str2num strange behaviour

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Elena Casiraghi 2019-8-23

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/477278-str2num-strange-behaviour

评论： Elena Casiraghi 2019-8-24

Dear, I have this code:

svmKernel ={'rbf','gaussian', 'linear','2','3','4'};
for k=1:numel(svmKernel)
    [p, status] = str2num(svmKernel{k});
    if ~status; t = templateSVM('Standardize',true,'KernelFunction',svmKernel{indK});                    
    else; t = templateSVM('Standardize',true,'KernelFunction','polynomial', 'PolynomialOrder', p); end        

WHY str2num returns p =

    Linear Function:
    Parameters: [1×1 struct]

and status = 1 when issuing [p,status] = str2num('linear')?

   [p, status] = str2num('linear')
Linear Function:
    Parameters: [1×1 struct]
status =
  logical
   1

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Elena Casiraghi 2019-8-23

Thanks!

This is a good information!

Elena Casiraghi 2019-8-23

在 MATLAB Online 中打开

By the way, This is my result

which str2num
C:\Program Files\MATLAB\R2018a\toolbox\matlab\strfun\str2num.m

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Answer 1

Stephen23 2019-8-23

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https://ww2.mathworks.cn/matlabcentral/answers/477278-str2num-strange-behaviour#answer_388743

编辑：Stephen23 2019-8-23

在 MATLAB Online 中打开

"str2num strange behaviour"

It is not strange behavior at all: str2num (unfortunately) relies on eval, as the str2num help clearly states "...str2num uses the eval function, which can cause unintended side effects when the input includes a function name." You provided str2num a function name to evaluate... you can easily find out which function it is evaluating:

which linear -all

As the str2num documentation recommends, you could avoid the whole problem simply by using str2double (which could be called before the loop), then using isnan.

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Steven Lord 2019-8-23

在 MATLAB Online 中打开

Another solution would be to avoid trying to convert that text data into numbers entirely.

svmKernel = {{'rbf'}, ...
    {'gaussian'}, ...
    {'linear'}, ...
    {'polynomial', 'PolynomialOrder', 2}, ...
    {'polynomial', 'PolynomialOrder', 3}, ...
    {'polynomial', 'PolynomialOrder', 4}};
for k=1:numel(svmKernel)
    t = templateSVM('Standardize',true,'KernelFunction', svmKernel{k}{:});
end

The command svmKernel{k} extracts the contents of each cell inside svmKernel in turn. Since I have cell arrays inside each of those cells, I can turn them into a comma-separated list. This is the technique shown in the "Function Call Arguments" example in the "How to Use the Comma-Separated Lists" item on that documentation page.

Elena Casiraghi 2019-8-24