gap-system

Working with holes in the lists [GAP SYSTEM]

I have a question regarding holes in the lists. Assume that I have the following code:

gap> l:=[2,3,,5];
[ 2, 3,, 5 ]
gap> for i in [1..Size(l)] do
> Print(l[i],"\n");
> od;
2
3
Error, List Element: <list>[3] must have an assigned value in
  Print( l[i], "\n" ); at *stdin*:13 called from 
<function "unknown">( <arguments> )
 called from read-eval loop at *stdin*:14
you can 'return;' after assigning a value

Is there some function to find if the i-th element of a list is hole or not before trying to access it? I am looking for something like this:

gap> for i in [1..Size(l)] do
> if IS_HOLE(l[i])=true then Print("Hole \n); else Print(l[i],"\n"); fi;
> od;

I've read the manual, but still have no clue how to deal with this. Can anybody suggest some solution to this problem, please?

Solution

  • I've checked again the documentation and finally found the answer to my question. Thanks @mike_pierce for the hint. I had to use IsBound function, which returns false for the 'hole' element. Here is the edited code from my example:

    gap> l:=[2,3,,5];;   
gap> for i in [1..Size(l)] do   
> if IsBound(l[i])=false then Print("Hole \n"); else Print(l[i],"\n"); fi;   
> od;   
2 
3 
Hole  
5