Sum two values coming from eval

Question

Davide Di Pasquale 2018-12-10

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/434905-sum-two-values-coming-from-eval

评论： Stephen23 2019-1-30

采纳的回答： Guillaume

Hi,

I would like to ask how to sum two numerical values, which coming from the eval fucntion

set(handles.A_edit,'string',eval(char(strcat('handles.forces.',gridnames(gridnum,1),'.total.B_IBE'))));

+

set(handles.A_edit,'string',eval(char(strcat('handles.forces.',gridnames(gridnum,1),'.total.C_IBE'))));

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Davide Di Pasquale 2018-12-10

Hi KSSV,

just picked up an existing code that used this eval function.

I have just to sum two force component.

Regards

Stephen23 2018-12-10

编辑：Stephen23 2018-12-10

"just picked up an existing code that used this eval function."

For more than ten years it has been recommended to avoid using eval to dynamically access structure fieldnames, since MATLAB 6.5:

https://blogs.mathworks.com/loren/2005/12/13/use-dynamic-field-references/

So whoever wrote that code is badly informed and uses poor programming practices. Do NOT learn from that code!

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Answer 1

Guillaume 2018-12-10

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https://ww2.mathworks.cn/matlabcentral/answers/434905-sum-two-values-coming-from-eval#answer_351473

在 MATLAB Online 中打开

Plenty of problems with your code.

For a start set as you use it does not return a value, so there is nothing to sum. I'm going to assume that's the output of eval you want to sum. Nothing is stopping you to sum these output before you call set.

2nd problem: Can the B_IDE ad C_IBE be summed? For that they would need to be numeric. But the string property of uicontrol expects text. Perhaps the sum should be converted to text, but you don't show that.

3rd problem: I don't see the point of the char. strcat already outputs a char array. Unless gridnames is a cell array of course, but in that case, clearly you don't know how to extract values from a cell array (using {}). In that case,

char(strcat('handles.forces.',gridnames(gridnum,1),'.total.B_IBE'))

should be:

strcat('handles.forces.',gridnames{gridnum,1},'.total.B_IBE')  %and if gridnames is a vector cell array 2d indexing is a waste of time.

And of course, 4th problem is the use of eval. It's completely unnecessary as well here as there's much simpler syntax to obtain the same.

In the end, I'm going to guess that what you want to do is:

B_total = handles.forces.(gridnames{gridnum}).total.B_ICE;
C_total = handles.forces.(gridnames{gridnum}).total.C_ICE;
set(handles.A_edit, 'string', num2str(B_total + C_total))

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Stephen23 2019-1-30

Davide Di Pasquale's "Answer" moved here:

Hi Guillaume,

Now the problem is solved:

    B_total = strcat(handles.forces.(gridnames{gridnum}).total.VISCOUSDRAG_TE);
    C_total = strcat(handles.forces.(gridnames{gridnum}).total.CD_vortd);
    set(handles.cd_edit, 'string', num2str(str2double(B_total)+ str2double(C_total)));

Thanks