Iterating through a string

[Steve]

Hi Guys,

I have a string which contains data elements separated by spaces. I also
have a function which returns the number of characters from the beginning of
the string for a given number of spaces. I am using a loop with strchr for
the number of spaces, and I was just wondering if there was a more efficient
(ie faster) way of achieving this. Any ideas please? Current function code
below:

int GetNewPosition( const std::string& DataStr, const int Spaces)
{
int i = 0; // space counter
char* b = const_cast<char *>(DataStr.c_st r()); // beginning of string

do {
b = strchr(b, ' '); // find next space
++i; ++b; // increment space counter and pointer
} while(i<Spaces && b != NULL);

if(b) return b - DataStr.c_str() ;
else return -1;
}
Any help is appreciated,
Thanks for reading,
Steve.

[Mabden]

"Steve" <st*******@hotm ail.com> wrote in message
news:Ohrxc.1911 $ud5.1728@newsf e4-gui...

Hi Guys,

I have a string which contains data elements separated by spaces.

You could break it apart using strtok()

[Steve]

Thanks folks,

This exercise just tells me no matter how long I think I've been doing C++,
I'll always be a n00b! Fancy overlooking the "case of NULL", D'Oh!!

Thanks for the code advice too, it's appreciated...t hough I'm beginning to
think It'd be better for the world if I gave up programming ;)

Steve.
"Steve" <st*******@hotm ail.com> wrote in message
news:Ohrxc.1911 $ud5.1728@newsf e4-gui...

Hi Guys,

I have a string which contains data elements separated by spaces. I also
have a function which returns the number of characters from the beginning of the string for a given number of spaces. I am using a loop with strchr for
the number of spaces, and I was just wondering if there was a more efficient (ie faster) way of achieving this. Any ideas please? Current function code
below:

int GetNewPosition( const std::string& DataStr, const int Spaces)
{
int i = 0; // space counter
char* b = const_cast<char *>(DataStr.c_st r()); // beginning of string

do {
b = strchr(b, ' '); // find next space
++i; ++b; // increment space counter and pointer
} while(i<Spaces && b != NULL);

if(b) return b - DataStr.c_str() ;
else return -1;
}
Any help is appreciated,
Thanks for reading,
Steve.

[Dave Townsend]

With regard to the first comment I originally posted, I thought there might
be a solution
using streams to parse the words out of a string, I looked a bit closer,
below is
my attempt. This works fine apart from one detail, perhaps somebody might
jump in, there doesn't seem to be a way to specify the separator chars of
the original stream, in the example, the last word is returned as "dog." not
"dog"
Other than that, its a wonderfully simple snippet of code, praise to STL!

I've not benchmarked this code, but I presume its going to be relatively
efficient
since the iterator will maintain the position in the stringstream as the
words are pulled out,
there will be some extra overhead I suspect from using the string class as
opposed to
plain characters, that's life.

The question is then, how to specify the separator/whitespace for the
stream.
#include <iostream>
#include <sstream>
#include <iterator>

using namespace std;
int main(int argc, char* argv[])
{
istringstream is("the quick brown fox jumped over the lazy dog.");

istream_iterato r<string> is_iter(is);
istream_iterato r<string> end;

for ( ; is_iter != end; ++is_iter)
{
string nextword = *is_iter;
cout << "next word is \"" << nextword << "\"\n";
}
return 0;
}

"Steve" <st*******@hotm ail.com> wrote in message
news:Ohrxc.1911 $ud5.1728@newsf e4-gui...

Hi Guys,

I have a string which contains data elements separated by spaces. I also
have a function which returns the number of characters from the beginning of the string for a given number of spaces. I am using a loop with strchr for
the number of spaces, and I was just wondering if there was a more efficient (ie faster) way of achieving this. Any ideas please? Current function code
below:

int GetNewPosition( const std::string& DataStr, const int Spaces)
{
int i = 0; // space counter
char* b = const_cast<char *>(DataStr.c_st r()); // beginning of string

do {
b = strchr(b, ' '); // find next space
++i; ++b; // increment space counter and pointer
} while(i<Spaces && b != NULL);

if(b) return b - DataStr.c_str() ;
else return -1;
}
Any help is appreciated,
Thanks for reading,
Steve.

[Daniel T.]

