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

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Raman
Raman 2014-10-7
编辑： Raman 2014-12-22
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?

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回答（1 个）

Stefan Wehmeier
Stefan Wehmeier 2014-10-30
I cannot reproduce this. If it happens (in which version ??), then it is clearly unexpected.
  1 个评论
Raman
Raman 2014-12-22
编辑：Raman 2014-12-22
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.

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