Is there any other way to do the job without using eval?
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Matlab experts say ' eval is evil'. I use eval in the case when I read Edit Text data in GUI and str2double is not applicable. For example, I need to type, read and save (in cell-array) an arbitrary array with arbitrary length, say A=[1,5,9]. I do:
set(handles.A_edit,'String','[1,5,9]')
C{1}=get(handles.A_edit,'String');
A=eval(C{1});
Is there any other way to do the job without using eval?
1 Kommentar
Akzeptierte Antwort
Azzi Abdelmalek
am 30 Dez. 2012
Bearbeitet: Azzi Abdelmalek
am 30 Dez. 2012
A=str2double(get(handles.A_edit,'String'))
9 Kommentare
Jan
am 30 Dez. 2012
Bearbeitet: Jan
am 30 Dez. 2012
@G A: subs concerns symbolical calculations and here eval is a completely different story: neither evil nor prone to errors, but the best method to evaluate a symbolical expression numerically.
I recommend neither str2num nor the direct usage of eval. Try this:
str2num('system(''cmd'')')
If you are stuck in the command shell interaction inside Matlab's command window, type "exit" to stop the nonsense. Equivalent system calls are used frequently to crack web-based services using a standard input masks. Although you think you can trust the users of your Matlab GUI, it is a good programming practice to avoid a string evaluation ever, under all circumstances and in general and at all.
When I need to input a vector in a GUI, I implement three edit fields like (sorry for the poor graphics):
[edit1] : [edit2] : [edit3]
Then I check if the users has inserted values for all three fields and create the vector by:
v1 = sscanf(get(edit1_H, 'String'), '%g', 1);
v2 = sscanf(get(edit2_H, 'String'), '%g', 1);
v3 = sscanf(get(edit3_H, 'String'), '%g', 1);
... checks with ISNAN ...
v = colon(v1, v2, v3);
Of course eval and str2num would handle '[1:10, 11, 13:17, 27]' also, but also 'sum = 23' and this assigns a value to a symbol, which is a built-in function and in debug mode this will lead to extremely surprising effects. If the user types 'o' instead of '0' by accident, the results may be strange also, when 'o' is a variable already and the evil eval evaluates the string.
But, sigh, parsing inputs cannot be 100% secure. See e.g. EVAL bug and even sprintf allowed to obtain root privileges in older Matlab versions. So really evil users will break your program without eval also, but at least this happens less likely by mistakes and typos.
Weitere Antworten (1)
Walter Roberson
am 30 Dez. 2012
T1 = regexprep(get(handles.A_edit,'String'), '\[|\]', ' ');
T2 = regexp(T1, '[,;\s]', 'split');
Acell = str2double(T2);
A = cell2mat(Acell);
11 Kommentare
Walter Roberson
am 31 Dez. 2012
T2 = regexp(T1, '[,;\s]+', 'split');
Note: with this modification, if the user enters (say) '[1,,,3]' then it would become [1 3] and the multiple delimiters would be ignored. If you want more error checking in the "no colon" section,
if regexp(S, '[\[,;\[]\s*[\[,;\[]')
error('multiple delimiters')
end
Mind you this will also reject '[[1],[2]]'. To do better, you need to define exactly which syntaxes you wish to permit.
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