In article <2i************ @uni-berlin.de>,
"John Harrison" <jo************ *@hotmail.com> wrote:

"Steve" <st*******@hotm ail.com> wrote in message
news:Ohrxc.191 1$ud5.1728@news fe4-gui...

Hi Guys,

I have a string which contains data elements separated by spaces. I also
have a function which returns the number of characters from the beginning

of

the string for a given number of spaces. I am using a loop with strchr for
the number of spaces, and I was just wondering if there was a more

efficient

(ie faster) way of achieving this. Any ideas please? Current function code
below:

int GetNewPosition( const std::string& DataStr, const int Spaces)
{
int i = 0; // space counter
char* b = const_cast<char *>(DataStr.c_st r()); // beginning of string

do {
b = strchr(b, ' '); // find next space
++i; ++b; // increment space counter and pointer
} while(i<Spaces && b != NULL);

if(b) return b - DataStr.c_str() ;
else return -1;
}

Your current function is bugged. If there are no spaces, then b == NULL,
then you increment b, then you test it for NULL. That isn't going to work.
You also have a completely gratuitous const_cast in your code.

It's quite a good example of how newbies worry about efficiency without
other more important considerations such as writing working code, and
writing clear code.

However strchr is probably the fastest way to do it, however as with all
these questions there is no official answer as to which method is fastest.
The C++ standard makes no mention of which functions or techniques are
faster than others, the only way to tell for sure is try different methods
and time them.

Here's my version, completely untested, so apologies in advance for any
bugs.

int GetNewPosition( const std::string& DataStr, int Spaces)
{
const char* start = DataStr.c_str() ;
const char* p = start;
for (int i = 0; i < Spaces; ++i)
{
if (p == NULL)
return -1;
p = strchr(p + 1, ' ');
}
return p - start;
}

I suspect your code above doesn't do what the OP wanted...

GetNewPosition( "", 0) should return -1 if I understand what he is trying
to do, yours returns 0. Care to try again? :-)

[Daniel T.]

In article <S-*************** *****@comcast.c om>,
"Dave Townsend" <da********@com cast.net> wrote:

With regard to the first comment I originally posted, I thought there might
be a solution
using streams to parse the words out of a string, I looked a bit closer,
below is
my attempt. This works fine apart from one detail, perhaps somebody might
jump in, there doesn't seem to be a way to specify the separator chars of
the original stream, in the example, the last word is returned as "dog." not
"dog"
That wasn't a requirement anyway.

Other than that, its a wonderfully simple snippet of code, praise to STL!

I've not benchmarked this code, but I presume its going to be relatively
efficient
since the iterator will maintain the position in the stringstream as the
words are pulled out,
there will be some extra overhead I suspect from using the string class as
opposed to
plain characters, that's life.

The question is then, how to specify the separator/whitespace for the
stream.
Good question...

#include <iostream>
#include <sstream>
#include <iterator>

using namespace std;
int main(int argc, char* argv[])
{
istringstream is("the quick brown fox jumped over the lazy dog.");

istream_iterato r<string> is_iter(is);
istream_iterato r<string> end;

for ( ; is_iter != end; ++is_iter)
{
string nextword = *is_iter;
cout << "next word is \"" << nextword << "\"\n";
}
return 0;
}

Your idea skips over all whitespace, not just spaces as the OP's code
did. I don't know if he would have a problem with that or not...

Otherwise, this works just as well as my custom iterator. What I did
would only be useful if one also implemented op-- (so you could go both
forward and backward) and/or a non-const version of op* (so you could
replace words.)

[Siemel Naran]

"John Harrison" <jo************ *@hotmail.com> wrote in message
news:2innbjFpab doU1@uni-

C++ should provide two overloads for strchr

const char* strchr(const char*, int);

and

char* strchr(char*, int);

Good point. Because the OP is using std::string it's clear he's using C++.

[John Harrison]

"Siemel Naran" <Si*********@RE MOVE.att.net> wrote in message
news:xN******** *************@b gtnsc05-news.ops.worldn et.att.net...

"John Harrison" <jo************ *@hotmail.com> wrote in message
news:2innbjFpab doU1@uni-

C++ should provide two overloads for strchr

const char* strchr(const char*, int);

and

char* strchr(char*, int);
Good point. Because the OP is using std::string it's clear he's using

C++.

Posting to comp.lang.c++ should also mean C++, although that isn't always
the case. Some posters profess to be coding in a mysterious language called
C/C++.

john