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I understand Mathematica can't assign the results of a Solve to the unknowns because there may be more than 1 solution. How can I assign the 4 values of following result to variables?

enter image description here

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Could you elaborate on what you are trying to achieve? In principle you can use Replace (or Part) to assign the values to variables. –  sebhofer Jun 11 '12 at 13:36
@sebhofer - I want to assign the first x value to a variable X1, then the y value to Y1, the other two to X2 and Y2. I just don't seem to get the hang of references yet. –  stevenvh Jun 11 '12 at 13:40

4 Answers 4

up vote 12 down vote accepted

You can do this :

s = Solve[y^2 == 13 x + 17 && y == 193 x + 29, {x, y}];
xx = s[[All, 1, 2]];
yy = s[[All, 2, 2]];

Now you can access solutions, this way xx[[1]], yy[[2]].

If you prefer to collect solutions in Array, there is another way :

X = Array[ x, {Length@s}];
Y = Array[ y, {Length@s}];
x[k_] /; MemberQ[ Range[ Length @ s], k] := s[[k, 1, 2]]
y[k_] /; MemberQ[ Range[ Length @ s], k] := s[[k, 2, 2]]

now X is equivalent to s[[All, 1, 2]], while Y to s[[All, 2, 2]], e.g. :

X[[1]] == x[1]
Y == s[[All, 2, 2]]

You do not have to use or even to define X and Y arrays, e.g.

{x[1], y[1]}
{(-11181 - Sqrt[2242057])/74498, 1/386 (13 - Sqrt[2242057])}

We've used Condition i.e. /; to assure definitions of x[i], y[i] only for i in an appropriate range determined by Length @ s, i.e. number of solutions.

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I think the OP is a complete beginner, and all he's looking for is ReplaceAll. This might be a bit too advanced for someone new to Mma. –  Szabolcs Jun 11 '12 at 13:57
@Szabolcs Literally there is the assignment tag, so he is rather looking for Set or SetDelayed applications. –  Artes Jun 11 '12 at 16:07
I think I'll use your first solution for now, until I get the hang of ReplaceAll. @Szabolcs: Yes, Mma virgin. Thanks, all. –  stevenvh Jun 12 '12 at 7:09
@stevenvh I think this answer can be also interesting for you : mathematica.stackexchange.com/questions/1819/… –  Artes Jul 12 '12 at 11:17

Usually you don't want to actually assign values to x and y, and you would use replacement rules instead:

sols = Solve[y^2 == 13 x + 17 && y == 193 x + 29, {x, y}];

{x, y} /. sols[[1]]

or for the second solution:

{x, y} /. sols[[2]]

If you really want to assign values to x and y globally, you could use:

Set @@@ sols[[1]]

but you must clear x and y before using another set:

Clear[x, y]
Set @@@ sols[[2]]

If you want to assign values to x and y within a Block you could do something like this:

Hold @@ {sols[[2]]} /. Rule -> Set /. _[vars_] :>
   Sin[x] + Sqrt[y] // N

This uses what I am calling the injector pattern to get the values into Block in the right syntax without it prematurely evaluating.

Related questions:

Getting rid of the “x ->” in FindInstance results

Using the output of Solve

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+1.Something related to your last comment and the injector pattern. –  Leonid Shifrin Jun 11 '12 at 14:05

If you really wish to assign solutions to variables, you can do something like this:

In[1]:= ClearAll[Subscript]


Subscript[y,2]=1/386 (13+Sqrt[2242057])

Then you can use the solutions for demonstration purposes:

enter image description here

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Update: Version 10 built-in function Values does value extraction conveniently for rules appearing in lists of arbitrary lengths and depths:

{{x1, y1}, {x2, y2}} = Values[Solve[y^2 == 13 x + 17 && y == 193 x + 29, {x, y}]]
(* {{(-11181-Sqrt[2242057])/74498,1/386 (13-Sqrt[2242057])}, 
    {(-11181+Sqrt[2242057])/74498,1/386 (13+Sqrt[2242057])}} *)

Another example:

(* {{1,2},{3},{{4}},{5,{6,{7}}}} *)

Original post:

{{x1, y1}, {x2, y2}} = Solve[y^2 == 13 x + 17 && y == 193 x + 29, {x, y}][[All, All, -1]]
(* {{(-11181 - Sqrt[2242057])/74498, 1/386 (13 - Sqrt[2242057])}, 
    {(-11181 + Sqrt[2242057])/74498, 1/386 (13 + Sqrt[2242057])}} *)

{x1, y2}
(* {(-11181- Sqrt[2242057]) / 74498, 1 / 386 (13 + Sqrt[2242057])} *)
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I'm sorry you've deleted your previous answer nevertheless suppresing duplicates we reduce increasing overall entropy of this site, +1. –  Artes Sep 19 '14 at 21:07
@artes, thank you for the upvote. I agree with your concern over excessive duplicates. The other Q/A is indeed a special case of this one. However, because of its special structure, some tricks that work there do not work here, e.g Last@@@Solve[...]. –  kguler Sep 19 '14 at 21:16

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