Matlab Programming Tips and Tricks

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Matlab Programming Tips and Tricks

• Samuel Cheng
• University of Oklahoma-Tulsa

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Myth

• Matlab is slow
• Think again!
• Matlab is extremely fast for matrix computation
• Matlab is an encapsulation of highly optimized
  Lapack and Blas numerical library

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Experiment 1

• Matrix multiplication for matrices of size 500 ?
  500
• C 4.08 sec
• Matlab 0.53 sec

size500 for (i0iltsizei) for
(j0jltsizej) for (k0kltsizek)
cijaikbkj
Dont expect your tiny C code will beat Matlab
for matrix operations! Matlab is slow only when
you make it so!
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Flow Control

• Matlab is a fully power programming language with
  flow control capability of other high level
  languages
• If-then-else
• for-end
• while-end
• try-catch-end

In most cases, you dont need them at all if you
do things right!
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Matlab

• Matlab is not Math lab
• It is Matrix lab
• Think everything in matrix

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Vectors and Matrices

• Row vectors
• Column vectors and transpose
• Matrices

gtgt X1,3,5,7,9
gtgt X13579 gtgt X1,3,5,7,9
gtgt Xzeros(2) X0,00,0 gtgt Xones(2)
X1,11,1 gtgt Xeye(2) X1,00,1
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Colon
gtgt X15 X1 2 3 4 5
Comment sign
gtgt X00.31 X0 0.3 0.6 0.9
gtgt Xones(2) gtgt X() ans 1 1 1 1
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Colon
gtgt X15 X1 2 3 4 5
Comment sign
gtgt X00.31 X0 0.3 0.6 0.9
gtgt Xones(2) gtgt X() ans 1 1 1 1
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Dot Products

• Every operation in Matlab is matrix operation by
  default
• Non-matrix operation is generalized to matrix
  operation as element-wise operation (sin, cos,
  etc.)
• To convert matrix operation to element-wise
  operation, add dot in front (.,.)

gtgt Aones(2) A.2 ans 1 1 1 1
gtgt Aones(2) A2 ans 2 2 2 2
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Most Important Tip

• Avoid loops by all means
• E.g. ysin(x), x0.1,0.2,,2pi
• Stupid code
• Better

for i12pi10

x0.1i y(i)sin(x) end
x0.10.12pi ysin(x)
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ndgrid

• Bad
• Much better

gtgt xid1yid1 gtgt for x-100.110 for
y-100.110 z(xid,yid)x3y2 yidyid1
end xidxid1 end
gtgt x,yndgrid(-100.110, -100.110) zx.3
y.2
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Batch File

• Batch file in Matlab is simple. Simply put your
  code in a file with extension .m
• Every symbols will be visible in the current
  workspace
• Good for initial development

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function

• Functions are exactly the same as batch files
  except they hide all variables except those you
  return

• Dont assume dimensions of your inputs (dont
  assume your input to be a column or row vector)
• Break your function into meaningful functions (a
  well-design program needs very few comments)
• Use functions whenever possible (instead of batch)

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function
gtgt type isprime1.m function boolisprime1(n) bool
1 for i2floor(sqrt(n)) if mod(n,i)0 bool
0 break end end gtgt boolisprime1(n) gtgt i ???
Undefined function or variable i.

• Exercise can you write the function above in 1
  line?

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Answer
gtgt type isprime2.m function boolisprime2(n) bool
all(mod(nones(floor(sqrt(n))-1,1)',
2floor(sqrt(n))))
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function pointer
gtgt add_at_(x,y) xy add _at_(x,y)xy gtgt
add(1,2) ans 3 gtgt addone_at_(x) add(1,x) gtgt
addone(3) ans 4
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size / length
gtgt Xones(4,5) gtgt size(X) ans 4 5 gtgt
size(X,2) ans 5 gtgt size(X()) ans 20 1 gtgt
length(X()) ans 20
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size / length
gtgt type better.m Ximread(mypic.png) nXsize(X
,1) nYsize(X,2) process(X,nX,nY)
gtgt type horrible.m Ximread(mypic.png) nX320
nY200 process(X,nX,nY)
gtgt type best.m Ximread(mypic.png) process_new
(X)
Dont put fixed values all over your codes! Ill
never change their values is not an excuse
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find

