(octave.info)Evaluation in a Different Context

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9.2 Evaluation in a Different Context
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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.

 -- : evalin (CONTEXT, TRY)
 -- : evalin (CONTEXT, TRY, CATCH)
     Like ‘eval’, except that the expressions are evaluated in the
     context CONTEXT, which may be either "caller" or "base".

     See also: Note: eval, Note: assignin.

 -- : assignin (CONTEXT, VARNAME, VALUE)
     Assign VALUE to VARNAME in context CONTEXT, which may be either
     "base" or "caller".

     See also: Note: evalin.