Re: string find
On Mar 29, 8:16 pm, Andre Kostur <nntps...@kostur.net> wrote:
"Gavin Deane" <deane_ga...@hotmail.com> wrote innews:1175185146.126237.61=
270@o5g2000hsb.googlegroups.com:
On 29 Mar, 17:12, "Mike Wahler" <mkwah...@mkwahler.net> wrote:
"Ko van der Sloot" <Ko.vanderSl...@uvt.nl> wrote in
messagenews:b313d$460bdd1f$89382996$2706@news1.tudelft.nl...
I was wondering which behaviour might be expected (or is required)
for the following small program.
I would expect that find( "a", string::npos ) would return
string::npos but is seems to be dependant on which string is
searched for.
On my system, using gcc 4.1.2, I get:
pos1=2
problem because pos1=0
I expected this
#include <iostream>
#include <string>
using namespace std;
int main(){
string use = "anu";
string::size_type pos1 = use.find_first_not_of( "an" );
cerr << "pos1=" << pos1 << endl;
pos1 = use.find( "a", string::npos );
if ( pos1 == string::npos )
cerr << "I expected this" << endl;
else
cerr << "problem because pos1=" << pos1 << endl;
pos1 = use.find( "q", string::npos );
if ( pos1 == string::npos )
cerr << "I expected this" << endl;
else
cerr << "problem because pos1=" << pos1 << endl;
}
Why are you giving 'npos' as the second argument to 'find()'?
Do you know the meaning of the second argument?
Do you know what 'npos' is for?
Might be an odd thing to want to do in practice,
Not really. Presumably, in practice, the position is the
results of an earlier find. And it's rather convenient to not
have to check. Consider something like the following:
std::string
getInBraces(
std::string const& s )
{
size_t pos1 = s.find( '{' ) ;
size_t pos2 = s.find( '}', pos1 ) ;
return pos2 == std::string::npos
? std::string()
: s.substr( pos1 + 1, pos2 - pos1 - 1 ) ;
}
It looks reasonable to me, and having to check before the second
call to find would make the code (slightly) more complicated.
but I'd be interested
in a definitive answer. My first thought was that maybe using npos as
the second argument to find has undefined behaviour, but I can't find
anything to back that up (doesn't mean it's not there of course). When
I compiled and ran the code using Visual Studio 2005, I got the output
I believe the OP ws expecting:
pos1=2
I expected this
I expected this
So I wonder whether the behaviour is undefined and I've just missed
where it says that, or has the OP found a compiler bug?
AFAIK, the second argument to find is an index into the string.
string::npos is not a valid index into a string, so I would expect the
same impact as specifying an out-of-bounds index to find.
What makes you say that? The standard doesn't place any such
restriction on it. In fact, the standard actually specifies as
a condition for not returning npos the fact that pos is in
range, which seems pretty clear to me. The argument is a
constraint on the return value; i.e. the value returned *must*
be <= pos (or npos, if a qualifying value cannot be found).
--
James Kanze (GABI Software) email:james.kanze@gmail.com
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