How does one parse strings in c/c++?

Question

I have a file that contains lines as below:

25 1 0 0 0 0
27 1 0 0 0 0
20 0 0 0 0 0
32 1 0 0 0 0
23 1 0 0 0 0
16 0 0 0 0 0
28 1 0 0 0 0

First, I store each line with value=1 in the second column as element of a string array. I call the stored elements back via a for-loop, and parse the contents of the element into integers. How does one do that in c++?

#include <vector> 
#include <iostream>
#include <string>
using namespace std;
   .....
   .....
   .....
   .....
char line[100];
vector<string> vec_line;
string Vline;
int trimVal, compa, nil0, nil1, nil2, nil3;
  ......
  ......
  ......
//then store lines as strings in vec_line as required by the premise above
// after that, retrieve the contents of the elements of the vector
for (int iline=0; iline<vec_line.size(); iline++) {
   cout<<"iline  "<<iline<<"  vec_line[iline]";
   //then I want to parse contents of each string element and store them integer formats
   sscanf(vec_line[iline], "%d %d %d %d %d %d", &trimVal, &compa, &nil0, &nil1, &nil2, &nil3); //how would one do this simple parsing task in c and c++?
}

Thank you.

Dan · Accepted Answer · 2012-08-31 13:07:22Z

2

sscanf is a C function, since you are already using C++ streams to output (and presumably read from the file?) you are better off using a stringstream to convert between strings and data types.

For example:

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

int main(int argc, char* argv[])
{
   stringstream sstr;
   string mystr = "123";
   int i;

   sstr << mystr;

   sstr >> i;

   cout << i << endl;

   return 0;
}

will output 123 to stdout. Of course you can also read from an ifstream using the >> operator directly into an int or double if you wish.

Hope this helps.

answered Aug 31, 2012 at 13:07

Dan

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Comments

hmjd · Accepted Answer · 2012-08-31 13:02:28Z

1

This is incorrect:

sscanf(vec_line[iline], "%d %d ...

as the first argument to sscanf() is a const char* and not a std::string. Change to:

sscanf(vec_line[iline].c_str(), "%d %d ...

Recommend checking the return value of sscanf() to ensure all expected assignments were made:

if (6 == sscanf(vec_line[iline].c_str(), "%d %d ...

edited Aug 31, 2012 at 13:02

answered Aug 31, 2012 at 12:56

hmjd

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Comments

Jerry Coffin · Accepted Answer · 2012-08-31 13:26:17Z

Unless I needed to store each row as a string, I'd do something more like this:

struct row { 
    static const int columns = 6;
    std::vector<int> column_data;

    // should we keep or ignore a row? Ignore if column 1 != 1.
    struct filter {
        bool operator()(row const &r) { return r.column_data[1] != 1; }
    };

    // read in a row.
    friend std::istream &operator>>(std::istream &is, row &r) {
        r.column_data.resize(row::columns);
        for (int i=0; i<row::columns; i++)
            is >> r.columns[i];
        return is;
    }

    // write a row out to a file.
    friend std::ostream &operator<<(std::ostream &os, row const &r) { 
        std::copy(r.column_data.begin(), r.column_data.end(),
                  std::ostream_iterator<int>(os, " "));
        return os;
};

Then reading and displaying the data would look something like this:

std::vector<row> data;

std::remove_copy_if(std::istream_iterator<row>(input_file),
                    std::istream_iterator<row>(),
                    std::back_inserter(data),
                    row::filter());

std::copy(data.begin(), data.end(), 
          std::ostream_iterator<row>(output_file, "\n"));

Or, if you just want to copy the correct rows from some input file to some output file, you can use the correct iterators to do that directly:

std::remove_copy_if(std::istream_iterator<row>(input_file),
                    std::istream_iterator<row>(),
                    std::ostream_iterator<row>(output_file),
                    row::filter());

rici · Accepted Answer · 2017-11-20 16:22:00Z

0

There is always lex and yacc (or Flex and Bison)

edited Nov 20, 2017 at 16:22

rici

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answered Aug 31, 2012 at 14:16

doron

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