# Difference between revisions of "Tips and tricks"

## 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

### A funny formatting trick with fprintf found by chance

Imagine that you want to create a text table with fprintf with 2 columns of 15 characters width and both right justified. How to do this thing?

That's easy:

If the variable Text is a cell array of strings (of length <15) with two columns and a certain number of rows, simply type for the kth row of Text

```fprintf('%15.15s | %15.15s\n', Text{k,1}, Text{k,2});
```

The syntax '%<n>.<m>s' allocates '<n>' places to write chars and display the '<m>' first characters of the string to display.

Example:

```octave:1> Text={'Hello','World'};
octave:2> fprintf('%15.15s | %15.15s\n', Text{1,1}, Text{1,2})
Hello |           World
```

### Load Comma Separated Values (*.csv) files

```A=textread("file.csv", "%d", "delimiter", ",");
inds = isnan(A);
B(!inds) = num2cell(A(!inds))
```

This gets you a 1 column cell array. You can reshape it to the original size by using the `reshape` function

The next version of octave (3.6) implements the `CollectOutput` switch as seen in example 8 here: http://www.mathworks.com/help/techdoc/ref/textscan.html

### Using Variable Strings in Octave Commands

For example, to plot data using a string variable as a legend:

Option 1 (simplest):

```legend = "-1;My data;";
plot(x, y, legend);
```

Option 2 (to insert variables):

```plot(x, y, sprintf("-1;%s;", dataName));
```

Option 3 (not as neat):

```legend = 'my legend';
plot_command = ['plot(x,y,\';',legend,';\')'];
eval(plot_command);
```

These same tricks are useful for reading and writing data files with unique names, etc.

### 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);
```