Graphics does not show anything with a Do inside

Questions
,
1

Can you please tell me what is wrong with the following code?

Manipulate[Module[{v},
v = {{0, 0}, {1, 0}, {1, 1}, {0, 1}};
Graphics[{Do[Line[{v[[i]], v[[1]]}], {i, n}]}]],
{n, {2, 3, 4}}]

I was hoping I get some lines as the output.
Thanks, Ferenc

2

Hi,

I guess that you want to use the Table command rather than the Do loop:

Manipulate[Module[{v}, v = {{0, 0}, {1, 0}, {1, 1}, {0, 1}}; Graphics[Table[Line[{v[[i]], v[[1]]}], {i, 1, n}]]], {n, {2, 3, 4}}]

Cheers,
M.

3

Yes, indeed, thank you. I see now that Graphics needs a list of primitives, and the output of Do is not a list. I have to get used to the Mathematica language and move away from procedural thinking. Thanks, Ferenc

4

Dear Ferenc,

you can, of course, also use the Do command to create a list. There are different ways to do this and it is certainly easier to use Table. If for some reason you need to use Do, then something like this might work for you

Manipulate[Module[{v}, v = {{0, 0}, {1, 0}, {1, 1}, {0, 1}}; 
Graphics[Reap[Do[Sow[Line[{v[[i]], v[[1]]}]], {i, 1, n}]][[2,1]]]], {n, {2, 3, 4}}]

M.