Tips and tricks

Preferences

Sometimes, Octave defaults are not teh best for someone's specific use. To change the defaults, use the following on the .octaverc file.

Changing default figure size

The default size of a figure may be appropriate for simple figures but not so much when using subplot for example. This can be changed though.

Code: change default figure size

set (0, 'DefaultFigurePosition', [1 get(0, "screensize")(4:-1:3) get(0, "DefaultFigurePosition")(4)]);

The value of DefaultFigurePosition must be a four element vector with the x and y coordinates for the figure, followed by its with and height. The code above sets the default image to be placed at the top of the monitor, with the width of the monitor and the same height previously set as default.

Tiny helper functions

This is a list of tiny helper functions (the equivalent of e.g., shell aliases), the kind one would have on its .octaverc file.

replace help with man

If you use octave too much, you'll find yourself trying to use help instead of man on bash. This function will fix that so you can use man in your octave instance (you can also do the opposite, create a help alias in bash but man has less characters).

Code: alias to help

function man (name) help (char (name)) endfunction

C++

Real matrix operations

This is a table of matrix operations commonly performed in Octave and their equivalents in C++ when using the octave libraries.

Operation	Octave	C++
add	A+B	A+B
subtract	A-B	A-B
matrix multiplication	A*B	A*B
element multiplication	A.*B	product(A,B)
element division	A./B	quotient(A,B)
transpose*	A'	A.transpose()
select element m,n of A**	A(m,n)	A(m-1,n-1)
select row N of A**	A(N,:)	A.row(N-1)
select column N of A**	A(:,N)	A.column(N-1)
extract submatrix of A	A(a:b,c:d)	A.extract(a-1,c-1,b-1,d-1)
absolute value of A	abs(A)	A.abs()
comparison to scalar***	A>2	mx_el_gt(A,2)
	A<2	mx_el_lt(A,2)
	A==2	mx_el_eq(A,2)
	A~=2	mx_el_ne(A,2)
	A>=2	mx_el_ge(A,2)
	A<=2	mx_el_le(A,2)
matrix of zeros	A=zeros(m,n)	A.fill(0.0)
matrix of ones	A=ones(m,n)	A.fill(1.0)
identity matrix	eye(N)	identity_matrix(N,N)
inverse of A	inv(A)	A.inverse()
pseudoinverse of A	pinv(A)	A.pseudo_inverse()
diagonal elements of A	diag(A)	A.diag()
column vector	A(:)	ColumnVector(A.reshape (dim_vector(A.length())))
row vector	A(:)'	RowVector(A.reshape (dim_vector(A.length())))
check for Inf or NaN	any(~isfinite(A))	A.any_element_is_inf_or_nan()
stack two matrices vertically	A=[B;C]	B.stack(C)
uniform random matrix	rand(a,b)	octave_rand::distribution("uniform"); octave_rand::matrix(a,b)
normal random matrix	randn(a,b)	octave_rand::distribution("normal"); octave_rand::matrix(a,b)
sum squares of columns	sumsq(A)	A.sumsq()
sum along columns	sum(A,1)	A.sum(0)
sum along rows	sum(A,2)	A.sum(1)
product along columns	prod(A,1)	A.prod(0)
product along rows	prod(A,2)	A.prod(1)
cumsum along columns	cumsum(A,1)	A.cumsum(0)
cumsum along rows	cumsum(A,2)	A.cumsum(1)
cumproduct along columns	cumprod(A,1)	A.cumprod(0)
cumproduct along rows	cumprod(A,2)	A.cumprod(1)
number of rows	size(A,1)	A.rows()
number of columns	size(A,2)	A.cols()

Notes:

• Transpose, addition, and multiplication operations also apply to RowVector, ComplexRowVector, ColumnVector, and ComplexColumnVector data types when the dimensions are in agreement.

• The difference is due to the fact that arrays are zero-based in C++, but one-based in Octave.

• The names of Octave internal functions, such as mx_el_gt, are not documented and are subject to change. Functions such as mx_el_gt may eventually be available at both the scripting level and in C++ under more common names such as gt.

Complex Matrix Operations

Operation	Octave	C++
conjugate tranpose	A'	A.hermitian()

General

Vectorizing Tricks

You can easily fill a vector with an index:

   for i=1:n, x(i) = i; end

   x = 1:n;

This works for expressions on the index by wrapping the index in an expression:

   for i=1:n, x(i) = sin(2*pi*i*f/r); end

   x = sin(2*pi*(1:n)*f/r);

You can also work with other vectors this way:

   for i=1:n, x(i) = sin(2*pi*y(i)*f/r); end

   x = sin(2*pi*y*f/r);

Conditionals in the for loop are a little bit tricky. We need to create an index vector for the true condition, and another for the false condition, then calculate the two independently.

   for i=1:n, if y(i)<1, x(i)=y(i); else x(i) = 2*y(i); endif

   idx = y < 1;
   x(idx) = y(idx);
   x(!idx) = 2*y(!idx);

FIXME: add the following

• examples from matrices
• tricks with sort and cumsum (e.g., hist, lookup)
• counter-examples such as a tridiagonal solver
• sparse matrix tricks
• tricks relying on fortran indexing

Other references

• MATLAB array manipulation tips and tricks by Peter Acklam: http://home.online.no/~pjacklam/matlab/doc/mtt/index.html
• The MathWorks: Code Vectorization Guide: http://www.mathworks.com/support/tech-notes/1100/1109.html