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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.
Previous: Calling a Function by its Name, Up: Evaluation [Contents][Index]