Qfyilyi

I have a weird issue that I don't know if I am miss reading the documentation or my computer is doing something strange.

I have an unordered_map. I want to iterate over the buckets of the unordered_map in bucket order. This part is important as I need the access to be relatively random. I searched over cplusplus.com and I found this. The code is as follows:

// unordered_map::bucket

#include <iostream>

#include <string>

#include <unordered_map>

int main ()

{

  std::unordered_map<std::string,std::string> mymap = {

    {"us","United States"},

    {"uk","United Kingdom"},

    {"fr","France"},

    {"de","Germany"}

  };



  for (auto& x: mymap) {

    std::cout << "Element [" << x.first << ":" << x.second << "]";

    std::cout << " is in bucket #" << mymap.bucket (x.first) << std::endl;

  }



  return 0;

}

The output I was expecting and wanted on my computer was

Element [us:United States] is in bucket #1

Element [de:Germany] is in bucket #2

Element [fr:France] is in bucket #2

Element [uk:United Kingdom] is in bucket #4

However the output I am getting is instead sorted by value type which is strange

Element [de:Germany] is in bucket #2

Element [fr:France] is in bucket #3

Element [uk:United Kingdom] is in bucket #1

Element [us:United States] is in bucket #1

I even tried replacing the value with a class that had no comparison operators and it still was able to sort them. Is this something with the way my computer is storing the map or is cplusplus.com outdated? I was able to iterate over the buckets through a loop like this:

for ( unsigned int i = 0; i < b.bucket_count(); ++i) {



    for ( hash_table::const_local_iterator image_iterator = 

    b.begin(i);image_iterator!= b.end(i); ++image_iterator ){

The only issue is that I need to be able to skip a certain number of values i.e. I only want 1 item for every 100 which requires complicated loops and is slow.

Any help would be appreciated. I can't seem to figure this out!

[EDIT]
With my code my unordered_map is actually unordered_map where point is a simple class that just has two member variables and no helper functions.

When I run the loop above on my map here is my output. I have linked a text file as it is a pretty long file here
What confuses me even more is that my Point class has no comparison operators. Could it be my insertion order that is causing this?

In std::unordered_map the bucket for a particular element is completely determined by the hash value computed on the key, std::unordered_map use as the corresponding specialization of std::hash as default hash function, which is implementation defined and is not even guaranteed to be the same throughout different program execution. see std::hash.

As @François Andrieux said in the comment section, order of iteration of std::unordered_map is not specified, hence, you cannot expect the same iteration behavior in all machines, e.g the output in my computer is:

Element [de:Germany] is in bucket #0

Element [fr:France] is in bucket #3

Element [uk:United Kingdom] is in bucket #4

Element [us:United States] is in bucket #4