Introduction To Matlab Class 3

1
Introduction To MatlabClass 3

• Instructors Hristiyan (Chris) Kourtev
• and Xiaotao Su, PhD

2
Variables

• Integers m 5 5
• Doubles (Floating pt) n 7.382
• Character strings c1 beep b, e,
  e, p c2 4
• Arrays of numbers arr1 4, 5, 8, m arr2
  m, n, 5.6, 0
• Arrays of strings str1 c1 blob same
  dimen.
• Concatenating arrays of numbers arr3 arr1,
  arr2
• Concatenating strings str2 c1,c2
• Matrices mat1 4, 5 6, 7 mat2 arr1
  arr2 same dimen.
• Cell Arrays (later on)

3
Boolean Expressions

• Boolean operands
• Boolean expressions either return 1 for true
  e.g. 5 5 or 0 for false e.g. 5 gt 9
• Put expressions in parentheses so they get
  evaluated firste.g. 0 (4lt5)

gt gt lt lt
equals not equal greater than greater than or equal to less than greater than or equal to and or
4
Loops (for and while)

• For loopfor index fromto do
  somethingend
• While loopwhile(condition) do something
  change something that affects value of
  conditionend

5
Loops (for and while) -- examples

• max_loops 5
• for index 1max_loops
• disp(index)
• end
• counter 1
• while(counter lt max_loops)disp(counter)
• counter counter 1
• end

nested loop example for k 1max_loops disp(k1
) for m 13 disp(m) end disp(k2) en
d outputs k1 m m m k2 k1 m m m k2 k1 mmm
k2 k1 m m m k2 k1 mmm k2
6
Commonly used functions

• rand - generates a random decimal number between
  0 and 1e.g. 0.3456 or 0.9993 or 0.0013 etc
• ceil(num) returns the next integer bigger than
  the inpute.g. ceil(5.56) ?6 or ceil(2.1) ? 3 or
  ceil(6) ? 6
• floor(num) returns the next integer, smaller
  than the inpute.g. floor(0.9) ? 0 or floor(-0.1)
  ? -1
• To generate a random number between 0 20
  ceil(rand20)

7
Commonly used functions -- continued

• m 1, 2, 3, 4 n 1, 2, 3, 4 5, 6, 7, 8 k
  9 8 0
• length(mat) returns the length of a vector or a
  matrixe.g. length(m) ? 4, length(n) ? 4,
  lenth(k) ? 3
• size(mat,dim) returns all the dimensions of a
  matrix/vectore.g. size(m) ? 1, 4, size(n) ?
  2, 4, size(k) ? 3, 1, size (n, 2) ? 4

8
Multiple Input/Output Functions

• Functions can have more than one input and more
  than one outpute.g. s size(mat, dim)
• Storing returned values in 2 or more separate
  variablese.g. x, y size(mat)
• Storing returned values in a vector/cell
  arraye.g. vals size(mat)

9
Collecting User Input Using it

• Take input from keyboardnum1input('what is the
  first number?')
• Validation checks - isstr(var)- isnum(var)
• Converting from strings to numbers and back-
  num2str(var)- str2num(var)

10
Screen drawing and psychtoolbox

• Screen Geometry

Origin
Coordinates are measured in
pixels.
X ----------- Positive -gt
Y
Positive
Max X and Y
11
Basic Display Loop in PsychToolbox (PTB)

• Open Window
• while some condition is true do the following
• draw something
• make some changes
• repeat
• Close screen

12
Basic Display Loop in PTB (code)

• draw_stuff.m
• Draws stuff on the screen
• clear all
• try
• which_screen0
• bg_color 0, 0, 0
• window_ptr, screen_dimensions
• Screen(which_screen, 'OpenWindow',bg_color)
  
• shape_dimensions screen_dimensions/2
• tic

while (toc lt 5) move shape
shape_dimensions shape_dimensions
0,5,0,5 draw shape
Screen(window_ptr, 'FillRect', 0,0,255,
shape_dimensions) flip buffer and
repeat Screen(Flip, window_ptr) end cl
ear Screen catch clear Screen end
13
Moving Shapes around the Screen

• Shapes are defined by an array of numbers e.g.
  sh1 10, 15, 40, 45 x1, y1, x2, y2
• To move a shape in the x direction by 5 pixels we
  need to increment both x coordinates by 5e.g.
  sh1(1) sh1(1) 5 sh1(3) sh1(3) 5 or use
  another array sh1 sh1 5, 0, 5, 0
• Same for moving in the y direction
• Increment both if you want to move diagonally

14
Task 1 Mousing Around

• Draw a green circle of radius 30 pixels, whose
  location on the screen is controlled by the mouse
• Change the color of the circle to red when a
  button is clicked and back to green when released
• Hints You will need to use the following
  functions
• Get input from the mousemouse_x, mouse_y,
  buttons GetMouse(window_ptr)
• Hide and show the windows cursorHideCursor,
  ShowCursor
• Draw a circleScreen(window, 'FillOval',
  dot_color, cursor_dim)
• If sum(buttons)gt0, a button has been clicked

15

• mousingAround.m
• clear all
• try
• which_screen0
• bg_color 0, 0, 0
• window_ptr, screen_dimensions
• Screen(which_screen, 'OpenWindow',
  bg_color)
• dot_r 20 radius of cursor
• HideCursor
• green 0, 255, 0
• red 255,0, 0
• tic

while(toclt5) mouse_x, mouse_y, buttons
GetMouse if(sum(buttons)gt0) dot_color
red else dot_color green end cursor
mouse_x-dot_r, mouse_y-dot_r, ...
mouse_xdot_r, mouse_ydot_r Screen(window_pt
r, 'FillOval', dot_color, cursor) Screen('Flip',
window_ptr) end clear Screen ShowCursor catch
clear Screen ShowCursor end
16
Skip Sync Tests

• Add to your program before opening the new
  screenScreen('Preference','SkipSyncTests', 1)

17
Direction/Velocity Vectors

• A vector has both magnitude and direction
• Direction x,y
• Magnitude v v(x2y2) sqrt(x2 y2)
  Pythagorean

x
v
y
x
18
Detecting Collisions With Borders

• rect1 10, 10, 50, 50 x1, y1, x2, y2
• screen_dimensions 0, 0, 1280, 1024
• Collision w/ top border if rect1(2) lt
  screen_dimensions(2)
• Collision w/ left border if rect1(1) lt
  screen_dimensions(1)
• Collision w/ bottom border if rect1(4) gt
  screen_dimensions(4)
• Collision w/ right border if rect1(3) gt
  screen_dimensions(2)

19
Task 2 Bouncing off the walls

• Modify your existing program to add 2 blue
  squares that start off in a random direction at
  speed 5 and 7 pixels respectively and bounce off
  the walls
• Hints
• Will need to use direction vectors
• Will need to use the Pythagorean theorem to
  calculate the direction vectors
• If a square bumps into the left or right walls,
  invert (multiply by -1) the x component of its
  velocity vector
• If a square bumps into the left or right walls,
  invert (multiply by -1) the y component of its
  velocity vector

20
Task 3 Click, Click

• Modify your program to
• Make the squares clickable. They should change
  color to purple when clicked.
• Hint Purple 255,0,255

21
if((mouse_xgtshape1_rect(1))...
(mouse_xlt shape1_rect(3))...
(mouse_ygt shape1_rect(2))...
(mouse_ylt shape1_rect(4)))
shape1_color purple end
if((mouse_xgtshape2_rect(1))...
(mouse_xltshape2_rect(3))...
(mouse_ygtshape2_rect(2))...
(mouse_yltshape2_rect(4)))
shape2_color purple end