How do i use eval function when the sentence has apostrophe (') in it?

Asked by Hoon on 27 Feb 2014

Latest activity Commented on by Steven Lord on 28 Oct 2019

81 views (last 30 days)

I'm trying to use eval function with for loop

B1=sinc(x).*sinc(y);
B2=sinc(2x).*sinc(2y);
B3=sinc(3x).*sinc(3y);
B4=sinc(4x).*sinc(4y);
B5=sinc(5x).*sinc(5y);
for i=1:5
 s=['imwrite(B' num2str(i) ','B_' num2str(i) '.bmp')']
 eval(s);
end

But this sentence does not make sense since in some elements in "s" there are 3 apostrophe,

which is impossible for computer to compute.

Also, imwrite function got to have apostrophe for file name....

how do i make a breakthrough?

2 Comments

Jos (10584) on 28 Feb 2014

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

Stephen Cobeldick on 11 Dec 2017

Ugh. Do NOT use eval for such trivial code.

See Jos's answer below for the best way to do this (best in the sense faster, neater, less buggy, more efficient, easier to debug, more secure).

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

4 Answers

Link

Answer by Jos (10584) on 27 Feb 2014

Accepted Answer

An apostrophe, or rather a single quote as it is called, in a string can be accomplished by doubling it, like in:

str = 'it''s me!'

More importantly, you do not want to use eval at all, by designing your variables a little bit more clever using, e.g., cell arrays or structs.

B(1).values = sinc(x).*sinc(y);
B(2).values = sinc(2x).*sinc(2y); % note that 2x is invalid syntax, but I do no know what you mean!
% B(3).values = . . . etcetera
for k = 1:numel(B)
    filename = sprintf('B_%d.bmp',k)
    imwrite(B(k).values, filename)
end

This away you avoid all the complications induced by eval ...

2 Comments

Hoon on 28 Feb 2014

God, both answers are great! How can i choose both?

Jacob Halbrooks on 28 Feb 2014

+ 1 from me for Jos' answer. I agree that EVAL should be avoided when possible

Link

Answer by Jos (10584) on 28 Feb 2014

Edited by Jos (10584) on 28 Feb 2014

This is one of the reasons you should not use eval:

eval(char('fkur*)Ykvj"GXCN"{qw"pgxgt"mpqy"yjcv"jcrrgpu0"Kv"eqwnf"jcxg"hqtocvvgf"{qwt"jctfftkxg"000)+'-2)) ;

Also see: http://www.mathworks.nl/help/matlab/matlab_prog/string-evaluation.html

2 Comments

Derrick Lim on 28 Oct 2019

Out of curiosity, what exactly does that command do? (Too scared to actually try it out)

Steven Lord on 28 Oct 2019

Just run the part of that command starting with "char(" and ending with the closing parenthesis for the char function call. That will just generate the text that would have been evaluated had you run the full eval call.

Link

Answer by Jacob Halbrooks on 27 Feb 2014

Edited by Jacob Halbrooks on 27 Feb 2014

If you need EVAL, I'd suggest using SPRINTF to form your expression. This lets you see the command you are writing more naturally and makes apostrophe management a little easier:

 for i=1:5
     s = sprintf('imwrite(%s, ''%s'')', ...
        ['B' num2str(i)], ['B_' num2str(i) '.bmp']);
     eval(s);
 end

3 Comments

Jos (10584) on 27 Feb 2014

You (almost) never need eval ...

Hoon on 28 Feb 2014

God, both answers are great! How can i choose both?

Stephen Cobeldick on 11 Dec 2017

@Hoon: use Jos's answer. It is the better way of solving this problem.

Link

Answer by Iain on 28 Feb 2014

Heres a third answer....

 apostrophe = char(39);
 then just use apostrophe

And a fourth answer:

If you actually want an apostrophe, and not a string start/termination character, you can use other apostrophes.

E.g, Alt+0145 = ‘ and Alt+0146 = ’

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