unordered_map hash code example
Example 1: map vs unordered_map in C++
| map | unordered_map
---------------------------------------------------------
Ordering | increasing order | no ordering
| (by default) |
Implementation | Self balancing BST | Hash Table
| like Red-Black Tree |
search time | log(n) | O(1) -> Average
| | O(n) -> Worst Case
Insertion time | log(n) + Rebalance | Same as search
Deletion time | log(n) + Rebalance | Same as search
::-> Use std::map when
1. You need ordered data.
2. You would have to print/access the data (in sorted order).
3. You need predecessor/successor of elements.
::-> Use std::unordered_map when
1. You need to keep count of some data (Example – strings) and no ordering is required.
2. You need single element access i.e. no traversal.
Example 2: insert in unordered_map
#include <iostream>
#include <string>
#include <unordered_map>
int main ()
{
std::unordered_map<std::string,double>
myrecipe,
mypantry = {{"milk",2.0},{"flour",1.5}};
std::pair<std::string,double> myshopping ("baking powder",0.3);
myrecipe.insert (myshopping);
myrecipe.insert (std::make_pair<std::string,double>("eggs",6.0));
myrecipe.insert (mypantry.begin(), mypantry.end());
myrecipe.insert ( {{"sugar",0.8},{"salt",0.1}} );
std::cout << "myrecipe contains:" << std::endl;
for (auto& x: myrecipe)
std::cout << x.first << ": " << x.second << std::endl;
std::cout << std::endl;
return 0;
}
Example 3: unordered_map c++
#include <bits/stdc++.h>
#include <iostream>
#include <map>
#include <unordered_map>
using namespace std;
int main() {
map<char, int> M;
unordered_map<char, int> U;
string s = "Sumant Tirkey";
for (char c : s) {
M[c]++;
}
for (char c : s){
U[c]++;
}
return 0;
}