Conversion to logical from sym is not possible. I want the first argument to be a logical argument which has && for non symbolic variables, In a piecewise function.

Hi,

I'm trying to execute this piecewise function

a = 10;
b = 4;
c = 2;
mu1 = 1;
sig1 = 0.5;

pdf1(x) = piecewise( a == 0 && b < 0 , -(1/(b*exp((c1 - x + b*mu1)^2/(2*b^2*sig1^2))*sqrt(2*pi)*sig1)),...
           a == 0 && b > 0, 1/(b*exp((c1 - x + b*mu1)^2/(2*b^2*sig1^2))*sqrt(2*pi)*sig1),...
a ~= 0 && (-b^2 + 4*a*c1 - 4*a*x) < 0,(exp((b - sqrt(b^2 - 4*a*c1 + 4*a*x) + 2*a*mu1)^2/(8*a^2*sig1^2)) + exp((b + sqrt(b^2 - 4*a*c1 + 4*a*x) + 2*a*mu1)^2/(8*a^2*sig1^2)))/(exp((b^2 + 2*a*b*mu1 + 2*a*(-c1 + x + a*mu1^2))/(2*a^2*sig1^2))*sqrt(2*pi)*sqrt(b^2 + 4*a*(-c1 + x))*sig1),...
 0);

for which i get the error Conversion to logical from sym is not possible. I realised this is because the first argument cannot have && or ||, however for my application its critical for this to be present. Any help is appreciated. TIA.

Answer 1

[Githin John]

You may try this code assuming it still satisfies your requirements.

syms a b c1 x;
assume(a==10);
assume(b==4);
assume(c1==2);
mu1 = 1;
sig1 = 0.5;
pdf1(x) = piecewise( a == 0 & b < 0 , -(1/(b*exp((c1 - x + b*mu1)^2/(2*b^2*sig1^2))*sqrt(2*pi)*sig1)),...
           a == 0 & b > 0, 1/(b*exp((c1 - x + b*mu1)^2/(2*b^2*sig1^2))*sqrt(2*pi)*sig1),...
a ~= 0 & (-b^2 + 4*a*c1 - 4*a*x) < 0,(exp((b - sqrt(b^2 - 4*a*c1 + 4*a*x) + 2*a*mu1)^2/(8*a^2*sig1^2)) + exp((b + sqrt(b^2 - 4*a*c1 + 4*a*x) + 2*a*mu1)^2/(8*a^2*sig1^2)))/(exp((b^2 + 2*a*b*mu1 + 2*a*(-c1 + x + a*mu1^2))/(2*a^2*sig1^2))*sqrt(2*pi)*sqrt(b^2 + 4*a*(-c1 + x))*sig1),...
 0);

or

syms a b c1 x;
mu1 = 1;
sig1 = 0.5;
pdf1(a,b,c1,x) = piecewise( a == 0 & b < 0 , -(1/(b*exp((c1 - x + b*mu1)^2/(2*b^2*sig1^2))*sqrt(2*pi)*sig1)),...
           a == 0 & b > 0, 1/(b*exp((c1 - x + b*mu1)^2/(2*b^2*sig1^2))*sqrt(2*pi)*sig1),...
a ~= 0 & (-b^2 + 4*a*c1 - 4*a*x) < 0,(exp((b - sqrt(b^2 - 4*a*c1 + 4*a*x) + 2*a*mu1)^2/(8*a^2*sig1^2)) + exp((b + sqrt(b^2 - 4*a*c1 + 4*a*x) + 2*a*mu1)^2/(8*a^2*sig1^2)))/(exp((b^2 + 2*a*b*mu1 + 2*a*(-c1 + x + a*mu1^2))/(2*a^2*sig1^2))*sqrt(2*pi)*sqrt(b^2 + 4*a*(-c1 + x))*sig1),...
 0);

Answer 2

[Walter Roberson]

(a == 0 && b < 0) == sym(true)

piecewise cannot accept a logical value as the first parameter, but compare to symbolic true works.

sym(a == 0 && b < 0)

just might work, but in some code I was doing recently sym() of the condition failed and I had to compare to sym(true)

[Josyula Gopala Krishna]

Strangely they don't work it throws the same error. Johns answer does work. He used

syms a b c1 x;
assume(a==10);
assume(b==4);
assume(c1==2);

This does work.