Fitting nonlinear model to data

I have been trying to fit nonlinear model to my data

Here are the codes:

X = [0.05 0.06 0.07 0.08 0.09 0.1 0.125 0.15 0.175 0.2 0.25 0.3 0.4 0.5 0.6 0.7 0.8 -0.1 -0.5 -0.8];%FAW40;
Y = [0.3 0.4 0.35 0.42 0.3 0.32 0.35 0.37 0.38 0.495 0.45 0.55 0.6 0.55 0.6 0.6 0.65 0.2 0.3 0.5]; %NDVI;
figure
% plot(FAW40_GS,NDVI_GS,'ok');
k = diff(x)<0; %select only data when the soil is drying
x = X(1:end,:);
y = Y(2:end,:);
plot(x(k(:,1),1),y(k(:,1),1),'ok')
hold all
plot(x(k(:,2),2),y(k(:,2),2),'+k')
plot(x(k(:,3),3),y(k(:,3),3),'dk')
set(gca,'XLim',[0 1])
set(gca,'XTick',(0:0.1:1))
%set(gca,'YLim',[0 1])
set(gca,'YTick',(0:0.1:1));
hold all
ylabel('\bf NDVI','fontsize',14);
xlabel('\bf FAW','fontsize',14);
slope = 0.2;
r = 0.40;
ypred = @(A, x)(A(1) ./ (A(2) + exp([(r-Y)*slope])));
A0 = [0.2;-0.5];
A1 = [ 0.3; -0.2];
opts = statset('nlinfit');
opts.RobustWgtFun = 'bisquare';
A_fit = nlinfit(x,y,ypred,A0,A1,opts);
hold all
plot(x',y','ko', x,ypred(A_fit,x'))
legend('Data','Fitted','location', 'northwest')

 _But when I did I get the error 
Error using nlinfit (line 185)
Requires a vector second input argument. Any help would be appreaciated.
 _

[Torsten]

nlinfit has at most 5 inputs, not 6.

[Sonisa]

I do not know what is the meaning of second input argument? Even I deleted the 6th input, I get the same error.

[Torsten]

x and y must have the same length (y is the measured value at time x). Your y has one element less than x.

And the other error remains. Either you specify A0 or A1 as initial guess for the parameters you want to determine.

[Sonisa]

I still get the same error. What does this error mean?

Error using nlinfit (line 185) Requires a vector second input argument.

[Torsten]

It means that the second argument in the call to nlinfit must be a vector. In your case, x is a scalar and y is empty.

Try

x = X;
y = Y;

instead of

x = X(1:end,:);
y = Y(2:end,:);

Best wishes

Torsten.

[Sonisa]

This also doesn't work.

[Matt J]

Please give us the version of the code that you are actually running. The code you have posted gives a different error,

Undefined function or variable 'x'.

Error in test (line 8)
k = diff(x)<0; %select only data when the soil is drying

[Sonisa]

x = 0.637259937 0.621294757 0.672668532 0.521265646 0.421903196 0.549649009 0.312588087 0.28327584 0.359424364 0.253502479 0.253985122 0.313290724 0.209563459 0.2319699 0.274603618 0.156334057 0.183523167 0.236265826 0.475258711 0.560432822 0.629973282 0.238506777 0.253712482 0.302720237

y = 0.611673879 0.672925747 0.681451013 0.577414406 0.702284322 0.680381636 0.470820795 0.554830443 0.577386675 0.405224584 0.534853857 0.530397181 0.391375854 0.42629674 0.366951257 0.344208072 0.360370785 0.342645703 0.365246711 0.471838895 0.41643422 0.401104282 0.397572023 0.411658118

[Matt J]

Nope. Those lines will not execute

>> x = 0.637259937  0.621294757  0.672668532 0.521265646  0.421903196  0.549649009 0.312588087 0.28327584  0.359424364 0.253502479  0.253985122  0.313290724 0.209563459  0.2319699  0.274603618 0.156334057  0.183523167  0.236265826 0.475258711  0.560432822  0.629973282 0.238506777  0.253712482 0.302720237
 x = 0.637259937  0.621294757  0.672668532 0.521265646  0.421903196  0.549649009 0.312588087 0.28327584  0.359424364 0.253502479  0.253985122  0.313290724 0.209563459  0.2319699  0.274603618 0.156334057  0.183523167  0.236265826 0.475258711  0.560432822  0.629973282 0.238506777  0.253712482 0.302720237
                 ↑
Error: Invalid expression. Check for missing multiplication operator, missing or unbalanced
delimiters, or other syntax error. To construct matrices, use brackets instead of
parentheses.

[Sonisa]

Look at this file and hope it will works.