First, from the documentation, ‘ modelfun must accept two input arguments, a coefficient vector and an array X—in that order—and return a vector of fitted response values. ’ So your function statement should be:
function f = lineshape_exchange(a,x)
Second, what sizes are your x and y arrays? I get the impression they’re matrices and not vectors. The nlinfit function can take matrix x values, but must return a vector of estimated function values, and y must be a vector as well. (The lsqcurvefit function in the Optimization Toolbox can take matrix independent variables and fit them to matrix dependent variables.)
