### 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.

evalin `(context, try)` ¶
evalin `(context, try, catch)` ¶
`[var1, …] =` evalin `(…)` ¶

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

  See also: eval, assignin.

assignin `(context, varname, value)` ¶

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

  See also: evalin.