C++ Parsing a line out of a large file - c++

Question

I have read an entire file into a string from a memory mapped file Win API
CreateFile( "WarandPeace.txt", GENERIC_READ, FILE_SHARE_READ, 0, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0 )
etc...
Each line is terminated with a CRLF. I need to find something on a line like "Spam" in the line "I love Spam and Eggs" (and return the entire line (without the CRLF) in a string (or a pointer to the location in the string) The original string cannot be altered.
EDITED:
Something like this:
string ParseStr( string sIn, string sDelim, int nField )
{
int match, LenStr, LenDelim, ePos, sPos(0), count(0);
string sRet;
LenDelim = sDelim.length();
LenStr = sIn.length();
if( LenStr < 1 || LenDelim < 1 ) return ""; // Empty String
if( nField < 1 ) return "";
//=========== cout << "LenDelim=" << LenDelim << ", sIn.length=" << sIn.length() << endl;
for( ePos=0; ePos < LenStr; ePos++ ) // iterate through the string
{ // cout << "sPos=" << sPos << ", LenStr=" << LenStr << ", ePos=" << ePos << ", sIn[ePos]=" << sIn[ePos] << endl;
match = 1; // default = match found
for( int k=0; k < LenDelim; k++ ) // Byte value
{
if( ePos+k > LenStr ) // end of the string
break;
else if( sIn[ePos+k] != sDelim[k] ){ // match failed
match = 0; break; }
}
//===========
if( match || (ePos == LenStr-1) ) // process line
{
if( !match ) ePos = LenStr + LenDelim; // (ePos == LenStr-1)
count++; // cout << "sPos=" << sPos << ", ePos=" << ePos << " >" << sIn.substr(sPos, ePos-sPos) << endl;
if( count == nField ){ sRet = sIn.substr(sPos, ePos-sPos); break; }
ePos = ePos+LenDelim-1; // jump over Delim
sPos = ePos+1; // Begin after Delim
} // cout << "Final ePos=" << ePos << ", count=" << count << ", LenStr=" << LenStr << endl;
}// next
return sRet;
}
If you like it, vote it up. If not, let's see what you got.

Answer 1

If you are trying to match a more complex pattern then you can always fall back to boost's regex lib.
See: http://www.boost.org/doc/libs/1_41_0/libs/regex/doc/html/index.html
#include <iostream>
#include <string>
#include <boost/regex.hpp>
using namespace std;
int main( )
{
std::string s;
std::string sre("Spam");
boost::regex re;
ifstream in("main.cpp");
if (!in.is_open()) return 1;
string line;
while (getline(in,line))
{
try
{
// Set up the regular expression for case-insensitivity
re.assign(sre, boost::regex_constants::icase);
}
catch (boost::regex_error& e)
{
cout << sre << " is not a valid regular expression: \""
<< e.what() << "\"" << endl;
continue;
}
if (boost::regex_match(line, re))
{
cout << re << " matches " << line << endl;
}
}
}

Answer 2

Do you really have to do it in C++? Perhaps you could use a language which is more appropriate for text processing, like Perl, and apply a regular expression.
Anyway, if doing it in C++, a loop over Prev_delim_position = sIn.find(sDelim, Prev_delim_position) looks like a fine way to do it.

Answer 3

system("grep ....");