Help with parsing a string.

baka101 · #1 02-Sep-2007, 17:36

I am trying to parse a user-entered string and separate it at all the spaces, but I don't know how to change the parameters to accept user-entered strings, just what string is programmed in for the string already.

I did not create this code, but I am trying to modify it to work like I want it to, so don't ask me the how the whole thing works.

Here is the code I am working on:

CPP / C++ / C Code:

//paerser class--------------------------------------------
class parser 
{
      std::string text;
      public:
      static const int max_field_size=100;

      explicit parser(const char *s) : text(s) {}

      void tokenize(char *fields[], int nf, const char breaker=' ') 
      {
           // parses internal string, breaking at instances of <breaker>
           // which are thrown away. Returns separated values as fields
           std::istringstream buffer(text);
           int f=0;
           while (f<nf) 
           {
                 buffer.getline(fields[f], max_field_size, breaker);
                  ++f;
           }
      }
};
//end of parser class--------------------------------------


int main()
{
...
cin >> string s;
parsef(s);
...
}

//string parsing Function----------------------------------
void parsef(string s) 
{

const char* userstring="RUN filename and this\n";
char **fields = new char*[3];
for (int i=0; i<3; ++i)
fields[i]=new char[parser::max_field_size];

parser parse(userstring);
parse.tokenize(fields, 3);
for (int i=0; i<3; ++i)
cout <<  "word " << i << ": " << fields[i] << endl;

}
//end string parsing Function------------------------------

Only what I put in for 'userstring' is parsed. I don't know how to get the string I pass to the parse function to be the string parsed.

I thinking it's something simple, but I just can't figure it out. Could someone help, please? And thanks.

davekw7x · #2 02-Sep-2007, 20:00

Quote:

Originally Posted by baka101

I am trying to parse a user-entered string

If I am understanting:
1. You need the user to input a std::string

2. The string from part 1 is passed as an argument to parsef

3. The function parsef declares an object of the parser class and initializes it with whatever the user entered in part 1.

4. You use the parser function "tokenize" to break up the string

5. You print the results.

So, you need to perform steps 1 and 2 in your main() function.

The parsef function will be modified to use information that was fed to it to do the tokenizing.

I would suggest getline() to read a std::string from the user, since parsef needs a std::string as an argument.

So the main program could look like:

CPP / C++ / C Code:

#include <iostream>
#include <string>
using namespace std;

void parsef(string s);
int main()
{
    string myString;
    cout << "Enter a string: ";
    getline(cin, myString);
    parsef(myString);

    return 0;

}

Now parsef is going to use its "s" parameter rather than the hard-coded "userstring" shown by your example

CPP / C++ / C Code:

void parsef(string s)
{

    //const char *userstring = "RUN filename and this\n"; // just delete this
    char **fields = new char *[3];
    for (int i = 0; i < 3; ++i)
        fields[i] = new char[parser::max_field_size];

    // Then what?
    parser parse(???);

You need to declare a parser object and initialize it with the string, but there is no constructor that takes a std::string as an argument. The only constructor takes a const pointer to char. (It uses a so-called "C-style" string, not a std::string.) What to do?

Answer: The std::string member function c_str() is a const pointer to char and it points to a "C-style" string equivalent to the std::string.

CPP / C++ / C Code:

    parser parse(s.c_str());

From here on the operation is the same.

Regards,

Dave

Footnote: The parsef function allocates memory for three strings. It should de-allocate the memory before returning to the calling function. So at the end of parsef, I would expect to see something like:

CPP / C++ / C Code:

    for (int i = 0; i < 3; i++) {
        delete [] fields[i];
    }
    delete fields;

}
//end string parsing Function------------------------------

As far as that goes, I'm not sure why fields was dynamically allocated anyhow; why not just declare it as an array of three pointers to char? Then allocate the three char arrays. Or, maybe there is a reason? I mean, either way is "correct", I just generally avoid dynamic memory allocation unless there is a good reason. I'm funny that way.

As far as that goes, I'm not sure why tokenize is designed to take an array of pointers to char; why not an array of std::strings (or a std::vector of std::strings).
Oh, well...

baka101 · #3 03-Sep-2007, 13:00

Thanks, I finally get it.

Peter_APIIT · #4 04-Sep-2007, 07:42

Can u post the program here ?

I need our help.

I really do understand what u discussed here.

Thanks for your help.

Peter_APIIT · #5 04-Sep-2007, 08:10

Can u post the program here ?

I need our help.

I really do understand what u discussed here.

Thanks for your help.

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search
Rate This Thread
Rate This Thread:

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Multiple questions for C++ project	devster420	C++ Forum	1	20-Apr-2007 22:26
Message Class	TransformedBG	C++ Forum	5	29-Nov-2006 22:28
C++ String Template Parsing	vikasbucha	C++ Forum	1	11-Jun-2006 03:14
variables return to previous value after i try to set them	nasaiya	MS Visual C++ / MFC Forum	2	14-Jun-2005 01:43
Help wit my source code compiler errors	Krandygrl00	C++ Forum	1	06-Jun-2005 09:14

Help with parsing a string.

Re: Help with parsing a string.

Re: Help with parsing a string.

Re: Help with parsing a string.

Re: Help with parsing a string.