• E.g. thresholding
• Bad
• Better

function xthreshold(x,th) for
xid1length(x) if abs(x(xid))ltth x(xid)0 en
d

• 1-line

function xthreshold(x,th) indfind(abs(x)ltth)
x(ind)0
x(abs(x)ltth)0
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exist
gtgt type threshold.m function xthreshold(x,th) if
exist(th,var) th1 end x(abs(x)ltth)0
Use exist to assign default variables for your
functions
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sprintf and eval
gtgt type bad.m Process(b1) Process(b2) Process(b
3) Process(b4) Process(b5)
General rule Write codes with low entropy
(deterministic)
gtgt type good.m for id15 eval(sprintf(Process(
bd),id)) end
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cell

• Cell arrays/matrices can store mixed content for
  each component

gtgt a1,12,a a 1 1x2 double a gtgt
a(2) ans 1x2 double gtgt a2 ans 1 2
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File I/O

• fopen
• fread
• fwrite
• fprintf
• save/load

More or less same as C
gtgt save tmp a b c gtgt load tmp gtgt save tmp2 a
-ascii
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Debugging

• Use dbstop to initiate debugger
• Use dbclear all to erase all stop points
• Shortcuts
• F10 step over
• F11 step in
• Shift-F11 run to next stop point
• F12 toggle stop point

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try/catch
gtgt xrand(1,99) gtgt for i1100
y(i)x(i)2 end ??? Index exceeds matrix
dimensions.
gtgt for i1100 try y(i)x(i)2 catch i
end end i 100
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Visualization Functions

• ezplot
• plot
• plot3
• mesh
• surf
• See help document

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Most Common FAQs for Figures

• Dont show axis

gtgt axis off

• Change axis ranges

gtgt axis(xstart xend ystart yend)

• Change axis font size

Click axis gtgt set(gca,fontsize,fontsize)

• Use Latex

gtgt title(\fracxy,interpreter,latex)
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GUI

• Help guide

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Interface with C

• Matlab can be slow with non-matrix operation
• Look for internal function in those cases
• Speed up by interfacing with C

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Profiling

• Using tic/toc
• Matlab has its own profiling tools
• help profile

gtgt Arand(500) gtgt tic AA toc Elapsed time
is 0.329000 seconds.
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Interface with OS

• Write out file
• Call command line excutable
• Read file

gtgt type interface.m function Imoutinterface(size
,parameter) Imgenerate_img(size) imwrite(Im,tmp
.png) system(sprintf(process tmp.png
d,parameter)) Imoutimread(tmp.png)
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Interface with Java
gtgt type SC.java public class SC public double
add(double a,double b) return ab
Compile source and create JAR file
gtgt !javac SC.java gtgt !jar cvf skeleton.jar
SC.class added manifest adding SC.class(in
250) (out 194)(deflated 22)
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Interface with Java
Import class
gtgt javaaddpath ./skeleton.jar gtgt import SC
Create and use object
gtgt a SC a Skeleton_at_32b427c1 gtgt
a.add(1,2) ans 3
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Interface with Java

• Always remember to set functions/members to
  public if you intent to access them through
  Matlab
• You would like to clear and re-import (and also
  remove and re-add path) jar file if you make
  changes to your java source
• If you do not use default package (for example,
  use package p), you should perform import p.SC
  instead of import SC

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Interface with C using Mex
gtgt type mex_skeleton.c include "mex.h" void
mexFunction(int nlhs, mxArray plhs, int nrhs,
const mxArray prhs) double y,x1,x2 int
nrows,ncols nrows mxGetM(prhs0) ncols
mxGetN(prhs0) plhs0
mxCreateDoubleMatrix(nrows,ncols,mxREAL) x1
mxGetPr(prhs0) x2 mxGetPr(prhs1) y
mxGetPr(plhs0) process(x1,x2,y)
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Interface with C using Mex
gtgt mex mex_skeleton.c gtgt x1rand(1,10) gtgt
x2rand(1,10) gtgt y mex_skeleton(x1,x2)
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Conclusions

• Matlab can be fast in terms of both development
  and actual running, an extremely good tool for
  prototyping
• Avoid loop in all means
• Getting help
• Use help often
• Google matlab command
• Email me or knock my door (try afternoon and
  evening)