[GAP Forum] A strange error

[Burkhard Höfling]

Dear Mathieu,

> Dear Gap forum,
> see below a program that creates a strange error.
> ---------------------------------------------
> n:=4;
> for i in [1..n]
> do
>   eMat:=4;
>   H:=List([1..n], x->x*i*eMat);
> od;
> ---------------------------------------------
> running it with "gap.sh TheProg" produce the following message:
> .
> .
> Syntax error: warning: unbound global variable in BugSearch line 5
>   H:=List([1..n], x->x*i*eMat);
>                              ^

this is absolutely normal and intended behaviour. Please note that  
this is only a warning and your code is absolutely correct.

What happens is the following. GAP first reads (parses) the entire  
`for' loop before executing it. While reading the GAP function x- 
 >x*i*eMat, it discovers that the variable `eMat' is (yet) undefined  
and produces a warning. After the `for' loop has been read, execution  
starts. Of course, the assignment `eMat:=4' takes place before eMat  
is needed in the function, so that the code runs correctly.

> The manual seems silent on the subject, but after a bit of  
> fiddling, I will guess that it's a funny sort of scoping problem.   
> Although scoping seems to be limited to "global" and "local  
> (function)", it might be that there is block scoping as well.   
> Since you are introducing 'eMat' within the "scope" of the for  
> loop, perhaps the variable name 'eMat' does not actually become  
> known until the loop is (textually) complete.

There is no block scoping in GAP; in particular, any global variable  
defined in a for loop remains defined outside the loop.

The reason for this behaviour is that in most cases, the warning  
occurs if one mis-types the name of a variable. In this way, such an  
error can be spotted before the function is being run.

In order to avoid this problem, simply define the variable before the  
`for' loop (by assigning a more or less meaningful value). In your  
case, you could even move the line `eMat := 4;' before the loop.

Cheers,

Burkhard.