Substitute 3 variables in this symbolic equation

Hi, i have a big symbolic equation containing w, x, y as the symbolic variables.

Something similar to http://www.mathworks.com/matlabcentral/answers/35371-error-while-converting-a-huge-symbolic-equation-to-double-question-edited-and-simplified-further however it contains w also.

I now want to substiute w in the equation also. The previous answer to my question was

subs(subs(Wnet, x, 5:5:30)', y, 1100:100:1500) subs(subs(SFC, x, 5:5:30)', y, 1100:100:1500)

However, i want to add w to it from 1:0.5:3 How do i do it?

Answer 1

[Christopher Creutzig]

And then, there's always

    >> [X, Y, W] = ndgrid(5:5:30, 1100:100:1500, 1:0.5:3);
    >> subs(Wnet, {x, y, w}, {X, Y, W})

[Alexander]

I like this one.

[Vaibhav]

Example of what i meant to say above through an example script.

clear

h=[1,2,3,4,5,]; %height

syms w x y

t0=w; %temperature (depends on h, there's no equation to it!)

p0=x; %pressure (depends on h, there's no equation to it!)

m=y; %another variable which is completely independent

Wnet=3.*t0 + 2.*p0 + m.^2;

x1=subs(subs(Wnet, {w,x}, {250:10:280,15000:5000:30000})', y, 0.5:0.5:3);

%means for h = 1,t0=250,p0=15000

%h = 2,t0=260,p0=20000

%so in real, h and y is what i want to get in the 5x5 array.

[Vaibhav]

h=[1,2,3,4] ***

i think the code i typed works :P

evaluating my answers

Answer 2

[Alexander]

Maybe in this case the suggestion from Walter scales better:

 [X, Y, Z] = ndgrid(5:5:30, 1100:100:1500, 1:0.5:3);
 WnetF = matlabFunction(Wnet, 'vars', [x, y, z]);
 WnetN = double(arrayfun(WnetF, X, Y, Z));

[Vaibhav]

i made a mistake in my question

Example of what i meant to say above through an example script.

clear

h=[1,2,3,4,5,]; %height

syms w x y

t0=w; %temperature (depends on h, there's no equation to it!)

p0=x; %pressure (depends on h, there's no equation to it!)

m=y; %another variable which is completely independent

Wnet=3.*t0 + 2.*p0 + m.^2;

x1=subs(subs(Wnet, {w,x}, {250:10:280,15000:5000:30000})', y, 0.5:0.5:3);

%means for h = 1,t0=250,p0=15000

%h = 2,t0=260,p0=20000........

%so in real, h and y is what i want to vary and substitute to get the 5x5 array.

Answer 3

[Christopher Creutzig]

Or, expanding on the thing you have:

    >> syms x y z
    >> f = 3*x + 2*y + z;
    >> subs(subs(subs(f, x, 1:4), y, (1:3)'), z, reshape(1:3,1,1,3))

    ans(:,:,1) =

         6     9    12    15
         8    11    14    17
        10    13    16    19

    ans(:,:,2) =

         7    10    13    16
         9    12    15    18
        11    14    17    20

    ans(:,:,3) =

         8    11    14    17
        10    13    16    19
        12    15    18    21

(For symmetry, you could also write the (1:3)' as reshape(1:3,1,3).)

Compared to Alexander's solution (which works just fine, afaics), this may (or may not) be faster, since it only inserts each x-value once. It is also a way in which you can get symbolic results, e.g., a result still containing a. (Alexander's solution has the advantage of doing most of the work in MATLAB, not in symbolics, which often is faster.)

[Vaibhav]

I made a mistake in expressing my question.

I have to substitute 3 variables, but i have to convert 2 variables to one.

i have a matrix for height which is h=[1,2,3,4,5]

Now for h=[1,2,3,4,5]

the values of w and x are

w=[0.5:0.5:3]

x=[5:5:30]

They correspond to the same values of h

y is as it is.

What i somewhat mean is

subs(subs(Wnet, {w,x}, {0.5:0.5:3,5:5:30})', y, 1100:100:1500)

subs(subs(SFC, {w,x}, {0.5:0.5:3,5:5:30})', y, 1100:100:1500)

How do i tackle this?

Answer 4

[Vaibhav]

The code i type worked :D

But still, i got to learn lots for your answers above.

Lots of codes and stuff to try and learn.

Thanks for the help guys =)

clear 
h=[1,2,3,4,5,]; %height
syms w x y
t0=w; %temperature (depends on h, there's no equation to it!) 
p0=x; %pressure (depends on h, there's no equation to it!) 
m=y; %another variable which is completely independent 

Wnet=3.*t0 + 2.*p0 + m.^2;
x1=subs(subs(Wnet, {w,x}, {250:10:280,15000:5000:30000})', y, 0.5:0.5:3);
%means for h = 1,t0=250,p0=15000 
%h = 2,t0=260,p0=20000 
%so in real, h and y is what i want to get in the 5x5 array.