something wrong in fit function

Question

Ilan Tal 2024-1-7

0
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https://ww2.mathworks.cn/matlabcentral/answers/2067286-something-wrong-in-fit-function

评论： Dyuman Joshi 2024-1-7

I am using matlab in a mooc and apparently missing some point. The code is

% Create the custom logistic fittype with fit parameters k and c
ft = fittype( 'y= 1./(1+exp(-k*(x-c)))',...
'Independent','x', 'Dependent','y',...
'Coefficients',...
('k','c'));

The error message is

File: solution.m Line: 9 Column: 5

Invalid expression. When calling a function or indexing a variable, use parentheses. Otherwise, check for mismatched delimiters.

The problem seems to be the comma between 'k' and 'c'. Since there are 2 parameters I need a comma between them. I have tried everything I can think of but I can't get past the error messages (which are slightly different depending on the exact format of the expression). In all cases it seems to be complaining about that comma.

I have never used matlab for an actual project, so maybe I am missing something obvious. I have been working on it for hours and can't get it to fly. Could someone with more experience please tell me what my actual error is? It makes no sense that it is really the comma.

Thanks,

ilan

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Answer 1

Dyuman Joshi 2024-1-7

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在 MATLAB Online 中打开

You can use either a cell array of char vectors or string array for multiple coefficients (or dependent/independent variables for that matter).

Remove the 'y = ' part, as that is not needed.

% Create the custom logistic fittype with fit parameters k and c
ft = fittype('1./(1+exp(-k*(x-c)))',...
    'Independent','x', 'Dependent','y',...
    'Coefficients', {'k', 'c'})
ft = 
     General model:
     ft(k,c,x) = 1./(1+exp(-k*(x-c)))
%Testing
x = linspace(1, 100);
k = randi(5)
k = 1
c = randi(5)
c = 5
y = 1./(1+exp(-k*(x-c)));
myfit = fit(x',y',ft)
Warning: Start point not provided, choosing random start point.
myfit = 
     General model:
     myfit(x) = 1./(1+exp(-k*(x-c)))
     Coefficients (with 95% confidence bounds):
       k =           1  (1, 1)
       c =           5  (5, 5)