subs(a,b,1) introduces variable b into a?

Hi,

This is a peculiar issue. I am using sym variables. My variable system_matrix has no dependence on the sym variable l, but when I run the line:

system_matrix = subs(system_matrix,l,10)

system_matrix all of a sudden has l introduced! if I run the same line again, it gets rid of it.

Code: -----------------

syms m b k1 k2 l rho U C

syms s

M = eye(2); K_H = [(k1+k2)/(m*b),((k1-k2)/(2*m))-C*rho*U^2/(2*m);6*(k1-k2)/(m*b^2),3*(k1+k2)/(m*b)-3*C*rho*U^2/(2*m*b)];

system_matrix = s^2.*M + K_H;

% substitute for values

system_matrix = subs(system_matrix,m,15);

system_matrix = subs(system_matrix,k1,5000);

system_matrix = subs(system_matrix,k2,4000);

system_matrix = subs(system_matrix,b,1);

system_matrix = subs(system_matrix,l,10); % for some reason this introduces new l's into the equation?

system_matrix = subs(system_matrix,C,1);

system_matrix = subs(system_matrix,rho,1.25);

----------------

end code

At the end of this expression, system_matrix has the variable l in it, even though it never did before!

Anyone know what is going on?

Answer 1

[Stefan Wehmeier]

I cannot reproduce this. If it happens (in which version ??), then it is clearly unexpected.

[Raman]

THis happens in 2012b. I simplified it to this:

syms m l;

system_matrix = [1,m/(10);1,1];

system_matrix = subs(system_matrix,l,10);

It appears that the function is replacing all instances of 10 with "l" instead of the other way.

I tried the same code in 2014a and it seems to be fixed.