9.2 Evaluation in a Different Context

Before you evaluate an expression you need to substitute the values of the variables used in the expression. These are stored in the symbol table. Whenever the interpreter starts a new function it saves the current symbol table and creates a new one, initializing it with the list of function parameters and a couple of predefined variables such as nargin. Expressions inside the function use the new symbol table.

Sometimes you want to write a function so that when you call it, it modifies variables in your own context. This allows you to use a pass-by-name style of function, which is similar to using a pointer in programming languages such as C.

Consider how you might write save and load as m-files. For example,

function create_data
  x = linspace (0, 10, 10);
  y = sin (x);
  save mydata x y
endfunction

With evalin, you could write save as follows:

function save (file, name1, name2)
  f = open_save_file (file);
  save_var(f, name1, evalin ("caller", name1));
  save_var(f, name2, evalin ("caller", name2));
endfunction

Here, ‘caller’ is the create_data function and name1 is the string "x", which evaluates simply as the value of x.

You later want to load the values back from mydata in a different context:

function process_data
  load mydata
  … do work …
endfunction

With assignin, you could write load as follows:

function load (file)
  f = open_load_file (file);
  [name, val] = load_var (f);
  assignin ("caller", name, val);
  [name, val] = load_var (f);
  assignin ("caller", name, val);
endfunction

Here, ‘caller’ is the process_data function.

You can set and use variables at the command prompt using the context ‘base’ rather than ‘caller’.

These functions are rarely used in practice. One example is the fail (‘code’, ‘pattern’) function which evaluates ‘code’ in the caller's context and checks that the error message it produces matches the given pattern. Other examples such as save and load are written in C++ where all Octave variables are in the ‘caller’ context and evalin is not needed.

Built-in Function: evalin (context, try, catch)

Like eval, except that the expressions are evaluated in the context context, which may be either "caller" or "base".

Built-in Function: assignin (context, varname, value)

Assign value to varname in context context, which may be either "base" or "caller".