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Matlab for Scientific Programming A Brief Introduction Mark Levene Follow the links to learn more! Many features will be demonstrated What can we achieve in 3 hours? • Demonstration of why you may consider to use Matlab and for what types of tasks. • Tips on getting you started as a Matlab programmer. • You will need to practice Malab at home, and/or when doing a project for which Matlab may be suitable. Matlab Resources There are many!! • Matlab tutorials and learning resources • Attaway, MATLAB A Practical Introduction to Programming and Problem Solving, Second Edition, Elsevier, 2012 (Introductory). • Banches, Text Mining with Matlab, Springer, 2012 (Intermediate). • Martinez et al., Exploratory Data Analysis with Matlab, Second Edition, CRC Press, 2011(Advanced). • Matlab claims over 1 million users world wide in 2012!! • There are many Matlab books. • Also see Matlab documentation centre. Why Matlab? Introductory Example • Example5_2 from Martinez – load iris • 3 classes of Iris to be distinguished by – sepal length, sepal width – petal length, petal width • Look at the “data” • Briefly discuss the kmeans algorithm for grouping data into k groups (here k=3); demonstrate help in Matlab What can you do in Matlab? • The normal things you can do in any other programming languages, but is interpreted and not strict in its typing to allow quick prototyping. • Has many built-in features to handle matrices, maths & stats, data analysis and plotting. • Has a wide range of toolboxes such as curve fitting, neural networks, bioinformatics, symbolic maths and finance. • Although Matlab is proprietary there are many open source toolboxes; see Matlab central. A simple function in Matlab function area = conearea(radius, height) area = pi/3 * radius^2 * height; end conearea(4,6.1) ans = 102.2065 • Matlab has the usual control flow that other languages have – use help when needed !! Vectorised code • Chapter 5 in Attaway for i=1:10 v(i) = i; end %create a vector v = 1:1:10 %start=1, increment=1,end=10 for i=1:10 w(i) = w(i)^2; end %standard loop w=w.^2; %vectorised code • Can use any other vector operations! w=log(v); %a vector can be an argument • Can query a vector using find: find(w>5); %returns indices satisfying condition Matrices and Linear Algebra • Chapter 12 in Attaway • Can also vectorise (try it out yourselves!) m = rand(3,3); %create a random 3x3 matrix • Matrix operations work as expected! n = m.*m %Matrix multiplication • You will need to revise your linear algebra to make use of these Matlab features. Example SVD and PCA (Eigenvector decomposition) • For example Singular Value Decomposition (SVD) – Look this up for maths details, • SVD has applications in many areas including Information Retrieval is easy in Matlab. m = rand(100,10); svd(m); • Principal Components Analysis (PCA) is a special case of SVD measuring the directions along which the variance is maximised. load filteredyeastdata; mapcaplot(yeastvalues, genes); Basic Statistics • Chapter 13 in Attaway x= [8 9 3; 10 2 3; 6 10 9]; mean(x) ans = 8 7 5 var(x) ans = 4 19 12 std(x) ans = 2.0000 4.3589 3.4641 • y = [9 10 10 9 8 7 3 10 9 8 5 10]; mode(y) ans = 10 median(y) ans = 9 • There is much more in the statistics toolbox Curve fitting • • • • • • • Example7_3 in Martinez Demonstrate cftool with (x,x) and (x,y) Example 9_2 in Martinez Demonstrate histogram Demonstrate cftool with (xk,nuk) normfit = [33,40,42,41,39,32]; linfit = [2,44,49,61,82,95] Maths • Chapter 15 in Attaway • Symbolic maths syms x y f = x^2 + y^2 + 2*x*y simplify(f) ans = (x + y)^2 expand(ans) f ezplot(x^2+2*x+2) % plot the function Solving equations solve(2*x^2+x-6) ans = -2 3/2 syms x a b c solve(a*x^2+b*x+c) ans = -(b + (b^2 - 4*a*c)^(1/2))/(2*a) -(b - (b^2 - 4*a*c)^(1/2))/(2*a) • Can try and solve more complex equations solve(exp(x)-3) ans = log(3) Calculus syms x diff(x^3,x) ans = 3*x^2 %differentiation int(3*x^2) ans = x^3 %integration • Can do much more, including solving differential equations. • Do not worry about the Maths as such, as in Matlab we pick up the tools as and when we need them. Summary • Matlab provides and easy-to-use, state-of-theart environment for scientific computing. • There are a wide variety of toolboxes for different applications, many of which are open source. • Matlab may not always be the most efficient solution but it is great for quick prototyping. • Matlab is not designed for general purpose programming, although it is a complete language.