3  0.2])

set(hp,'Style','listbox')
Sliders

hs = uicontrol('Style','slider', ...
          'Position', [200 200 150 20], ...
          'Callback','get(gcbo,''value'')')


set(hs,'CallBack', ...
  'set(gcf,''Color'',[0 0 get(gcbo,''Value'')])')


In practice, the Callback string is usually a call to some user-written function with gcbo as the argument.

All the work is done inside the function.
GUIDE
GUIDE is the GUI Design Environment.

Allows interactive layout of a GUI window, including menus, graphics, text boxes, etc.


  help guide


Creates a .fig file to store layout information.

Creates an editable .m file to load the .fig file and hold associated callback routines.


Image Data

clear all

load durer

whos

image(X)

colormap(map)

axis image

axis off

set(gca, 'Position', [0 0 1 1])

colormap(autumn)

brighten(0.5)
Reading Image Files

  imfinfo('/afs/cs/usr/dst/brain.jpg')

  brain = imread('/afs/cs/usr/dst/brain.jpg');
  whos brain

The uint8 datatype  holds unsigned bytes.

  image(brain)

  colormap(bone)

  axis image

  colormap(bone(256))

  zoom on
Mouse Input

   getline('closed')

Click the left button to enter points.  Click the right mouse button to end.  Return value is a matrix of points defining the polygon.


   p1 = getptr(gcf)

   setptr(gcf,'hand')

   p2 = getptr(gcf)

   set(gcf,p1{:})

The p{:} notation expands the contents of the cell array p into multiple arguments to set.

Image Manipulation

Toolboxes
Toolboxes contain collections of related 
functions that extend the basic MATLAB language.

The matlab toolbox contains a variety of libraries that implement MATLAB features such as graphing and matrix functions.

    help graph2d
    help matfun

The images and stats toolboxes contain 
routines for image processing and statistical calculation.

The ver command displays version information for all the toolboxes currently installed.

    ver

    Search Path

You can control the search path MATLAB uses to find functions and data files.

    path


    addpath('/afs/cs/usr/dst/bump')


    what bump


    bump
Thinking in MATLAB
pts = 0:pi/160:12*pi;

data = sin(pts) + sin(0.3*pts) + cos(3*pts);

clf, hold on, plot(data)

thresh = mean(data) + var(data);
b = data > thresh;

bstart = ~b(1:end-1) & b(2:end);
bend = b(1:end-1) & ~b(2:end);

pstart = find(bstart);
pend = 1+find(bend);

xcoords = [pstart; pend; NaN*pstart];
ycoords = repmat(thresh,size(xcoords));

plot(xcoords(:),ycoords(:),'r','Linewidth',2)

function birthday(emp)
    emp.age = emp.age + 1

function emp = birthday(emp)
   emp.age = emp.age + 1;

function bmap(im)
  clf, zoom on
  image(im),   axis image, axis off
  d = double(im);

  m = [bone(256); autumn(50)];
  colormap(m)

  coords = getline('closed');
  [x,y] = meshgrid(1:size(im,1), ...                  1:size(im,2));
  z = inpolygon(x,y, ...
             coords(:,1), coords(:,2));
  d(z) = d(z) + 50;

  image(d), axis image, axis off