MIT OpenCourseWare http://ocw.mit.edu 18.085 Computational Science and Engineering IFall 2008 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.A Matlab Cheat-sheet (MIT 18.06, Fall 2007) Basics: save 'file.mat' save variables to file.mat load 'file.mat' load variables from file.mat diary on record input/output to file diary diary off stop recording whos list all variables currenly defined clear delete/undefine all variables help command quick help on a given command doc command extensive help on a given command Defining/changing variables: x = 3 define variable x to be 3 x = [1 2 3] set x to the 1×3 row-vector (1,2,3) x = [1 2 3]; same, but don't echo x to output x = [1;2;3] set x to the 3×1 column-vector (1,2,3) A = [1 2 3 4;5 6 7 8;9 10 11 12]; set A to the 3×4 matrix with rows 1,2,3,4 etc. x(2) = 7 change x from (1,2,3) to (1,7,3) A(2,1) = 0 change A2,1 from 5 to 0 Arithmetic and functions of numbers: 3*4, 7+4, 2-6 8/3 multiply, add, subtract, and divide numbers 3^7, 3^(8+2i) compute 3 to the 7th power, or 3 to the 8+2i power sqrt(-5) compute the square root of –5 exp(12) compute e12 log(3), log10(100) compute the natural log (ln) and base-10 log (log10) abs(-5) compute the absolute value |–5| sin(5*pi/3) compute the sine of 5π/3 besselj(2,6) compute the Bessel function J2(6) Arithmetic and functions of vectors and matrices: x * 3 multiply every element of x by 3 x + 2 add 2 to every element of x x + y element-wise addition of two vectors x and y A * y product of a matrix A and a vector y A * B product of two matrices A and B x * y not allowed if x and y are two column vectors! x .* y element-wise product of vectors x and y A^3 the square matrix A to the 3rd power x^3 not allowed if x is not a square matrix! x.^3 every element of x is taken to the 3rd power cos(x) the cosine of every element of x abs(A) the absolute value of every element of A exp(A) e to the power of every element of A sqrt(A) the square root of every element of A expm(A) the matrix exponential eA sqrtm(A) the matrix whose square is A Transposes and dot products:x.', A.' the transposes of x and A Constructing a few simple matrices:rand(12,4) a 12×4 matrix with uniform random numbers in [0,1) randn(12,4) a 12×4 matrix with Gaussian random (center 0, variance 1) zeros(12,4) a 12×4 matrix of zeros ones(12,4) a 12×4 matrix of ones eye(5) a 5×5 identity matrix I (“eye”) eye(12,4) a 12×4 matrix whose first 4 rows are the 4×4 identity linspace(1.2,4.7,100) row vector of 100 equally-spaced numbers from 1.2 to 4.7 7:15 row vector of 7,8,9,…,14,15 diag(x) matrix whose diagonal is the entries of x (and other elements = 0) Portions of matrices and vectors: x(2:12) the 2nd to the 12th elements of x x(2:end) the 2nd to the last elements of x x(1:3:end) every third element of x, from 1st to the last x(:) all the elements of x A(5,:) the row vector of every element in the 5th row of A A(5,1:3) the row vector of the first 3 elements in the 5th row of A A(:,2) the column vector of every element in the 2nd column of A diag(A) column vector of the diagonal elements of A Solving linear equations:A \ b for A a matrix and b a column vector, the solution x to Ax=b inv(A) the inverse matrix A–1 [L,U,P] = lu(A) the LU factorization PA=LU eig(A) the eigenvalues of A [V,D] = eig(A) the columns of V are the eigenvectors of A, and the diagonals diag(D) are the eigenvalues of A Plotting:plot(y) plot y as the y axis, with 1,2,3,… as the x axis plot(x,y) plot y versus x (must have same length) plot(x,A) plot columns of A versus x (must have same # rows) loglog(x,y) plot y versus x on a log-log scale semilogx(x,y) plot y versus x with x on a log scale semilogy(x,y) plot y versus x with y on a log scale fplot(@(x) …expression…,[a,b])plot some expression in x from x=a to x=b axis equal force the x and y axes of the current plot to be scaled equally title('A Title') add a title A Title at the top of the plot xlabel('blah') label the x axis as blah ylabel('blah') label the y axis as blah legend('foo','bar') label 2 curves in the plot foo and bar grid include a grid in the plot figure open up a new figure window x', A' the complex-conjugate of the transposes of x and A dot(x,y), sum(x.*y) …two other ways to write the dot product x' * y the dot (inner) product of two column vectors x and y x * y' the outer product of two column vectors x and