Re: Function handles for nonexisting functions

[John W. Eaton]

On 22-Apr-2009, Judd Storrs wrote:

| On Wed, Apr 22, 2009 at 8:50 PM, John W. Eaton <address@hidden> wrote:
| 
| > Create directories d1 and d2.  Put the following fucntion in
| > d1/crash.m:
| >
| >  function crash ()
| >    'd1/crash'
| >
| > and put the following function in d2/crash.m:
| >
| >  function crash ()
| >    'd2/crash'
| >
| > Now start Matlab in the directory that contains d1 and d2, and run the
| > following commands:
| >
| >  fh = @crash
| >  fh ()
| >  cd d1
| >  fh ()
| >  cd ../d2
| >  fh ()
| 
| 
| >> fh = @crash
| 
| fh =
| 
|     @crash
| 
| >> fh ()
| ??? Undefined function or variable 'crash'.
| 
| >> cd d1
| >> fh ()
| 
| ans =
| 
| d1/crash
| 
| >> cd ../d2
| >> fh ()
| 
| ans =
| 
| d2/crash

So if the function handle is not defined initially, the definition
is not sticky?  It changes depending on what function is currently
visible?

|  clear all
| >  fh = @crash
| >  cd ..
| >  fh ()
| >  cd d1
| >  fh ()
| 
| 
| >> clear all
| >> fh = @crash
| 
| fh =
| 
|     @crash
| 
| >> cd ..
| >> fh ()
| 
| ans =
| 
| d2/crash
| 
| >> cd d1
| >> fh ()
| 
| ans =
| 
| d2/crash

In this case, there is a function available when the handle is created
and it appears to keep its original value, so that's different from
the first case, and more like what I would expect a function handle to
do.

| Now, while Matlab is still running, change the definition of
| > d2/crash.m to be
| >
| >  function crash ()
| >    'foo'
| >
| > and then continue your Matlab session with the following commands:
| >
| >  fh ()
| >  cd ..
| >  fh ()
| >  cd d2
| >  fh ()
| 
| 
| >> fh ()
| 
| ans =
| 
| d2/crash
| 
| >> cd ..
| >> fh ()
| 
| ans =
| 
| d2/crash
| 
| >> cd d2
| >> fh ()
| 
| ans =
| 
| foo

Hmm...  So the new value is not picked up until you cd back to the
directory where it was first defined?  Does that make sense?  Seems
odd to me.

| > clear all
| >  addpath (pwd)
| >  cd ..
| >  fh = @crash
| >  fh ()
| 
| 
| >> clear all
| >> addpath(pwd)
| >> cd ..
| >> fh = @crash
| 
| fh =
| 
|     @crash
| 
| >> fh ()
| 
| ans =
| 
| foo

OK, the function is found through the path.

| Now, while Matlab is still running, change the definition of
| > d2/crash.m to be
| >
| >  function crash ()
| >    'bar'
| >
| > and then continue your Matlab session with the following commands:
| >
| >  fh ()
| >  cd d1
| >  fh ()
| >  cd ../d2
| >  fh ()
| 
| 
| >> fh ()
| 
| ans =
| 
| bar
| 
| >> cd d1
| >> fh ()
| 
| ans =
| 
| bar
| 
| >> cd ../d2
| >> fh ()
| 
| ans =
| 
| bar

So when the function is found through an absolute path element, the
new definition is found immediately?

Ugh.  Is it just me, or does this seem like a lot of twisty special
cases?

jwe