eval is giving wrong results

Question

Igor de Britto 2012-3-25

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/33411-eval-is-giving-wrong-results

评论： Stephen23 2014-10-15

Hi, all

I do have a function running within some loops and create variables with dynamically generated names based on the values of the loops.

The result of my function is a 1000 x 50 matrix of integers. I want to plot the mean of each collumn. Therefore, here's the code

% Distribuição de espécies em grupos
for tipoQSG = tipoQSGmin:tipoQSGmax
    for gradienteQSG = gradienteQSGmin:gradienteQSGmax
% Interações interespecíficas
for tipoSIS = tipoSISmin:tipoSISmax
    for quantidadeSIS = quantidadeSISmin:quantidadeSISmax
        for concentracaoSIS = concentracaoSISmin:concentracaoSISmax
            for sinaisSIS = sinaisSISmin:sinaisSISmax
                for aleatorizacaoSIS = aleatorizacaoSISmin:aleatorizacaoSISmax
% Efeito de grupos funcionais
for tipoSEG = tipoSEGmin:tipoSEGmax
    for sinalSEG = sinalSEGmin:sinalSEGmax
MPDF = myfunc(tipoQSG, gradienteQSG, tipoSIS, quantidadeSIS, concentracaoSIS, sinaisSIS, aleatorizacaoSIS, tipoSEG, sinalSEG)

end end end end end end end end end

I have no problems doing what I need if I type manually, for instance:

plot(mean(MPDF_111111111))

However, when I try to do it in a dynamic way, it fails.

% Distribuição de espécies em grupos
for tipoQSG = tipoQSGmin:tipoQSGmax
    for gradienteQSG = gradienteQSGmin:gradienteQSGmax
% Interações interespecíficas
for tipoSIS = tipoSISmin:tipoSISmax
    for quantidadeSIS = quantidadeSISmin:quantidadeSISmax
        for concentracaoSIS = concentracaoSISmin:concentracaoSISmax
            for sinaisSIS = sinaisSISmin:sinaisSISmax
                for aleatorizacaoSIS = aleatorizacaoSISmin:aleatorizacaoSISmax
% Efeito de grupos funcionais
for tipoSEG = tipoSEGmin:tipoSEGmax
    for sinalSEG = sinalSEGmin:sinalSEGmax
      % Checa-se a execução satisfatória do cenário adotado anteriormente. Se
      % ele já tiver sido executado em número suficiente de vezes,
      % ignora-se-o e passa-se ao próximo.
          % Checa se os resultados existem.

% dynamical plot attempt eval(['plot(' 'mean(' 'sprintf(''MPDF_%d%d%d%d%d%d%d%d%d'', tipoQSG, gradienteQSG, tipoSIS, quantidadeSIS, concentracaoSIS, sinaisSIS, aleatorizacaoSIS, tipoSEG, sinalSEG)' ')' ')' ])

        % manual plot attempt
        plot(genvarname(sprintf('MPDF_%d%d%d%d%d%d%d%d%d', tipoQSG, gradienteQSG, tipoSIS, quantidadeSIS, concentracaoSIS, sinaisSIS, aleatorizacaoSIS, tipoSEG, sinalSEG)))
        hold on
        % Try to see if the variable name might be wrong
        eval(['sprintf(''MPDF_%d%d%d%d%d%d%d%d%d'', tipoQSG, gradienteQSG, tipoSIS, quantidadeSIS, concentracaoSIS, sinaisSIS, aleatorizacaoSIS, tipoSEG, sinalSEG)' ]) == MPDF_111111111
    end
end
                end
            end
        end
    end
end
    end
end

Why does it returns me a vector for the mean(MPDF_111111111), but a single value for the for the eval equivalent? How can I remedy this problem?

Thanks!

2 个评论
显示无隐藏无

Igor de Britto 2012-3-25

Managed to get MPDF_111111111 and the loop variables on the question!

Stephen23 2014-10-15

In case the message got lost: DO NOT USE EVAL for calling trivial functions like sprintf. This is a topic that has been discussed a thousand times before...

Eval isn't fast, it is slower than just calling any function directly.
Eval obscures the code intent. Totally.
Eval is not compiled for optimized running. Every call has to eval all over again!
Eval makes debugging almost impossible.
Eval can produce different outputs in normal and debug modes.
Eval can create and overwrite variables in workspaces.
Eval is often associated with other practices that are not an efficient use of MATLAB... sequential variable names, for example.

These topics have been covered many times in MATLAB's official documentation, blogs and other discussions:

http://blogs.mathworks.com/loren/2005/12/28/evading-eval/

http://www.mathworks.com/matlabcentral/answers/51946-systematic-do-not-use-global-don-t-use-eval

http://www.mathworks.com/help/matlab/matlab_prog/string-evaluation.html

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Daniel Shub 2012-3-25

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/33411-eval-is-giving-wrong-results#answer_41974

在 MATLAB Online 中打开

I don't really understand your question and we cannot run your code ...

It looks like:

plot(genvarname(sprintf('MPDF_%d%d%d%d%d%d%d%d%d', tipoQSG, gradienteQSG, tipoSIS, quantidadeSIS, concentracaoSIS, sinaisSIS, aleatorizacaoSIS, tipoSEG, sinalSEG)))

is trying to plot a string

plot('MPDF_111111111')

is odd.

Here

eval(['sprintf(''MPDF_%d%d%d%d%d%d%d%d%d'', tipoQSG, gradienteQSG, tipoSIS, quantidadeSIS, concentracaoSIS, sinaisSIS, aleatorizacaoSIS, tipoSEG, sinalSEG)' ]) == MPDF_111111111

you seem to be comparing

eval('MPDF_111111111') == MPDF_111111111

which again is odd.

Overall I guess my answer is: DON'T USE EVAL!!!!

It makes debugging a pain.

Taking another look: Are you trying to do

eval('plot(mean(MPDF_111111111))')