How can I use a function answer as a variable in another function?

I need to make a function dXdt. One of the variables is N. N is found with the function dNdt, so N has multiple answers, 145 x 1 is the array made. which doesnt work very well as a variable. I cant do a nested function because N is needed and N is not in the function of the original equation. How would I use the solution of dNdt as a variable? Would I need a for loop? And if so how would I go about doing that for an array so large?

[millercommamatt]

does dXdt only accept scalars?

[dpb]

"N has multiple answers, 145 x 1 is the array made. which doesnt work very well as a variable"

N is a perfectly valid variable name for a vector array -- what's wrong with just passing it to the function?

We would have to see the definition of the function dXdt; it would, of course, have to know what to do with the vector input (but it'll have to be able to handle a145 single inputs otherwise which, if you think a vector is cumbersome, just try writing that!!! :) )