Assignment Problem

[Thomas D. Dean]

I think an earlier post went away...

I am having problems with this, again.

restart; with(geom3d):
data:={ [1,2,3,4],
[5,6,7,8],
[9,10,11,12]
};
######################
## both these fail
for i from 1 to nops(data) do
point(A, data[i][2 .. 4]);
P[i] := A;
print(i, A) end do;
for i from 1 to nops(data) do print(i,form(P[i]),coordinates(P[i])); end do;
##

The problem is in the assignment to P[i]. All the P[i]'s have the same
value.

I am doing this wrong. All the p[i]'s have the same value. The
description in Heck says this. I tried P[i]:=evaln(A) but this did not
correct the problem.


p := [seq(evaln(point(A, data[i][2..4])), i = 1..nops(data))];
seq(print(i, coordinates(p[i])), i = 1..nops(data));

This produced the correct values.

What is wrong with the loop approach?

Tom Dean

[Mate]

In geom3d the points (and other objects) are implemented
as names with associated properties (attributes).

In the first part you re-use the same name (A) three times.
Of course, only the last definition survives.

The second part is equivalent to

for i from 1 to nops(data) do

P[i]:='point'(A, data[i][2 .. 4]);

end do;
for i from 1 to nops(data) do print(i,form(P[i]),coordinates(P[i]));
end do;

Now, P[i] (and also A) is redefined each time it is used.
This is not efficient and in geom3d only a single point (name), A, is
defined.
Probably you will have big problems later using P[i] with other
functions.

The correct solution would be to define the points P1,P2,... using

for i from 1 to nops(data) do

point(P||i, data[i][2 .. 4]) end do;

Now,

for i from 1 to nops(data) do print(i,form(P||i),coordinates(P||i));
end do;

works ok.


Mate