Is there any other way to do the job without using eval?

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?

[G A]

By the way, what about subs?

>> a='[1,2,3]';

>> subs(a)

ans =

     1     2     3

Answer 1

[Azzi Abdelmalek]

A=str2double(get(handles.A_edit,'String'))

[G A]

 a='[1,2,3]'

a =

[1,2,3]

>> str2double(a)

ans =

   NaN

[Azzi Abdelmalek]

Use

Str2num

[G A]

thanks, it does work

>> a='[1,2,3]'

a =

[1,2,3]

>> str2num(a)

ans =

     1     2     3

[Jan]

@G A: str2num is based on an eval also, such that it does not match the match the restrictions of the question.

[G A]

And, I suspect, subs as well?

[G A]

Jan, which way then is better to use: str2num, which is, as you say, based on eval or directly eval?

[G A]

>> a='1:100000000'

a =

1:100000000

>> tic;b=eval(a);toc  
Elapsed time is 0.307041 seconds. 
>> tic;c=str2num(a);toc
Elapsed time is 0.313607 seconds.

So eval is slightly faster.

[Jan]

@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.

[G A]

Thanks, Jan!

In some cases I also imput vectors using three fields as you have shown. However I did not think that so many hidden problems with str2num and eval exist.

Answer 2

[Walter Roberson]

T1 = regexprep(get(handles.A_edit,'String'), '\[|\]', ' ');
T2 = regexp(T1, '[,;\s]', 'split');
Acell = str2double(T2);
A = cell2mat(Acell);

[G A]

 T1 = regexprep('[1,2,3,4]', '\[|\]', ' ')

T1 =

 1,2,3,4

>> T2 = regexp(T1, '[,;\s]', 'split')

T2 =

    ''    '1'    '2'    '3'    '4'    ''

>> Acell = str2double(T2)

Acell =

   NaN     1     2     3     4   NaN

>> A = cell2mat(Acell)

Cell contents reference from a non-cell array object.

[Walter Roberson]

Revised:

T1 = regexprep(get(handles.A_edit,'String'), '\[|\]', '');
T2 = regexp(T1, '[,;\s]', 'split');
A = str2double(T2);

This would need more work if you expected the user to be able to enter two-dimensional arrays instead of vectors.

[G A]

Thanks, !

How to deal with this kind of input:

'2:2:10' 
or
'1:10'

instead of

'[2,4,6,8,10]' ?

[Walter Roberson]

if any(S == ':')
  T2 = regexp(S, ':', 'split');
  if length(T2) == 2 || (length(T2) == 3 & isempty(T2{2}))
    T2(2:3) = {'1' T2{3}};
  end
  if length(T2) == 1
    error('not enough roughage for colon')
  elseif length(T2) == 3
    A = str2double(T2{1}) : str2double(T2{2}) : str2double(T2{3});
  else
    error('too much roughage for colon')
  end
else
  %no colon to process
end

[G A]

Thanks! I can now use your code as new function A=str2num1(S)

[G A]

the code works allright with S='2:2:10' but with S='1:5' generates an error

Index exceeds matrix dimensions.

Error in teststr2num1>str2num1 (line 4)

    T2(2:3) = {'1' T2{3}};

[Walter Roberson]

Hmmm. Okay.

if length(T2) == 2
  T2(2:3) = {'1' T2{2}}
elseif length(T2) == 3 & any(cellfun(@isempty, T2))
  error('Cannot colonize empty fields.')
end

[G A]

Now it works OK. Thanks again, Walter! Whole function:

function A=str2num1(S)
if any(S == ':')
  T2 = regexp(S, ':', 'split');
    if length(T2) == 2
  T2(2:3) = {'1' T2{2}};
    elseif length(T2) == 3 && any(cellfun(@isempty, T2))
  error('Cannot colonize empty fields.')
    end
  if length(T2) == 1
    error('not enough roughage for colon')
  elseif length(T2) == 3
    A = str2double(T2{1}) : str2double(T2{2}) : str2double(T2{3});
  else
    error('too much roughage for colon')
  end
else
  %no colon to process
T1 = regexprep(S, '\[|\]', '');
T2 = regexp(T1, '[,;\s]', 'split');
A = str2double(T2);
end

[G A]

one problem

g='[1  2  3  4]';%double spaces

G=str2num1(g)
G =

     1   NaN     2   NaN     3   NaN     4

d='[1,  2,  3,  4]';%spaces

D=str2num1(d)

D =

     1   NaN     2   NaN     3   NaN     4

[Walter Roberson]

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.

[G A]

Thanks, Walter, and Happy New Year!