FIltering CSV data using C++

Question

Sorry for asking a question that many may think has already been asked.

I have a very long CSV data file (dat.csv) with 5 columns. I have another short CSV (filter.csv) file with 1 column.

Now, I only need to extract columns from dat.csv where column-1 matches with that of column-1 of filter.csv.

I would usually do this in BASH using sed/awk. However, for some other reasons I need to do this within a C++ file. Can you suggest an efficient way to do this?

Sample Data:

data.csv

ID,Name,CountryCode,District,Population

3793,NewYork,USA,NewYork,8008278
3794,LosAngeles,USA,California,3694820
3795,Chicago,USA,Illinois,2896016
3796,Houston,USA,Texas,1953631
3797,Philadelphia,USA,Pennsylvania,1517550
3798,Phoenix,USA ,Arizona,1321045
3799,SanDiego,USA,California,1223400
3800,Dallas,USA,Texas,1188580
3801,SanAntonio,USA,Texas,1144646

filter.csv

3793
3797
3798

Answer 1

This .csv sorting library might help:

http://www.partow.net/programming/dsvfilter/index.html

You could merge the columns of both tables into one larger table, and then query for matches in the new table (where column 1 of table A is and column 1 of table B is). Or maybe that library has functions for comparing tables.

Answer 2

Here are some tips:

1. The stream from which you're reading the data needs to ignore the commas, so what it should to is set comma characters to whitespace using the std::ctype<char> facet imbued in its locale. Here's an example of modifying the classification table:

   struct ctype : std::ctype<char>
   {
   private:
       static mask* get_table()
       {
           static std::vector<mask> v(classic_table(),
                                      classic_table() + table_size);
   
           v[','] &= ~space;
           return &v[0];
       }
   public:
       ctype() : std::ctype<char>(get_table()) { }
   };
   

2. Read the first csv. file line-wise (meaning std::getline()). Extract the first word and compare it with an extraction from the second .csv file. Continue this until you reach the end of the first file:

   int main()
   {
       std::ifstream in1("test1.csv");
       std::ifstream in2("test2.csv");
   
       typedef std::istream_iterator<std::string> It;
   
       in1 >> comma_whitespace;
       in2 >> comma_whitespace;
   
       std::vector<std::string> in2_content(It(in2), It());
       std::vector<std::string> matches;
   
       while (std::getline(in1, line))
       {
           std::istringstream iss(line);
           It beg(iss);
   
           if (std::find(in2_content.begin(),
                         in2_content.end(), *beg) != in2_content.end())
           {
               matches.push_back(line);
           }
       }
   }
   
   // After the above, the vector matches should hold all the rows that
   // have the same ID number as in the second csv file
   

   comma_whitespace is a manipulator which changes the locale to the custom ctype defined above.

   _{Disclaimer: I haven't tested this code.}