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.
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