C++ unordered_map weird behaviour - c++

Question

I was solving leetcode problem (Pairs of Songs With Total Durations Divisible by 60) and my solution below uses a map, when I change it to an unordered_map and print the elements inside the loop; the number of elements are much more than the input
class Solution {
public:
int numPairsDivisibleBy60(vector<int>& time) {
map<int, int> mod_d;
for(auto el : time) {
if(mod_d.count(el % 60) == 0) {
mod_d[el % 60] = 1;
}else mod_d[el % 60]++;
}
int ans = 0, i = 1;
//cout << "Size: " << mod_d.size() << "\n";
for(auto el : mod_d) {
int f = el.first, s = el.second;
cout << f << " " << 60 - f << "\n";
ans += mod_d[(60 - f) % 60] * (((60 - f) % 60) == f ? s - 1 : s);
}
return ans / 2;
}
};
Sample Input Test: [15, 63, 451, 213, 37, 209, 343, 319]
And the output is as follows:
3 57
15 45
19 41
29 31
31 29
33 27
37 23
41 19
43 17
45 15
57 3
The number of elements printed should be only 8 inside the loop, but with an unordered_map, the elements are much more.
The code that is not working well is below:
class Solution {
public:
int numPairsDivisibleBy60(vector<int>& time) {
unordered_map<int, int> mod_d;
for(auto el : time) {
if(mod_d.count(el % 60) == 0) {
mod_d[el % 60] = 1;
}else mod_d[el % 60]++;
}
int ans = 0, i = 1;
//cout << "Size: " << mod_d.size() << "\n";
for(auto el : mod_d) {
int f = el.first, s = el.second;
cout << f << " " << 60 - f << "\n";
ans += mod_d[(60 - f) % 60] * (((60 - f) % 60) == f ? s - 1 : s);
}
return ans / 2;
}
};
The only difference is the usage of unordered_map instead of a map
And it prints the elements wrongly as:
19 41
43 17
37 23
33 27
31 29
29 31
3 57
41 19
15 45
41 19
3 57
29 31
31 29
57 3
33 27
37 23
43 17
17 43
19 41
23 37
27 33
Anyone knows why is this odd behavior happening?

Answer 1

Alright I now understand, thank you so much all for the help. As per this link that shows the different ways to access a map, I see that using [] operator creates the elements if they are not there in the map and that is my mistake. When I should use std::map::at to retrieve map element
Fix
Checking if the element exist at first and using at() to access it
class Solution {
public:
int numPairsDivisibleBy60(vector<int>& time) {
unordered_map<int, int> mod_d;
for(auto el : time) {
if(mod_d.count(el % 60) == 0) {
mod_d[el % 60] = 1;
}else mod_d[el % 60]++;
}
int ans = 0, i = 1;
cout << "Size: " << mod_d.size() << "\n";
for(auto el : mod_d) {
int f = el.first, s = el.second;
cout << f << " " << i++ << "\n";
if(mod_d.count((60 - f) % 60) > 0) ans += mod_d.at((60 - f) % 60) * (((60 - f) % 60) == f ? s - 1 : s);
}
return ans / 2;
}
};