Asked by Susan
on 10 Apr 2019

Hi MATLAb guys,

I am stucking at some point and need your help.

Do you know how it is possible to find a relationship between two variables in one equations? for example y = ax+b, (a,b) are given, (x,y) are variables. I would like matlab give me x = (y-b)/a.

I have wrote the following code, but it doesn't work out well. Any idea how to fix it? Thanks in advance

Input: n_W, n_L, W_net1, m_net1, m_net2, beta

Output: W_net2

funX=@(W, W_net1, m_net1) (2*(1 - 2*W))/((1 - 2*W)*(1 + W_net1) + W*W_net1*(1 - (2*W)^m_net1));

funY=@(Z, W_net2, m_net2) (2*(1 - 2*Z))/((1 - 2*Z)*(1 + W_net2) + Z*W_net2*(1 - (2*Z)^m_net2));

funW=@(X,Y,n_W,n_L) (1 - ((1 - X)^(n_W - 1))*(1 - Y)^n_L);

funZ=@(X,Y,n_W,n_L)(1 - ((1 - X)^n_W)*(1 - Y)^(n_L - 1));

funS=@(X,Y,n_W,n_L,beta)(Y*((1 - Y)^(n_L - 1))*(1 - X)^(n_W))-beta;

fun1=@(X,Y,W,Z,n_W,n_L,W_net2, m_net2, W_net1, m_net1, beta) ([funX(W,W_net1,m_net1); funY(Z,W_net2,m_net2) ; funW(X,Y,n_W,n_L); funZ(X,Y,n_W,n_L) ; funS(X,Y,n_W,n_L,beta)]);

n_W = 3;

n_L = 4;

beta = 0.7;

m_net2 = 4;

W_net1 = 8;

m_net1= 4;

fun2=@(P) (P-fun1(P(1),P(2),P(3),P(4),n_W,n_L,beta,m_net1,W_net1,m_net2));

InitialGuess=[0;0;0;0];

fsolve(fun2,InitialGuess)

P.S. The approach is

1) knowing beta, from S ===> relationship between X and Y can be found.

2) from X and W ===> X and Y can be found numerically

3) based on Y, W_net2 can be derived

Answer by John D'Errico
on 10 Apr 2019

syms a x b y

EQ = y == a*x + b;

isolate(EQ,x)

ans =

x == -(b - y)/a

Walter Roberson
on 16 Apr 2019

I will not get to this right away; I have something else I need to do tonight.

Susan
on 16 Apr 2019

Susan
on 18 Apr 2019

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Answer by Susan
on 12 Apr 2019

I have got some silly questions. Sorry in advance if they are super simple. Could anyone kindly help me to figure them out?

1) As for a variable that I am looking for, MATLAB gives me

W_net2 = 1665452806855272103430873159617883565696871742382633107303767067803714653521912534085508640394758232435502244395851440048832512/13020781553015561664280552453950494568153170161082649667490158510277680133267873702693313295722419712822602179610396291511542125

How is it possible that I simply get 0.1279 instead of this very long result? Why doesn't MATLAB do the division?

2) When I use solve(), there is a "z" in results. According to my underestanding, by using vpa() I should get rid of z, but I don't. Any idea?

Thanks in advance.

Walter Roberson
on 17 Apr 2019

Any relationship between n_L and nL ? Any relationship between n_W and nW ?

Walter Roberson
on 17 Apr 2019

funY = @(Y, X) double( subs(solY(1,1), sym('X'), X) - Y);

Susan
on 17 Apr 2019

Thank you so much Walter!

And sorry for the mess. Yes, n_L = nL and n_W = nW

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Answer by Susan
on 22 Apr 2019 at 15:02

Hey MATLAB experts,

Can anyone kindly help me to write down the functions in 'solve function' for below equations? Thanks in advance.

ivec = 1 : Nw;

jvec = 1 : Nl;

X = zeros(Nw, K);

Y = zeros(Nl, K);

W = zeros(Nw, K);

Z = zeros(Nl, K);

S = zeros(Nl, K);

for k = 1 : K

for i = ivec

X(i, k) = 2*(1 - 2*W(i,k))/((1 - 2*W(i,k))*(1 + W_net1(i,k)) + W(i,k)*W_net1(i,k)*(1 - (2*W(i,k))^m_net1(i,k)));

ii = setdiff(ivec, i);

tW1 = prod( 1 - X(ii, k) );

tW2 = prod( 1 - Y(jvec, k) );

W(i,k) = 1 - tW1 * tW2;

end

for j = jvec

Y(j, k) = 2*(1 - 2*Z(j,k))/((1 - 2*Z(j,k))*(1 + W_net2(j,k)) + Z(j,k)*W_net2(j,k)*(1 - (2*Z(j,k))^m_net2(j,k)));

i = ivec;

tZ1 = prod(1 - X(i, k));

jj = setdiff(jvec, j);

tZ2 = prod(1 - Y(jj, k));

tZ3 = tZ2 * tZ1;

Z(j,k) = 1 - tZ3;

S(j,k) = Y(j, k) * tZ3;

end

end

Susan
on 10 May 2019 at 14:51

Walter,

Could you please kindly help me to solve this issue?

Many thanks in advance

Walter Roberson
on 10 May 2019 at 18:33

Sorry, I do no know when I will be well enough to address this.

Susan
on 10 May 2019 at 18:38

No worries. Get well soon :)

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Answer by Susan
on 26 Apr 2019 at 20:56

Hi Walter,

In one of the comments above you mentioned that "The tests I am doing effectively recover from NaN, so I know that is not the reason we cannot find a root." Did you do any specific things that the test recover from Nan?

The reason I am asking is that I am using fmincon() to solve two optimization problems (the objective function is the same but I optimize the objfun w.r.t. two different variables), and regardless of what I am selecting the initial values, I ended up with this error

"Error using sqpInterface

Objective function is undefined at initial point. Fmincon cannot continue."

Thanks in advance.

Walter Roberson
on 26 Apr 2019 at 22:07

Susan
on 26 Apr 2019 at 22:17

Thank you! I will use another name for this variable then! Thanks again

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