Finding a character in a string?

Question

Johny 2013-12-3

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https://ww2.mathworks.cn/matlabcentral/answers/108376-finding-a-character-in-a-string

评论： Jos (10584) 2013-12-3

采纳的回答： The Matlab Spot

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so I have the following line of code:

hashtag=lower(input('Which hashtag would you like information on?','s'));

This prompts the user to input a hashtag, like say '#helloworld'. However, the program runs even without the user putting the hashtag. So it runs even if they only input "helloworld", which presents problems for me later on. How can I get the code to stop reading and present an error message if there is no hashtag symbol in the user's input? Apparently the # character means something in matlab as well which is causing me problems.

I have this so far but it seems not to work:

 if k=(strfind(hashtag,'#')))
    error('Please include the hashtag')
 end

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Answer 1

The Matlab Spot 2013-12-3

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using start anchor with regexp will ensure that you '#' as the first character of you input string.

hashtag=lower(input('Which hashtag would you like information on?','s'));
if(isempty(regexp(hashtag,'^#','once')))
    error('Please include the hashtag')
end

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Johny 2013-12-3

This answer is more robust than the one above, so best answer goes to you. I didn't know the ^ symbol did anything at all actually. I learned something!Thanks to both of you guys though.

Jos (10584) 2013-12-3

using regexp for this problem is like killing a mosquito with a nuclear missile ...

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Answer 2

sixwwwwww 2013-12-3

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Dear Johny, try this:

hashtag=lower(input('Which hashtag would you like information on?','s'));
if isempty(strfind(hashtag, '#'))
    error('Please include the hashtag')
end

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Answer 3

Jos (10584) 2013-12-3

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Per definition a hashtag starts with a # sign followed by one or more letters or numbers

So, quite simply, check all these three requirements:

str = lower(input('Which hashtag would you like information on?','s'));
if numel(str)<2 || str(1) ~= '#' || ~all(ismember(str(2:end),['a':z' '_' '0':'9']))
   error('Input is not a valid hashtag') ;
end