How to call a value from a file within an equation?

I have the equation :

aggRF(n)=(-1.3e-6.*(('CH4(n)'-'CH4(0'))/2)-8.2e-6*n+.043)*(sqrt(CH4(n)-sqrt(CH4(0))))

How do I call certain values from the file (attached). CH4 is the species, the number is the year. I can type CH4(#) in and it gives me the correct value but how do I do this in an equation?

Answer 1

[dpb]

Define anonymous function--

fnAggRF=@(n)(-1.3e-6.*(('CH4(n)'-'CH4(0'))/2)-8.2e-6*n+.043)*(sqrt(CH4(n)-sqrt(CH4(0))));

Then, simply

n=23;                % example 'n' vaue
AggRF=fnAggRF(n);    % evaluate for n

NB: You will have to have read the .mat file and have CH4 in memory when the function handle is created to build in the constants for CH4(0); if you have more than one file or CH4 array the function must be recreated with that new/updated array in memory at the time; otherwise the function will still reflect the original values; they are not dynamically updated if the variables upon which the anonymous function is dependent are changed. See the documentation for anonymous functions for more detail.

To avoid the above, you could always make the argument list to include the array...

fnAggRF=@(CH4,n)(-1.3e-6.*(('CH4(n)'-'CH4(0'))/2)-8.2e-6*n+.043)*(sqrt(CH4(n)-sqrt(CH4(0))));

By including CH4 in the argument list, it becomes a locally-scoped dummy argument in the expression and is not the same variable as that existing in the workspace or function space.

ERRATUM:

Matlab arrays are one-based; CH4(0) must be CH4(1), sorry...

fnAggRF=@(n)(-1.3e-6.*(('CH4(n)'-'CH4(1'))/2)-8.2e-6*n+.043)*(sqrt(CH4(n)-sqrt(CH4(1))));

[Nesha Wright]

Thank you! I could be wrong but this will only work if I am just needing to solve with CH4 though, I have 47 species in total in the same equation. I have attached a slightly longer version:

 aggRF(n) = (-1.3e-6.*(('CH4(n)'-'CH4(0'))/2)-8.2e-6*n+.043)...%CH4
    *(sqrt(CH4(n)-sqrt(CH4(0))))... 
    +9-8.0e-6*((CO2(n)-CO2(0))/2)+4.2e6*((N2O(n)-N2O(0))/2)...%N2O
    -4.9e-6+.117)*(sqrt(N2O(n)-sqrt(N2O(0))))...

So the only way to do this is to create a function for each of the 47?

I am also getting the following error when I use your answer for the single line. Thoughts?

"Error using @(n)-1.3e-6.*(('CH4(n)'-'CH4(0'))/2-8.2e-6*n+.043*(sqrt(CH4(n)-sqrt(CH4(0)))) Matrix dimensions must agree."

[dpb]

"... getting the following error"

You've put quotes around the first two variable expressions; you just need the variable itself:

   aggRF(n) = (-1.3e-6.*(('CH4(n)'-'CH4(0'))/2)-8.2e-6*n+ ...

should be

 aggRF(n) = (-1.3e-6.*((CH4(n)-CH4(0))/2)-8.2e-6*n+ ...

[dpb]

"the only way to do this is to create a function for each of the 47?"

No; but you didn't put that in the original problem specification and the crystal ball is back in the shop for repair (yet) again... :)

You do need to expand beyond just an anonymous function, however, and probably to reconsider the data organization to make it more generic.

I'll stick with this comment for just now to give the feedback; I had meetings in town and I'll ponder and come back later on to look at the expanded Q? in more depth.

[Nesha Wright]

You're right, my bad. Thanks so much! I'm slowly making ground with Matlab.

[dpb]

Is not the full equation simply a summation of the same functional form for each of M components with only a differing coefficient (some zero, perhaps)?

If so, the most efficient way to store and solve would be to hold all the components and coefficients as 2D arrays and solve as array summation.

See if can't put the system into such a regular order such that all coefficients and components are just 2D arrays...

[Nesha Wright]

On it! THANK YOU!!!!

[dpb]

As an expansion of the above idea but with a little more structure than just a "plain" 2D array with everything identified only by row and column numbers, one could build a structure with fields containing the material and coefficients and manipulate it; then one has both the commonality of the one data structure but also has way to refer to elements either by name or variables.

Similar to that also could be the table which is essentially an ordered and named 2D array that can also be operated on globally.