about the interator of map

This is the exercise of TC++PL ch6
(*2) Read a sequence of possibly whitespaceseparated
(name,value) pairs, where the name is a
single whitespaceseparated
word and the value is an integer or a floatingpoint
value. Compute
and print the sum and mean for each name and the sum and mean for all
names. Hint: ?6.1.8.

struct Stat
{
    Stat():sum_(0.0),count_(0)
        {
        }
    double sum_;
    int count_;
};
using std::string;
using std::cin;
using std::cout;
using std::endl;
typedef std::map<string,Stat> Data;
typedef Data::iterator pData;

void collect_data(Data &stats)
{
    string name;
    while(cin >> name)
    {
        if( name == "Quit")
        {
            return ;
        }
        float value;
        cin >> value;
        stats[name].sum_ += value;
        ++stats[name].count_;
    }

}
void print_data(const Data& stats)
{
    double sum = 0;
    int count = 0;
    for(Data::iterator p = stats.begin(); p != stats.end(); ++p)//when
i modify iterator to const_iterator it can work
    {
        std::cout << (*p).first
                  << " sum =" <<(*p).second.sum_
                  << ", mean = "
                  <<(*p).second.sum_/(*p).second.count_
                  << endl;
        sum += p->second.sum_;
        count += p->second.count_;
    }
    cout << endl;
    cout << " Globle sum: " << sum << endl;
    cout << " Globle count: " << count << endl;
}
int main()
{
    Data stats;
    collect_data(stats);
    print_data(stats);
    return 0;
}

The code will cause compile error, I wonder the reason:
bzhu@TY-PC /g/WorkingDir/tcplex/ch6
$ make
g++ -g -o6.3.exe 6.3.cpp
6.3.cpp: In function `void print_data(const Data&)':
6.3.cpp:39: error: conversion from
`std::_Rb_tree_const_iterator<std::pair<const std::string, Stat> >' to
non-scalar type `std::_Rb_tree_iterator<std::pair<const std::string,
Stat> >' requested
make: *** [6.3.exe] Error 1

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