crashing

Hm, the code seems to crash in merge_stats function (i marked it red) and i have no idea why? When i enter only one name and value, it's ok, but when i enter more then one name and value it crashes...

Code:

#include <iostream>
#include <map>
#include <numeric>
#include <set>
#include <string>

using namespace std;

typedef multiset<double> SubData;
typedef map<string, SubData> Data;

void collect_data(Data &stats)
{
     string name;
     while(cin >> name) {
         double val;
         cin >> val;
         stats[name].insert(val);
     }
}

void print_setstats(SubData const &stats)
{
     size_t count = 0;
     double sum = 0.0, median;
     
     for(SubData::const_iterator p = stats.begin(); p != stats.end(); p++, count++) {
         if(count == (stats.size() - 1) / 2) {
             median = *p;
             if(stats.size() % 2 == 0) {
                 SubData::const_iterator q = p;
                 q++;
                 median = (median + (*q))/2.0;
             }
         }
         sum += *p;
     }
     cout << "sum: " << sum
          << ", mean: " << sum/ (double) count
          << ", median: " << median << endl;
}

void merge_stats(SubData &dst, SubData &src)
{
     SubData::iterator hint = dst.begin();
     
     for(SubData::iterator p = src.begin(); p != src.end(); p++) {
             hint = dst.insert(hint, *p);
             src.erase(p);
     }
}

void print_stats(Data &stats)
{
     SubData global_set;
     
     for(Data::iterator p = stats.begin(); p != stats.end(); p++) {
         cout << (*p).first << ": ";
         print_setstats((*p).second);
         merge_stats(global_set, (*p).second);
     }
     cout << "Global stats: ";
     print_setstats(global_set);
}

int main()
{     
      Data stats;
      collect_data(stats);
      print_stats(stats);
      return 0;
}

I don't know enough about multisets to say, but this looks dangerous:

Code:

     for(SubData::iterator p = src.begin(); p != src.end(); p++) {
             hint = dst.insert(hint, *p);
             src.erase(p);
     }

Are you sure that erase()'ing at the iterator doesn't invalidate it?

True, i figured that just out. Thanks. I'll look it up...

Typical correct way of erasing from a container whilst iterating over it:

Code:

for (it = c.begin(); it != c.end();)
{
    if (condition(*it))
        it = c.erase(it);
    else
        ++it;
}

works with all container types, though in a vector it is O(n^2).
Hence for a vector the erase/remove_if idiom is often used instead.

Originally Posted by iMalc

works with all container types

No, it only works with all standard sequence container types. Rather:

Code:

it = c.erase(it);

should be:

Code:

c.erase(it++);