How do I manipulate the variable with respect to which IFFT is computed?

How do I manipulate the variable with respect to which FFT or IFFT is computed in MATLAB? Say for example my function has two variables, but I only want to find the IFFT treating one of them as frequency. Basically I want to know how MATLAB's ifft function decides the variable with respect to which the integral of the transform will be found. Specifically, I need to compute the following integral :

How do I go about this? Thanks!

Answer 1

[Todd Leonhardt]

From MATLAB's fft function documentation:

fft(X,[],DIM) or fft(X,N,DIM) applies the fft operation across the dimension DIM.

Online fft documentation

MATLAB's ifft function behaves analogously.