FreeMat
|
Section: Flow Control
The keyboard
statement is used to initiate an interactive session at a specific point in a function. The general syntax for the keyboard
statement is
keyboard
A keyboard
statement can be issued in a script
, in a function
, or from within another keyboard
session. The result of a keyboard
statement is that execution of the program is halted, and you are given a prompt of the form:
[scope,n] -->
where scope
is the current scope of execution (either the name of the function we are executing, or base
otherwise). And n
is the depth of the keyboard
session. If, for example, we are in a keyboard
session, and we call a function that issues another keyboard
session, the depth of that second session will be one higher. Put another way, n
is the number of return
statements you have to issue to get back to the base workspace. Incidentally, a return
is how you exit the keyboard
session and resume execution of the program from where it left off. A retall
can be used to shortcut execution and return to the base workspace.
The keyboard
statement is an excellent tool for debugging FreeMat code, and along with eval
provide a unique set of capabilities not usually found in compiled environments. Indeed, the keyboard
statement is equivalent to a debugger breakpoint in more traditional environments, but with significantly more inspection power.
Here we demonstrate a two-level keyboard
situation. We have a simple function that calls keyboard
internally:
key_one.m
function c = key_one(a,b) c = a + b; keyboard
Now, we execute the function from the base workspace, and at the keyboard
prompt, we call it again. This action puts us at depth 2. We can confirm that we are in the second invocation of the function by examining the arguments. We then issue two return
statements to return to the base workspace.
--> key_one(1,2) [key_one,3]--> key_one(5,7) [key_one,3]--> a ans = 5 [key_one,3]--> b ans = 7 [key_one,3]--> c ans = 12 [key_one,3]--> return ans = 12 [key_one,3]--> a ans = 1 [key_one,3]--> b ans = 2 [key_one,3]--> c ans = 3 [key_one,3]--> return ans = 3