Re: Plot the results of Findroot

• To: mathgroup at smc.vnet.net
• Subject: [mg122949] Re: Plot the results of Findroot
• From: Kontopoulos Dimitris <dimitris.jp at gmail.com>
• Date: Fri, 18 Nov 2011 06:22:31 -0500 (EST)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com
• References: <7BB0E96E72E84E41B24A1FF8ED13F5E6131286E80D@IE2RD2XVS581.red002.local>

```hello and sorry for my late reply.

i just wanted to thank you so much for your help,
i ouldn't have made it alone! :)

Dimitris

On Nov 10, 2011, at 6:20 PM, Alexei Boulbitch wrote:

> Hello, Dimitris,
> One way to do what you want may be like the following. Since you do
example of two primitive equations:
>
> Clear[y1, y2, x1, x2, n, d];
> y1[n_] := x1 + x2 == n;
> y2[d_] := x1 - x2 == d;
>
> Let us define a function finding its roots for any n and d:
>
> fR[{n_, d_}] := FindRoot[{y1[n], y2[d]}, {x1, -1}, {x2, -2}];
>
> Let us check for the following list pairs {n, d} is {{1, 2}, {1, -1},
{3, -2}}. Map the function fR onto this list:
>
> Map[fR, {{1, 2}, {1, -1}, {3, -2}}]
>
> {{x1 -> 1.5, x2 -> -0.5}, {x1 -> 0., x2 -> 1.}, {x1 -> 0.5,
>   x2 -> 2.5}}
>
> Now we can package this all into one function. Its argument is a list
of pairs {n, d}:
>
> solving[lst_List] :=
>   Module[{fR, y1, y2},
>    y1[n_] := x1 + x2 == n;
>    y2[d_] := x1 - x2 == d;
>    fR[{n_, d_}] := FindRoot[{y1[n], y2[d]}, {x1, -1}, {x2, -2}];
>    Map[fR, lst]
>    ];
>
> Now to address your second question I create an arbitrary list, lst,
with the same n=1, and arbitrary d and solve it:
> lst = Table[{1, RandomInteger[{-5, 5}]}, {10}]
> sol = solving[lst]
>
> {{1, 0}, {1, -5}, {1, -1}, {1, -1}, {1,
>   5}, {1, -2}, {1, -4}, {1, -4}, {1, 5}, {1, 3}}
>
> {{x1 -> 0.5, x2 -> 0.5}, {x1 -> -2., x2 -> 3.}, {x1 -> 0.,
>   x2 -> 1.}, {x1 -> 0., x2 -> 1.}, {x1 -> 3., x2 -> -2.}, {x1 -> -0.5,
>    x2 -> 1.5}, {x1 -> -1.5, x2 -> 2.5}, {x1 -> -1.5,
>   x2 -> 2.5}, {x1 -> 3., x2 -> -2.}, {x1 -> 2., x2 -> -1.}}
>
> In order to plot x1=x1({1, d}) let us now construct the list of
pairs {d,x1}. Take the i,2-nd element of the list lst and the solution
for x1 (that is the i,1,2-the element) of the list of the solutions and
plot it:
>
> lstForPlot = Table[{lst[[i, 2]], sol[[i, 1, 2]]}, {i, 1,
Length[lst]}]
>
> {{0, 0.5}, {-5, -2.}, {-1, 0.}, {-1, 0.}, {5,
>   3.}, {-2, -0.5}, {-4, -1.5}, {-4, -1.5}, {5, 3.}, {3, 2.}}
>
>
> ListPlot[lstForPlot]
>
> You get a list of pairs {d,x1} plotted as the result.
> There is a possible weak point here: if your equations are non-linear,
it is not necessary that your initial values for FindRoot are always
good.
>
> Hope this helps, have fun, Alexei
>
>
> Hello everyone,
>
> I have a system of 21 simultaneous equations where i want to calculate
> the values of x1, x2...x21 and i am trying to evaluate it by using
> FindRoot.
>
> I have 2 problems,
> 1) I want to tell FindRoot to calculate this set for a range of values
> for 2 parameters (n and d) that are found in the equations
> 2) I want to plot the results of x1, x2,...x20 for the range of the
> parameters.
>
> i have named each of the 21 equations y1, y2, y3.... y21 and this is
> how i wrote the command for FindRoot:
>
> FindRoot[{y1, y2, y2, y4, y5, y6, y7, y8, y9, y10, y11, y12, y13, y14,
>    y15, y16, y17, y18, y19, y20,
>   x1 + x2 + x3 + x4 + x5 + x6 + x7 + x8 + x9 + x10 + x11 + x12 + x13 +
>      x14 + x15 + x16 + x17 + x18 + x19 + x20 + x21 == 1}, {{x1,
>    0.0272275467214981, 0, 1}, {x2, 0.0271731, 0, 1}, {x3, 0.04279, 0,
>    1}, {x4, 0.042534, 0, 1}, {x5, 0.0548527, 0, 1}, {x6, 0.054198, 0,
>    1}, {x7, 0.0627144, 0, 1}, {x8, 0.0614572, 0, 1}, {x9, 0.066180, 0,
>     1}, {x10, 0.064208, 0, 1}, {x11, 0.075882, 0, 1}, {x12, 0.062862,
>    0, 1}, {x13, 0.06160, 0, 1}, {x14, 0.058231, 0, 1}, {x15,
>    0.0230354, 0, 1}, {x16, 0.0250354, 0, 1}, {x17, 0.055199, 0,
>    1}, {x18, 0.026367, 0, 1}, {x19, 0.028367, 0, 1}, {x20, 0.051339,
>    0, 1}, {x21, 0.028735, 0, 1}}]
>
>
> Now every time i want to find the solution for each n and d i manually
> type their values
> e.x.
> n=10^4
> d=10^6
>
> and then i get the answer from FindRoot for all the x1,x2,....x21
>
> What i want to do is calculate x1,x2,...x21 for n ranging from 10^4 to
> 10^9 and d ranging from 10^4 to 10^11.
>
> And in the end I want to plot the results (for a given d) in a graph
> where the y axis is one of the x1,x2,..x21 and the x axis is n
>
> I tried creating a table first so i can plot its contents but it
> doesn't seem to be working....
> Table[n, d,
> Evaluate[MyFunction[n, d]], {n, 10^4, 10^9, 10^4}, {d, 10^4, 10^9,
>   10^11}]
>
> I would appreciate all the tips you can give me
>
> Dimitris
>
> Alexei BOULBITCH, Dr., habil.
> IEE S.A.
> ZAE Weiergewan,
> 11, rue Edmond Reuter,
> L-5326 Contern, LUXEMBOURG
>
> Office phone :  +352-2454-2566
> Office fax:       +352-2454-3566
> mobile phone:  +49 151 52 40 66 44
>
> e-mail: alexei.boulbitch at iee.lu
>
>

```

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