How to identify whether or not the keys in a std::unordered_map have experienced hash collisions?
That is, how to identify if any collision chaining is present?
Asked
Active
Viewed 1,100 times
7
Greg
- 8,175
- 16
- 72
- 125
-
I suspect you want to enforce some policy if it is the case, then ask the unordered_map to enforce it don't try to force it from client code. Check if max_load_factor member function solves the underling question. – Giuseppe Puoti Sep 10 '17 at 06:59
1 Answers
11
You can use the bucket interface and its bucket_size method.
std::unordered_map<int, int> map;
bool has_collision = false;
for(size_t bucket = 0; bucket < map.bucket_count(); bucket++) {
if(map.bucket_size(bucket) > 1) {
has_collision = true;
break;
}
}
Pete Becker
- 74,985
- 8
- 76
- 165
Aerle
- 136
- 1
- 3
-
If two elements happens to be in the same bucket it does not mean that there was a hash collision. – Revolver_Ocelot Sep 10 '17 at 06:30
-
4@Revolver_Ocelot -- sure it does; that's the definition of a collision. – Pete Becker Sep 10 '17 at 12:16
-
2a simple `if (map.bucket_count() < map.size())` can be done to quickly check if there are any collisions to avoid the for loop in many cases – phuclv Dec 26 '19 at 02:21
-
@phuclv there can be empty buckets, so a total count check won't suffice. – rustyx Jan 24 '23 at 09:17