Jacobian of equation with left and right hand sides
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I have a program where the user inputs an equation in the form of a string:
'2*a + 3*b = 5*c' % (just an example, it can be any linear equation)
in sequence I list the variables and take the Jacobian:
allvars = symvar(input);
J = jacobian(input, allvars);
However, because of the "=" in the equation my output is:
J = [ 2 = 0, 3 = 0, 0 = 5]
and instead I *need*** it to be:
J = [ 2, 3, -5]
How can I solve this? Having the user input '2*a + 3*b - 5*c' is not an option.
I tried looking for a rhs/lhs (right/left hand side) function but there aren't any, the @children function is available only in R2012a and greater.
2 Comments
Matt Kindig
on 9 Jul 2013
Hmmm, interesting question. Maybe you'll need to parse the input string and move everything to the left hand side first. Something like this might work:
input = '2*a + 3*b = 5*c'; %for example
sides= regexp(input, '=', 'split'); %chop by equals sign
%replace with expression where everything is on left hand side
modified = sprintf('%s - (%s)', sides{1}, sides{2});
allvars = symvar(modified);
J = jacobian( modified, allvars); %this should give the correct answer.
Accepted Answer
Daniel
on 24 Feb 2014
1 Comment
Karan Gill
on 9 May 2017
Starting R2017a, The "lhs" and "rhs" functions are available. See my answer below.
More Answers (2)
Matt J
on 9 Jul 2013
Could you do something like
>> str='2*a + 3*b = 5*c';
>> newstr=[strrep(str,'=','-(') , ')']
newstr =
2*a + 3*b -( 5*c)
0 Comments
Karan Gill
on 9 May 2017
Edited: Karan Gill
on 17 Oct 2017
Starting R2017a, The "lhs" and "rhs" functions are available. See:
Here's a toy example.
>> syms a b c d
>> eqn = a+b == c+d
eqn =
a + b == c + d
>> lhs(eqn)
ans =
a + b
>> rhs(eqn)
ans =
c + d
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