problem with transform

[CFLam]

Hello all,
I need to read in from a csv file and store in an integer array, after read in the line and split the line with boost::split, I got a vector<string> LineItems. I can convert it to integer array by:

    for (int i=0; i<=10; i++){
     WorkArr[i]=stoi(LineItems[i]);
    }

(I know the size and format of the csv therefore I can use an array to store.)
However, I would like to use transform to perform this task:

    transform(LineItems.begin(),LineItems.end(),WorkArr,stoi);

But compiler said that don't know which overloaded version of stoi to use (I am using VC++).
I know I can workaround in this way:

    int MyStoI(string & ss) return stoi(ss);
    transform(LineItems.begin(),LineItems.end(),WorkArr,MyStoI);

But this introduce another function MyStoI, is there any method to perform this without introduce new function ?
Thanks a lot.

Regds

LAM Chi-fung

[Peter87]

You can use a lambda.

transform(LineItems.begin(), LineItems.end(), WorkArr, [](auto& x){ return stoi(x); });

[CFLam]

Yes, it works, thanks a lot.

[lastchance]

@CFLam
If you are absolutely certain that:
(a) your file contains ONLY integer (or only double) data;
(b) there are no empty cells;
then you could circumvent the string conversions completely and read straight into a numerical array.

(If you can't meet those conditions then it is safer to read into strings and convert where possible.)

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

//======================================================================

/*
   Reads a SINGLE LINE containing ...
   ... an UNKNOWN NUMBER of ...
   ... SINGLE PUNCTUATION-SEPARATED values ...
   ... into a vector<T> of CONSISTENT NUMERICAL type
*/
template <typename T> istream &operator >> ( istream &in, vector<T> &V )       
{                                                                              
   string line;
   T item;
   char separator;

   V.clear();                          // In case V had anything in on entry
   getline( in, line );                // If you don't know how many values per line, you must ISOLATE ONE LINE
   stringstream ss( line );            // Create an internal stream from this line only

   while ( ss >> item )                // Whilst the next item ON THIS LINE is NUMERICAL
   {
      V.push_back( item );             // ... put it in the vector ...
      ss >> separator;                 // ... and clear ANY following non-white-space character (probably none at end of line)
   }

   return in;                          // This will allow input to continue on the main stream
}

//======================================================================

template <typename T> vector< vector<T> > readCSV( string filename )
{
   vector< vector<T> > matrix;
   vector<T> row;
   ifstream in( filename );   if ( !in ) { cout << "Can't open " << filename << '\n';   return matrix; }
   while ( in >> row ) if ( !row.empty() ) matrix.push_back( row );
   return matrix;
}

//======================================================================

int main()
{
   vector< vector<int> > matrix = readCSV<int>( "input.csv" );

   cout << "File read:\n";
   for ( auto row : matrix )
   {
      for ( auto e : row ) cout << e << "  ";
      cout << '\n';
   }
}

//====================================================================== 

input.csv:

1,2,3,4,
5,6,
7,8,9,10

Output:

File read:
1  2  3  4  
5  6  
7  8  9  10  

[CFLam]

@lastchance, the csv contain empty cell and mix string with integer, but I know exactly which cell contain string and which for integer, therefore I need to convert them explictly ~_~.