提高Matlab代码效率(improveMatlabPerformance)

1、Matlab code performanceMatlab code performance肖昌炎肖昌炎湖南大学湖南大学 自动化系自动化系Part I: Techniques for Improving PerformancePart I: Techniques for Improving Performance Preallocating Arraysticx = 0;for k = 2:1000000 x(k) = x(k-1) + 5;endtocElapsed time is 0.301528 seconds.ticx = zeros(1, 1000000);for k = 2:100

2、0000 x(k) = x(k-1) + 5;endtocElapsed time is 0.011938 seconds.Use the appropriate preallocation function for the kind of array you want to initialize: zeros for numeric arrays cell for character arraysPreallocating a Nondouble Matrix A = int8(zeros(100); A = zeros(100, int8); Assigning VariablesIf y

3、ou need to store data of a different type, it is advisable to create a new variable. Changing the class or array shape of an existing variable slows MATLAB down because it takes extra time to process.You should not assign a real value to a variable that already holds a complex value, and vice versa.

4、 Assigning a complex number to a variable that already holds a real number impacts the performance of your program.X = 23; .- other code - .X = A; % X changed from type double to char .- other code - Using Appropriate Logical OperatorsOperator Description&, | Perform logical AND and OR on arrays

5、 element by element&, | Perform logical AND and OR on scalar values with short-circuitingIn if and while statements, it is more efficient to use the short-circuit operators, & and |:if (isnumeric(varargin1) & (ischar(varargin2)Additional Tips on Improving PerformanceAdditional Tips on Im

6、proving Performance Split large script files into smaller ones, having the first file call the second if necessary. Construct separate functions (or local functions and nested functions) from the larger chunks of code. If you have overly complicated functions or expressions, use simpler ones to redu

7、ce size. Simpler functions often make good utility functions that you can share with others. Use functions instead of scripts because they are generally faster. Vectorize your code to take advantage of the full functionality of MATLAB. Use a sparse matrix structure to vectorize code without requirin

8、g large amounts of memory. Avoid running large processes in the background while executing your program in MATLAB. Avoid overloading MATLAB built-in functions on any standard MATLAB data classes because this can negatively affect performance.Part II: VectorizationPart II: Vectorization Using Vectori

9、zation Indexing Methods for Vectorization Array Operations Logical Array Operations Matrix Operations Ordering, Setting, and Counting Operations Functions Commonly Used in VectorizingUsing VectorizationUsing Vectorizationi = 0;for t = 0:.01:10 i = i + 1; y(i) = sin(t);End=t = 0:.01:10;y = sin(t);%co

10、mputes the cumulative sum of a vector at every fifth element:x = 1:10000;ylength = (length(x) - mod(length(x),5)/5;y(1:ylength) = 0;for n= 5:5:length(x) y(n/5) = sum(x(1:n);End=x = 1:10000;xsums = cumsum(x);y = xsums(5:5:length(x);Indexing Methods for VectorizationIndexing Methods for Vectorization

11、Subscripted Indexing%1D arrayA = 6:10;A(3 5)%2D arrayA = 11 12 13; 14 15 16; 17 18 19A(2:3,2:3) Linear IndexingA = 11 12 13; 14 15 16; 17 18 19;A(6)A(3,1,8)A(3;1;8)Note: Use the functions sub2ind and ind2sub to convert between subscripted and linear indices. Logical IndexingMATLAB selects elements o

12、f A that contain a 1 in the corresponding position of the logical matrix:A = 11 12 13; 14 15 16; 17 18 19;A(logical(0 0 1; 0 1 0; 1 1 1)Array OperationsArray Operationsfor n = 1:10000 V(n) = 1/12*pi*(D(n)2)*H(n);End% Vectorized CalculationV = 1/12*pi*(D.2).*H;Logical Array OperationsLogical Array Op

13、erationsD = -0.2 1.0 1.5 3.0 -1.0 4.2 3.14;%You can directly exploit the logical indexing power of MATLAB to select the valid cone volumesVgood = V(D = 0);%MATLAB can compare two vectors of the same size, allowing you to impose further restrictions.V(V = 0) & (D H)Matrix OperationsMatrix Operati

14、onsx = -2:0.2:2;y = -1.5:0.2:1.5;X,Y = meshgrid(x,y);F = X.*exp(-X.2-Y.2);=%Constructing MatricesA = ones(5,5)*10;v = 1:5;A = repmat(v,3,1)A = repmat(1:3,5,2)B = repmat(1 2; 3 4,2,2)A = 97 89 84; 95 82 92; 64 80 99;76 77 67;. 88 59 74; 78 66 87; 55 93 85;dev = bsxfun(minus,A,mean(A)Ordering, Setting

15、, and Counting OperationsOrdering, Setting, and Counting Operations Eliminating Redundant Elementsx = 2 1 2 2 3 1 3 2 1 3;x = sort(x);difference = diff(x,NaN);y = x(difference=0)=y=unique(x); Counting Elements in a Vectorx = 2 1 2 2 3 1 3 2 1 3;x = sort(x);difference = diff(x,max(x)+1);count = diff(

16、find(1,difference)y = x(find(difference)count_nans = sum(isnan(x(:);count_infs = sum(isinf(x(:);Functions Commonly Used in VectorizingFunctions Commonly Used in VectorizingallTest to determine if all elements are nonzeroanyTest for any nonzeroscumsumFind cumulative sumdiffFind differences and approx

17、imate derivativesfindfind indices and values of nonzero elementsind2sub Convert from linear index to subscriptsipermuteInverse permute dimensions of a multidimensional arraylogical Convert numeric values to logicalmeshgridGenerate X and Y arrays for 3-D plotsndgrid Generate arrays for multidimension

18、al functions and interpolationspermuteRearrange dimensions of a multidimensional arrayprodFind product of array elementsrepmat Replicate and tile an arrayReshape Change the shape of an arrayshiftdimShift array dimensionssortSort array elements in ascending or descending ordersqueezeRemove singleton

19、dimensions from an arraysub2ind Convert from subscripts to linear indexsumfind the sum of array elementsPart III Matrix Indexing in MATLABPart III Matrix Indexing in MATLAB Indexing Vectorsv = 16 5 9 4 2 11 7 14;v(3) % Extract the third elementv(1 5 6) % Extract the first, fifth, and sixth elementsv

20、(3:7) % Extract the third through the seventh elementsv2 = v(5:8 1:4) % Extract and swap the halves of vV(end) % Extract the last elementv(2:end-1) % Extract the second through the next-to-last elements Indexing Matrices with Two SubscriptsA = magic(4) A(2,4) % Extract the element in row 2, column 4

21、A(2:4,1:2) A(3,:) % Extract third row A(:,end) % Extract last column Linear Indexing A(6 12 15) idx = sub2ind(size(A), 2 3 4, 1 2 4) If I want to shift the rows of an m-by-n matrix A by k places, I use A(:,n-k+1:n 1:n-k). But what if k is a function of the row number? That is, what if k is a vector of length m? Is there a quick and easy way to do this?% index vectors for rows and columns p = 1:m; q = 1:n; % index matrices