[GAP Forum] A Question about For Loops

[Alexander Konovalov]

Dear Minghui Liu,

On 11 Feb 2014, at 11:31, Minghui Liu <matliumh at gmail.com> wrote:

> Dear All,
> 
> In GAP I would like to create a free group with 200 generators and call the
> nth generator dn. I tried the following commands but it does not work:
> 
> gap> F:=FreeGroup(200);
> gap> a:=[1..200];
> gap> for x in a do
>> dx:=F.x;
>> od;
> 
> The error message is

The reason is that GAP parses F.x as an access to the component 'x' of the
object F. Clearly that does not work in this context. To enforce evaluation
of x to get F.1, F.2 etc, use F.(x), for example

gap> F:=FreeGroup(200);
<free group with 200 generators>
gap> a:=[1..200];
[ 1 .. 200 ]
gap> for x in a do
> dx:=F.(x);
> od;
gap> dx;
f200

The difference between r.name and r.(name) constructions is explained in
"29.2 Accessing Record Elements" in the reference manual.

Hope this helps
Alexander