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→Programs, Libraries, and Packages: add recipe to check if package is missing
## Image IO with support for png files
## Image IO with support for png files
any (cellfun (@(x) ismember ("png", x), {imformats.ext}))
any (cellfun (@(x) ismember ("png", x), {imformats.ext}))
=== Find if a package is installed ===
==== Problem ====
You have a program that uses different functions or behaves different
depending on the availability of specific packages.
==== Solution ====
Use {{codeline|pkg ("describe", pkg-name)}} like so:
if (! isempty (pkg ("describe", "foo")))
## use functions from package foo, the prefered way
elseif (! isempty (pkg ("describe", "bar")))
## use functions from package bar, not so optimal
else
## default case
endif
==== Discussion ====
It's not recommended to use this if the only purpose is to then fail
in the absence of the package. In such case, simply try to load the package
and Octave will already give a error message that is informative enough.
There is only purpose to check this, if there is something different to
do if a package is missing. The same is true for catching an error from
{{codeline|pkg load}}. If you only catch an error to then throw it again
then you might as well not catch it in the first place.
## This contraption doesn't add anything. If 'pkg load' fails, it
## will already give an error message to the user.
try
pkg load foo;
catch
error ("failed to load foo: %s", lasterr ());
end_try_catch
## Again, doesn't add anything. The failure of 'pkg load' is enough
if (isempty (pkg ("describe", "foo")))
error ("program: package foo is not installed");
endif
Beware that an installed package is not always a guarantee that a function
will be available. Some packages may disable functions at build time, or
specific functions may have specific runtime requirements.
== Structures ==
== Structures ==