Error while running the funcyion

While running my function, I am getting following error. PLease help me

    Error using fit>iFit (line 340)

    Complex value computed by model function, fitting
    cannot continue.

    Try using or tightening upper and lower bounds on
    coefficients.

    Error in fit (line 108)
    [fitobj, goodness, output, convmsg] = iFit(
    xdatain, ydatain, fittypeobj, ...

[Walter Roberson]

Which model are you fitting against? What is the range of your x and y data? Have you put in any range constraints?

[Giru Mishra]

Model is:

function  y = lc(x,A,c,b,d)

y = zeros(size(x));

for i = 1:length(x)

          y(i)=(-b)*(log10(x(i))-log10(c))+log10(A);
  end

and then calling it as:

t = fittype('lc(x,A,c,b,d)')

f=fit(x,y,ft,'startpoint',[10 -4 1 2])

[Giru Mishra]

range of data is: x--> -4 to 6 y--> -5 to -2

[Giru Mishra]

and data is in log.. i.e. the data to be fitted is in log

Answer 1

[Matt J]

If c is allowed to be negative, then your model function will produce complex numbers via log10(c). Use the 'Lower' and 'Upper options to specify bounds.