Rearrange Variables in an equation

I declare the variables using sym or syms then i have an equation a+2*b=1; I want MATLAB to give me a=1-2*b; or alternatively b=(1-a)/2;

is there a way to do this?

Also, There maybe more than 2 variables Equation maybe linear or non-linear Equation can change at each iteration. Thanks

Answer 1

[Star Strider]

This is one way:

        syms a b
        eqn = (a + 2*b == 1)
        v_a = solve(eqn, a)
        v_b = solve(eqn, b)

producing this output:

v_a =

1 - 2*b

v_b =

1/2 - a/2

I suggest you avoid expressions such as:

        [a b] = solve(eqn, a, b)

even though it is valid syntax, because instead of solving the equation for ‘a’ and then solving it independently for ‘b’, it solves it for ‘a’ first and then solves the resulting equation for ‘b’ in terms of an introduced variable ‘z’. That produces a multi-line warning in red (that I will not reproduce here), and the results:

a =

1 - 2*z

b =

z

that does not make sense to me. I consider this to be a bug, and I would prefer it to be an option rather than the default, but that is not my decision.

[Mech Princess]

Ok. got it. There was a small error in the syntax. The following code works. Thank you so much

syms a b eqn = ('a + 2*b = 1') v_a = solve(eqn, a) v_b = solve(eqn, b)

[Mech Princess]

Sorry, one more issue regarding this. My variables alpha1, alpha2 and alpha3 are defined by sym and not syms. I need to do this as the no of alpha's is determined by the input data size. This case 3.

data = [-1 0 1;-1 +1 -1]'; alpha = sym('alpha',[1 3]); %output "[ alpha1, alpha2, alpha3]" st = 'alpha*data(:,2)=0' %output "alpha2 - alpha1 - alpha3 = 0" but I dont get this. instead, its something else v_2 = solve(st,alpha(1)) %doesnt work because the previous line is wrong

Do you have any suggestions for this? Thanks

[Star Strider]

This:

        data = [-1 0 1;-1 +1 -1]';         
        alpha = sym('alpha',[1 3]); 
        advct = alpha * data(:,2)
        v_2 = solve(advct == 0, alpha(1))

gives me these:

advct =

alpha2 - alpha1 - alpha3

v_2 =

alpha2 - alpha3

Is that the result you were hoping for?

Answer 2

[Nitesh khanna]

how to replace the variables a, b, c, d by variables w, x, y, z for; in eqn

eqn=a+b+c+d==9;

will this command work

eqn=subs(eqn, [a,b,c,d],[w,x,y,z])

Answer 3

[Bill Tubbs]

As explained here by Paul, you can use the isolate function for this:

>> syms a b
>> eqn1 = b == (1 - a) / 2;
>> isolate(eqn1, a)

ans =

a == 1 - 2*b

>> isolate(eqn1, b)

ans =

b == 0.5000 - 0.5000*a

[Walter Roberson]

.. Though mind you, that was not an option back in 2012 !

[Star Strider]

Definitely!

The isolate function was introduced in R2017a, 5 years after this thread.

I’ve used isolate extensively in my Answers since it was introduced, when its use was appropriate.