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This is probably a very simple question, but I couldn't find a duplicate.
As everybody knows, {x, y} + v gives {x + v, y + v}. But if I intend v to represent a vector, for example if I am going to substitute v -> {vx, vy} in the future, then the result {x + v, y + v} is meaningless.
How I can indicate to Mathematica that v is not a scalar and functions like Plus should not treat it as one? I tried setting $Assumptions = v ∈ Vectors[2] but that didn't help.
You can use Inactivate
pl = Inactivate[{x, y} + v, Plus]
As an indication to not being "active" the plus sign is dimmed
You can still use a (partially) inactivated expression
ti = pl * 8
and Activate it whenever you want
Activate[ti]
One workaround would be ClearAttributes[Plus,Listable] (The Plus is threading because of the Listable attribute). If you need the Listable attribute in another part of your code, you would need to run SetAttributes[Plus,Listable] to put the attribute back in.
ClearAttributes[Plus,Listable];
{x, y} + v
(* v + {x, y} *)
Or, you could simply use Defer (a bit tricky to use):
Defer[{x, y} + v]
(* {x, y} + v *)
Alternatively, you could use Hold (or HoldForm) and ReleaseHold:
ReleaseHold[Hold[{x, y} + v] /. v -> {vx, vy}]
(* {vx + x, vy + y} *)
Listable makes {a,b}+{c,d} not evaluate as expected.
$\endgroup$
– Ruslan
Sep 1 '16 at 20:59
I like to wrap lists in some suitable function to block this evaluation. MatrixForm is good because it looks pretty, but an undefined function would also work.
a = MatrixForm[{x, y}] + MatrixForm[v]
Then, once you have concrete definitions for your arrays, take away the MatrixForm:
b = a /. v -> {1, 2}
b /. MatrixForm -> Identity
(* {1 + x, 2 + y} *)
You may use Indexed to represent v as its components in the calculation.
{x, y} + (Indexed[v, #] & /@ Range[2])
(* {x + Indexed[v, {1}], y + Indexed[v, {2}]} *)
Later when v is assigned the Indexed components will become Part values of v.
v = {1, 2};
{x, y} + (Indexed[v, #] & /@ Range[2])
(* {1 + x, 2 + y} *)
As a function
f[t_] := {x, y} + (Indexed[t, #] & /@ Range[2])
Hope this helps.
If there's a specific addition operation one wants to block, one can just apply a custom function instead, for example
{x, y} ~plus~ v
(* plus[{x, y}, v] *)
and replace it with Plus when one is ready:
{x, y} ~plus~ v /. v -> {vx, vy} /. plus -> Plus
(* {vx + x, vy + y} *)
Another option, which doesn't require activation/replacement-by-identity/manual-indexing/etc. is to create a custom Plus function, which only evaluates for lists:
vectorPlus[a_?ListQ,b_?ListQ]=a+b;
With it you have:
vectorPlus[x, {q,r,s}]
vectorPlus[{q,r,s}, y]
vectorPlus[{q,r,s}, y] /. y->{a,b,c}
vectorPlus[{q,r,s}, {x,y,z}]
vectorPlus[x, {q, r, s}]
vectorPlus[{q, r, s}, y]
{a + q, b + r, c + s}
{q + x, r + y, s + z}
InactivatePlusbecause there are other computations you want to do? $\endgroup$ – march Dec 24 '15 at 3:51PiecewiseExpand[f[t, 0, 2]]for my Koch snowflake parametrization, because I couldn't figure out make the answers here work for it. But seeing all the different approaches inspired a solution of my own that did work out (though still not as elegant as I would like). $\endgroup$ – Rahul Dec 25 '15 at 2:51