CODEWITHSUNDEEP
cscanf
velocirabbit
velocirabbit

Reputation: 706

Why does this scanf() conversion actually work?

Ah, the age old tale of a programmer incrementally writing some code that they aren't expecting to do anything more than expected, but the code unexpectedly does everything, and correctly, too.

I'm working on some C programming practice problems, and one was to redirect stdin to a text file that had some lines of code in it, then print it to the console with scanf() and printf(). I was having trouble getting the newline characters to print as well (since scanf typically eats up whitespace characters) and had typed up a jumbled mess of code involving multiple conditionals and flags when I decided to start over and ended up typing this:

(where c is a character buffer large enough to hold the entirety of the text file's contents)

scanf("%[a-zA-Z -[\n]]", c);
printf("%s", c);

And, voila, this worked perfectly. I tried to figure out why by creating variations on the character class (between the outside brackets), such as:

[\w\W -[\n]]
[\w\d -[\n]]
[. -[\n]]
[.* -[\n]]
[^\n]

but none of those worked. They all ended up reading either just one character or producing a jumbled mess of random characters. '[^\n]' doesn't work because the text file contains newline characters, so it only prints out a single line.

Since I still haven't figured it out, I'm hoping someone out there would know the answer to these two questions:

I know this probably isn't a good usage for scanf, but since it comes from a practice problem, real world convention has to be temporarily ignored for this usage.

Thanks!

Upvotes: 2

Views: 161

Answers (2)

M.M
M.M

Reputation: 141586

If you include - inside the [ (other than at the beginning or end) then the behaviour is implementation-defined.

This means that your compiler documentation must describe the behaviour, so you should consult that documentation to see what the defined behaviour is, which would explain why some of your code worked and some didn't.

If you want to write portable code then you can't use - as anything other than matching a hyphen.

Upvotes: 0

Vaughn Cato
Vaughn Cato

Reputation: 64308

You are picking up numbers because you have " -[" in your character set. This means all characters from space (32) to open-bracket (91), which includes numbers in ASCII (48-57).

Your other examples include this as well, but they are missing the "a-zA-Z", which lets you pick up the lower-case letters (97-122). Sequences like '\w' are treated as unknown escape sequences in the string itself, so \w just becomes a single w. . and * are taken literally. They don't have a special meaning like in a regular expression.

Upvotes: 3

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