Dear Forum,
One of the exercises ( um, by Stephen Kohl I think ) gives the following
solution for drawing an Ulam spiral of primes. I am not sure whether the
"SaveAsBitmapPicture"
function requires a special package. Does it? In either case,
is there a document covering all the "export as graphic" tools
in GAP?
- Rohit Gupta
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UlamSpiral := function ( size, filename )
local spiral, smallprimes, n, p, r,
middle, edgelength, edgepos, direction, i, j, zero, one;
smallprimes := Filtered([2..size],IsPrimeInt);
spiral := NullMat(size,size,GF(2));
if size mod 2 = 0 then middle := [size/2,size/2];
else middle := [(size + 1)/2,(size + 1)/2]; fi;
zero := Zero(GF(2)); one := One(GF(2));
spiral[middle[1]][middle[2]] := one;
for p in smallprimes do
i := middle[1]; j := middle[2];
edgelength := 2; edgepos := 1; direction := 0; r := 1;
for n in [2..size^2] do
if direction = 0 then j := j + 1;
elif direction = 1 then i := i + 1;
elif direction = 2 then j := j - 1;
elif direction = 3 then i := i - 1; fi;
r := r + 1;
edgepos := edgepos + 1;
if r = p then
if n > p then spiral[i][j] := one; fi;
r := 0;
fi;
if edgepos = edgelength then
direction := (direction + 1) mod 4;
if direction in [0,2] then edgelength := edgelength + 1; fi;
edgepos := 1;
fi;
od;
od;
SaveAsBitmapPicture(spiral,filename);
end;
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