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Hoon
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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
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

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).

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4 Answers

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

God, both answers are great! How can i choose both?
+ 1 from me for Jos' answer. I agree that EVAL should be avoided when possible

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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)) ;

  2 Comments

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

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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

You (almost) never need eval ...
God, both answers are great! How can i choose both?
@Hoon: use Jos's answer. It is the better way of solving this problem.

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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 = ’

  0 Comments

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