Hello,
I have following questions regarding lists.
1. Is it safe to pass lists as parameters to functions ?
2. Is it safe to return lists from functions ?
And also additional question.
3. Is it allowed to define sub-functions inside a function ? I have seen such
example in lib/alglie.gi - there is subfunction in RootSystem function. But
when I tried I see error.
Regarding first 2 questions. Since assignment
a:=b;
for list do not create new list, I assume that list is really a pointer
(however this is never mentioned in GAP documentation). So if I declare local
variable in function, create list inside function and return it. Then pointer
to a list is returned but local memory of the function can be overwritten now.
So my list is not safe !?
Here is example
create_mat:=function (n)
local i, m;
m:=[];
for i in [1..n] do
Add(m, [i..i+n-1]);
od;
return m;
end;;
a:= create_mat(5);
# what is it <a> now ? Is it matrix (list) or just pointer to piece of memory
which can be overwritten ?
I am asking because I am receiving strange errors when working with matrices.
The error is saying that boolean is found where list is expected !?
For interested persons I attach my code in example.txt file.
So far everything worked OK (almost) with my matrix GAP code.
Regards,
Marek Mitros
# For matrix m returns coordinates [x,y] of all elements equal to <a> e.g. a=1.
# Assuming that in each row there is only one element equal to 1
mat_ones:=function(m,a)
local i,j, l,x,y;
l:=[];
for i in [1..Length(m)] do
if ForAny(m[i], x->x=a) then
Add(l, [i, Position(m[i], a)]);
fi;
od;
return l;
end;;
# received pair of numbers from range 1..24, returned 0..3
group24:=function(p)
local s;
#Print(p, " ", p[1]," ", p[2], "\n");
s:=Set([Int(p[1]/8),Int(p[2]/8)]);
if Length(s)=1 then
return 0;
else # 01, 02, 12
return s[1]+s[2];
fi;
end;;
# here is ..... piece of code inside another function
# mm is matrix 28*28
ll:=mat_ones(mm,1); # something is wrong here
#pp:=First(ll, x->IsSubset([1..24],x)); #pair of indexes from set
[1..24]
pp:=ll[Position(List(ll, x->IsSubset([1..24],x)), true)];
#Print(pp, IsList(pp),"\n");
f[3]:=group24(pp);
# I see error message that pp is not list but boolean !?
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