jonathanasdf

Serious Dabbler

scanf specifying set of valid characters

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Today, as I was being frustrated over yet again the absence of a scanf function that reads into an std::string, or more generally of the fact that most of the input/output functions still deal with char* and then need to be converted to std::string, I stumbled upon this neat little fact that was not mentioned on the cplusplus reference page, and that is the ability to specify a set of characters to extract using scanf instead of %s.

Quote from the man page of scanf:

[ –  Matches a nonempty sequence of characters from the specified set of accepted characters; the next pointer must be a pointer to char, and there must be enough room for all the characters in the string, plus a terminating null byte. The usual skip of leading white space is suppressed. The string is to be made up of characters in (or not in) a particular set; the set is defined by the characters between the open bracket [ character and a close bracket ] character. The set excludes those characters if the first character after the open bracket is a circumflex (^). To include a close bracket in the set, make it the first character after the open bracket or the circumflex; any other position will end the set. The hyphen character – is also special; when placed between two other characters, it adds all intervening characters to the set. To include a hyphen, make it the last character before the final close bracket. For instance, [^]0-9-] means the set “everything except close bracket, zero through nine, and hyphen”. The string ends with the appearance of a character not in the (or, with a circumflex, in) set or when the field width runs out.

Example: scanf(“%[a-c]”, buffer) would only extract the characters that are a, b, and c until a character that isn’t a, b, or c is found.

I’ve never heard this mentioned anywhere, so I thought I’d share this finding. Sadly this is still very primitive compared to regex. Anyways, hopefully we can in the near future move away from scanf and towards better functions.

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Written by jonathanasdf

January 27, 2011 at 11:38 PM

Posted in C++

Tagged with ,

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