Error : Output argument "y" (and maybe others) not assigned during call to "N".

Question

Kiran Kumar Asokan 2019-9-7

0
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https://ww2.mathworks.cn/matlabcentral/answers/479336-error-output-argument-y-and-maybe-others-not-assigned-during-call-to-n

评论： Walter Roberson 2019-9-8

I am currently working on building B spline curves basis function which is a recursive function. I am always getting this error Output argument "y" (and maybe others) not assigned during call to "N". Please let me know how to sort this.

function y = N(k,i,t,m)
%k is the order 
%i is the pointer for the control point 
%t (knot value) is the parameter used for defining a point on the curve
%m is the knot vector
% for the knot value t to be in between the ti and ti+k
if t < m(1,i+k+1)
    if t > m(1,i+1)
               
if k == 1
   y = 1;
end
for h = 2:1:k
    
   y = ((((t - m(1,i+1))/ (m(1,i+h)- m(1,i+1)))* N(h-1,i,t,m)) + (((m(1,i+h+1) - t)/ (m(1,i+h+1)- m(1,i+2)))* N(h-1,i+1,t,m)));
   
end 
    else 
        y = 0;
    end 
end
end

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Stephen23 2019-9-8

"Please let me know how to sort this."

Use consistent code indentation.

Your code is very badly indented. Badly indented code is one way that beginners hide bugs in their code, by making the logic of the code hard to understand.

Make your code easier to understand by using consistent indexing (e.g. the MATLAB default indexing: select all code text, ctrl+i). Then the reason for your unassigned output is obvious.

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Answer 1

Star Strider 2019-9-7

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在 MATLAB Online 中打开

The easiest way is to assign a value to ‘y’ at the outset. It will be re-assigned if the if blocks allow it.

Example —

function y = N(k,i,t,m)
y = NaN;
%k is the order 
... rest of the function code ...
end

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Kiran Kumar Asokan 2019-9-8

Screenshot 2019-09-08 at 12.24.42 AM.png

I am still getting random outputs. I have attached the required function below. Can anyone help me know whats wrong ?

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Answer 2

Walter Roberson 2019-9-7

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Reformat your code:

if t < m(1,i+k+1)
    if t > m(1,i+1)
                       
        if k == 1
           y = 1;
        end
        for h = 2:1:k
            
           y = ((((t - m(1,i+1))/ (m(1,i+h)- m(1,i+1)))* N(h-1,i,t,m)) + (((m(1,i+h+1) - t)/ (m(1,i+h+1)- m(1,i+2)))* N(h-1,i+1,t,m)));
           
        end 
    else 
        y = 0;
    end 
end

Notice that the else is associated with the inner if t > m(1,i+1) and so is not invoked in the case where t < m(1,i+k+1) is false. Therefore if t < m(1,i+k+1) is false, nothing is assigned to y.