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I’ve got this simple loop that prints out the solutions to 2 simple equations for various values of parameters $a$ and $b$.

I need to add a command that would display the parameter values that lead to the solution which is closest (in absolute way) to $x = 0.21$, $y = 0.34$. In my particular case it happens when $a=3$ and $b=2$ (which leads to $x=2/9$, $y=1/3$ exactly), but how to get this {a=3,b=2} answer? Thank you.

q := Do[sol = Solve[{a*x + y == 1, -3*x + b*y == 0}, {x, y}]; 
Print[{x /.sol[[1]], y /.sol[[1]]}], {a, 2, 3, 1}, {b, 2, 3, 1}]
q
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NMinimizesolves it directly in one step, assuming (a | b) \[Element] Integers: 

NMinimize[{1, {x == .21, y == 0.34, a*x + y == 1, -3*x + b*y == 0,Element[{a, b}, Integers]}}, {x, y, a, b}]
(*{1., {x -> 0.21, y -> 0.34, a -> 3, b -> 2}}*)
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  • $\begingroup$ Thank you, this seems to work fine. Just a question, in NMinimize you wrote "1" at the beginning. Why is it needed? Shouldn't be the function to be minimized subject to constraints? $\endgroup$ – Alex Aug 18 '19 at 13:08
  • $\begingroup$ That means a pseudo minimization of "1" with several constraints. Often the solver of NMinimize ist very robust. $\endgroup$ – Ulrich Neumann Aug 18 '19 at 17:59
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Not sure this is what you are looking for, but this is how I would approach the problem:

sol = Solve[{a*x + y == 1, -3*x + b*y == 0}, {x, y}]

NMinimize[Total@Abs[{0.21, 0.34} - sol[[1, ;; , 2]]], {a, b}]

{5.86753*10^-11, {a -> 3.14286, b -> 1.85294}}

These are the coefficients {a,b} that lead to the closest values of x,y under the metric I'm using, that is just a minimisation of the total of the distances between {x,y} and the target values.

If you instead want integer values, you could try:

sol = Solve[{a*x + y == 1, -3*x + b*y == 0}, {x, y}];
solfun[{a_, b_}] = sol[[1, ;; , 2]];
coeff = Tuples[{2, 3}, 2];
list = solfun /@ coeff;
min = MinDetect[Total /@ Abs[# - {0.21, 0.34} & /@ list]];

{{3, 2}}

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You can try this :

sol := Solve[{a*x + y == 1, -3*x + b*y == 0}, {x, y}];
Minimize[
 {Abs[.21 - x /. sol[[1]]] + Abs[.34 - y /. sol[[1]]], 
  1 <= a <= 3 && 1 <= b <= 3}, {a, b}, Integers]

with output

{0.0188889, {a -> 3, b -> 2}}.

Or, better, optimize simultaneously over all 4 variables :

Minimize[
 {Abs[.21 - x] + Abs[.34 - y],
  a*x + y == 1 
   && -3*x + b*y == 0 
   && 1 <= a <= 3 
   && 1 <= b <= 3
   && {x, y} \[Element] Reals
   && {a, b} \[Element] Integers},
 {x, y, a, b}]

{0.0188889, {x -> 0.222222, y -> 0.333333, a -> 3, b -> 2}}

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  • $\begingroup$ Thank you. But I would like to rather not rely on Minimize, but be able to "manually" vary a and b. This can be done by altering the step size in a command {a, 2, 3, 1}, {b, 2, 3, 1}. I should also apologize and correct myself because I said earlier "lead to the solution which is closest (in absolute way) to x=0.21, y=0.34". The abs diff should satisfy a predetermined tolerance level, say absolute difference should be less and equal to 0.01. Minimize is great b/c it can help to narrow down where a and b lie; then I could try to change the step size in varying a/b, to satisfy the tolerance. $\endgroup$ – Alex Aug 18 '19 at 12:34
  • $\begingroup$ @Alex If what you want is a tolerance, why not make it a constraint? $\endgroup$ – A.G. Aug 18 '19 at 23:53

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