Why will my function not run?

Question

James Crowe 2017-11-20

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/368176-why-will-my-function-not-run

评论： Walter Roberson 2020-12-18

Hi all, why will my function not run? If p = 1 I want to find the determinant of A, if p = 2 I want to find the trace of A and if p doesn't equal 1 or 2 I want to display the error message. Is my code correct, or is there a better way to do it? And why does the function not run. Thank you!

function [det(A),tr(A)] = myFunction(A,p)
if p == 1
  det(A) = det(A);
elseif p == 2
  tr(A) = trace(A);
else 
  disp ('Invalid input')
end

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Answer 1

Walter Roberson 2017-11-20

1
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https://ww2.mathworks.cn/matlabcentral/answers/368176-why-will-my-function-not-run#answer_292184

在 MATLAB Online 中打开

function [detA,trA] = myFunction(A,p)
if p == 1
  detA = det(A);
elseif p == 2
  trA = trace(A);
else 
  disp ('Invalid input')
end

However, you are going to have the problem that in each case you are not assigning to one of the output variables. You should check: does the assignment expect you to return the two possibilities to different variables or to just one output?

   function result = test(something)
     if something == 1
       result = 15;
    elseif something == 2
       result = -10;
    else
       result = 'What?'
    end

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Vlad Atanasiu 2020-12-18

在 MATLAB Online 中打开

So: If any outputs of a function are undefined, then Matlab throws an error.

Write a function were the first outputs are not defined:

function [a,b,c] = undefined_output(x)
c = x;

Call the function:

try
    [a,b,c] = undefined_output('no error');
    disp(c)
catch
    disp('error')
end

Answer:

error

Where is this behavior specified in the Matlab documentation?

Walter Roberson 2020-12-18

在 MATLAB Online 中打开

Not exactly correct. There is only an error if an unassigned output position is assigned to a variable or used in an expression.

try
    [a,b] = undefined_output('no error c unused');
    disp(b)
catch
    disp('error')
end
b okay
try
    [a,b,c] = undefined_output('no error c');
    disp(b)
    disp(c)
catch
    disp('error')
end
error
function [a,b,c] = undefined_output(x)
a = x;
b = 'b okay';
end