Undefined function or variable 'isnan'

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Carver Nabb 2019-1-7

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https://ww2.mathworks.cn/matlabcentral/answers/438669-undefined-function-or-variable-isnan

评论： Walter Roberson 2021-4-19

采纳的回答： Walter Roberson

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I am met with the error "Undefined function or variable 'isnan'" when running this code:

%counter = 1:4
%a is a 220x4 cell
%data is a 220x4 double
for i = size(a, 1)
    pairs = [];
    pairs(:,1) = counter;
    pairs(:,2) = data(i,:);
    pairs = pairs(~isnan(pairs(:,2),:));
    slope(i) = polyfit(pairs(:,1),pairs(:,2),1);
end

I can't seem to figure out exactly why. Is the way I'm using the NOT operator?

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Carver Nabb 2019-1-7

Screen Shot 2019-01-07 at 2.54.36 PM.png

nanren888 2019-1-7

As Walter says; It is complaining about the two-parameter isnan(x,y) that you've invented :)

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Walter Roberson 2019-1-7

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You have

pairs = pairs(~isnan(pairs(:,2),:));

You want

pairs = pairs(~isnan(pairs(:,2)),:);

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Walter Roberson 2019-1-7

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It is an oddity of the parser, which has some messy behaviour for colon.

functionname(argument, :)

will disable recognition of functionname as a function and will look for a variable with that name, probably resulting in the misleading error message.

A = reshape(1:60, 2, []);
A(1:2)
%1:2 gets computed as [1 2] first, then the datatype of A is looked up
%and since it is a variable, subsref is invoked to select the first two elements
A('1:2')
%'1:2' is a vector of characters, equivalent to char([49 58 50]) .
%The numeric equivalent is taken, [49 58 50] and are used as subscripts into A
A(:)
%the : is transformed into subsref() with type '()' and index {':'}
%which is treated by subsref as a reshape request
A(':')
%the ':' with the character scalar ':' is transformed into subsref() with
% type '()' and index {':'} which is treated as a reshape request
A(1, :)
%this is transformed into subsref() with type '()' and index {1, ':'}
%which is treated by subsref as a subset indexing request
A(1, ':')
%this is transformed into subsref() with type '()' and index {1, ':'}
%which is treated by subsref as a subset indexing request
A(1, '1:2')
%this is transformed into subsref() with type '()' and index {1, '1:2'}
%which is treated by subsref as a subset indexing request with
%the '1:2' being treated as equivalent to [49 58 50]

but

functionname(argument, ':')

is not treated as a subsref request.

Carver Nabb 2019-1-8

Thank you, this does produce non-zero values. However, they are not the correct slopes, for example, I know the first slope ought to be 3.6, and the last, -4.6. However the first value returned with this code is 113.

Carver Nabb 2019-1-8

I figured it out! Walter and Jan, thank you so much for the help. I do appreciate it.

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Jan 2019-1-7

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Although I assume that Walter hit the point already, I suggest to use teh debugger to identify such problems. Type in the command window:

dbstop if error

Now run the code again until it stops. Check the currently processed line by evaluating it piecewise:

% pairs = pairs(~isnan(pairs(:,2),:))
pairs(:,2)
~isnan(pairs(:,2),:)  % <- This should fail already
pairs(~isnan(pairs(:,2),:))

As Walter has said already, isnan(x, :) is meaningful only, if it is an array. Otehrwise using the colon as second input is not valid, if the function isnan() is meant. The error message "Undefined function or variable 'isnan'" is not really clear here. But splitting the commands into parts using the debugger helps to understand the problem.