How to read numbers from an ASCII file (C++)

Question

I need to read in data files which look like this:

* SZA: 10.00
 2.648  2.648  2.648  2.648  2.648  2.648  2.648  2.649  2.650  2.650
 2.652  2.653  2.652  2.653         


        

Answer 1

Simple solution using STL algorithms:

#include 
#include 
#include 
#include 

struct data
{
   float first; // in case it is required, and assuming it is 
                // different from the rest
   std::vector values;
};

data read_file( std::istream& in )
{
   std::string tmp;
   data d;
   in >> tmp >> tmp >> d.first;
   if ( !in ) throw std::runtime_error( "Failed to parse line" );

   std::copy( std::istream_iterator( in ), std::istream_iterator(),
         std::back_inserter(d.values) );

   return data;
}

If you really need to use an array, you must first allocate it (either dynamically or statically if you know the size) and then you can use the same copy algorithm

// parsing the first line would be equivalent
float data[128]; // assuming 128 elements known at compile time
std::copy( std::istream_iterator(is), std::istream_iterator(), 
      data );

But I would recommend using std::vector even in this case, if you need to pass the data into a function that takes an array you can always pass it as a pointer to the first element:

void f( float* data, int size );
int main()
{
   std::vector v; // and populate
   f( &v[0], v.size() ); // memory is guaranteed to be contiguous
}