Re: Vector reserve in a for_each

From:
"Alf P. Steinbach" <alfps@start.no>
Newsgroups:
comp.lang.c++
Date:
Sat, 24 Feb 2007 07:35:57 +0100
Message-ID:
<54a4meF1vgd9sU1@mid.individual.net>
* Chris Roth:

I have a vector of vectors:
    vector< vector<double> > v;
and have initialized it with:
    v( 5 );
as I know I will have 5 columns of data. At this point, I read text file
data into each of the the vectors using push_back. I know that I will be
reading in 5000 elements into each vector, so I use reserve:
    ifstream f( "file.txt" );
    if(f.is_open())
    {
        for( vector< vector<double> >::iterator itr = v.begin();
             itr != v.end(); ++itr )
        {
            itr->reserve(5000);
        }
        double d;
        while(f >> d)
        {
            m_data[0].push_back( d );
            f >> d;
            m_data[1].push_back( d );
            f >> d;
            m_data[2].push_back( d );
            f >> d;
            m_data[3].push_back( d );
            f >> d;
            m_data[4].push_back( d );
        }
    }
However, could I use a for_each to set the reserve of the vectors?


You could, but a loop is much cleaner.

However, I'd use an indexing loop, not an iterator-based loop.

Like

   for( size_t i = 0; i < v.size(); ++i ) { v.at(i).reserve( 5000 ); }

Or is
there a different/better way to read in the 5 column text data?


Sure: define "better".

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