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From: vd09+@andrew.cmu.edu (Vincent M. Del Vecchio)
Newsgroups: comp.sys.mac.programmer
Subject: Re: New Think C user
Message-ID: <EbguSSy00awQErA91F@andrew.cmu.edu>
Date: 9 Feb 91 07:41:18 GMT
References: <21005@sri-unix.SRI.COM>
Organization: Carnegie Mellon, Pittsburgh, PA
Lines: 42
In-Reply-To: <21005@sri-unix.SRI.COM>

mxmora@sri-unix.SRI.COM (Matt Mora) writes:
> I have a beginning C programming question. This question came up in my 
> C class and the instructor didn't now the anwser. Why do you have to use
> a double percent sign in a string literal to have it print a percent sign 
> when all logic would indicate that a backslash percent sign should work?
> 
A subtle difference.  The thing you need to know is that \a, \b, \n, \\, etc.,
get interpreted by the compiler.  "foo\n" is represented as 4 characters after
compilation.  I'm not sure what K&R says about "\%", for example, or other
combinations of "\<char>" which aren't specifically covered.  I imagine that it
would either be compiled to { '\\', '%' } or just { '%' } but I'm not sure
which.  The important point is that the backslashes are simply a device to
represent otherwise unrepresentable characters to the compiler.

The % operands, on the other hand, have no significance to the compiler and are
perfectly legitimate parts of strings.  For example, you can
char a[10] = "%";
char *b = "d";
int i = 12;
printf(strcat(a,b),i);

and you will get "12".  On the other hand, you can't do
char a[10] = "\";
char *b = "n";
printf(strcat(a,b));

since a "\n" is actually a single character (here the compiler would complain
that the string constant "\" is unterminated).

The % operands only have special meaning to printf and scanf.  When printf sees
a "%" in your format string, it looks at the next couple of characters to see
what kind of data you are sending it.

printf could have been written to accept "\\%" (2 characters) as an instruction
to print a single percent, but that would involve making '\\' a special
character as well, and within printf, '\\' is not a special character.  Also,
then you would have to give it "\\\\%" to get it to print \% on your screen.
Also, printf follows the more general convention that when you have a sort of
escape character (like '\\' at the compiler level, or '%' at the printf level),
doubling it removes any special meaning the character has.

Hope this answers your question...