Programming in Matlab

1
Programming in Matlab

• TSRT01, Lecture 2

2
Outline

• The Matlab programming environment
• The Search Path
• startup file example
• m-file scripts and functions
• Flow Control statements
• Function functions
• A simulink example

3
The programming environment

• The working directory is controlled by
• gtgt dir
• gtgt cd catalogue
• gtgt pwd
• The matlab search path defines where matlab
  searches for m-files
• gtgt path
• gtgt addpath
• gtgt pathtool
• gtgt which function

4
The programming environment

• Executable m-files are located in the current
  working directory or somewhere on the search
  path.
• The function/variable duality causes Matlab to
  search for an identifier according to the scheme
• 1. variable in current workspace
• 2. built-in variable
• 3. built-in m-file
• 4. m-file in current directory
• 5. m-file on search path

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The programming language

• Script-files contain Matlab lines just as you
  type them inside Matlab

fact.m
YFACT(N) Factorial n!12...n y prod(1n)
Tip Start with a comment line To type the file
(script of function), do gtgt type fact
6
The programming language

• Function m-files have local variables and always
  start with function

fact.m
function yfact(n) YFACT(N) Factorial
n!12...n y prod(1n)
The first consecutive comment rows are displayed
by the command gtgt help fact
7
Flow control - selection

• The if-elseif-else construction

if ltrelational expressiongt ltcommandsgt elseif
ltrel. expr.gt ltcommandsgt else ltrel. expr.gt
ltcommandsgt end
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Flow control - selection

• Relational operators , , gt, lt, (and),
  (or), (not), xor.
• Logical tests isempty, any, all

if isempty(who) disp(Empty workspace) else
who end
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Flow control - selection

• The if-elseif-else construction

fact.m
function yfact(n) YFACT(N) Factorial
n!12...n if nargin lt 1 error(no input
assigned) elseif n lt 0 error(input must be
non-negative) elseif abs(n-round(n)) gt eps
error(input must be integer) end y prod(1n)
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Flow control - repetition

• for-loops

for kltvectorgt ltstatementsgt end
The commands are executed repeatedly and the
variable k is given each value in the loop-vector
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Flow control - repetition

• Display table of factorials

for k112 disp(num2str(k), ,...
num2str(prod(1k))) end
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Flow control - repetition

• Display table of factorials

function yfact(n) YFACT(N) Factorial
n!12...n y1 for k2n y yn end
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Repetition Animation demo

• Construct a script m-file that saves the movie,
  frame by frame, in the variable M

logomovie.m
logo dtheta 10 for k 1360/dtheta M(k)
getframe(gcf) camorbit(dtheta,0) end
Note Prior to Matlab 5.3 the movie needs to be
initated by the moviein command
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Animation demo

• gtgt movie(gcf,M)

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Flow Control - Cond. Repetition

• while-loops

while ltcond. statementgt ltstatementsgt end
The commands are executed repeatedly as long as
the conditional statement evaluates to true
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Flow Control - Cond. Repetition

• What is the largest factorial Matlab can compute?
  Compare the implementations

k1 While prod(1k)Inf, kk1 end k
for k1100000 if prod(1k)Inf, k, return,
end end
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Flow Control - Cond. Repetition

• Newtons method for solving nonlinear equations
• Solve the equation

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Flow Control - Cond. Repetition
newton.m
function x,n newton(x0,tol,nmax) X,N
NEWTON(X0,TOL,MAXIT) x x0 n 0 while (n lt
nmax) (abs(x-x0)gttoln0) n n 1 x0
x x x - (exp(-x)-sin(x))/...
(-exp(-x)-cos(x)) end

• gtgt x,nnewton(0,1e-3,10)

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Function functions

• General purpose functions take other m-files as
  input, e.g. solution of ODEs.
• Several methods are provided in Matlab for
  solving ordinary differential equations
• gtgt help ode45
• a Runge-Kutta method of medium order for
  non-stiff problems

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Example differential equation
General ODE This example is model of an
electronic circuit with a nonlinear vacuum tube,
called van den Pol equation.
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Example differential equation

• Construct an m-file function for the right-hand
  side of

fvdp.m
function dxdt fvdp(t,x) dxdt fvdp(t,x)
van der Pol equation dxdt x(2,) ...
(1-x(1,).2).x(2,)-x(1,)
Solve the ODE in the time span 0lttlt10 gtgt t,x
ode45(fvdp,0,10,20) gtgt plot(t,x) gtgt
plot(x(,1),x(,2))
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Customize startup

• Create a regular text file with extension .m,
  e.g. with the commando
• gtgt edit

startup.m
My startup file addpath(C\MATLABR11\work\proje
ct) disp(Welcome to Matlab)
Save this m-file somewhere on the search path, it
will execute automatically when Matlab is launched
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Summary

• User added m-files must be stored in the current
  directory or on the search path
• The functions have local variables while scripts
  operate on the global workspace
• Flow control is handled by if-statements,
  for-loops and while-loops
• General purpose functions take other functions as
  input argument