Jacobian of equation with left and right hand sides

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.

[Matt Kindig]

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.

[Daniel]

Hi Matt,

Thank you for your swift reply.

Currently I'm doing something like this as a temporary fix but I was hoping to achieve a solution using the symbolic toolbox.

Answer 1

[Daniel]

Oh, I didn't realize I hadn't given this thread closure.

I went with Matt Kindig's approach of making the symbolic equation completely left sided, this is a minimum example:

sumblocks = {'y = x1 + x2'}
sides = regexp(sumblocks, '=', 'split');
for i=1:size(sides,1)
    sumblocks{i} = sprintf('%s - (%s)', sides{i}{1}, sides{i}{2});
end

The output given in "sumblocks" will be a left-sided equation.

[Karan Gill]

Starting R2017a, The "lhs" and "rhs" functions are available. See my answer below.

Answer 2

[Matt J]

Could you do something like

>> str='2*a + 3*b = 5*c';
>> newstr=[strrep(str,'=','-(') , ')']

newstr =

2*a + 3*b -( 5*c)

Answer 3

[Karan Gill]

Starting R2017a, The "lhs" and "rhs" functions are available. See:

https://www.mathworks.com/help/symbolic/lhs.html

https://www.mathworks.com/help/symbolic/rhs.html

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