unordered map not working

[old light]

Hi, how do I create an unordered map and modify the values? I've tried google, though none of the solutions worked. I'm trying to create a unordered map with all values in a string initialized to 0. Then I want to add 1 for every count of the letter. Any help is appreciated Thanks!

Code:

    unordered_map<string, int> letter_count, needle_count;
    for (int i = 0; i < H.size(); i++)
    {
        letter_count.emplace(H[i], 0).first->second = 0;
        needle_count.emplace(H[i], 0).first->second = 0;
    }

    for (int i = 0; i < N.size(); i++)
    {
        letter_count[H[i]] += 1;
        needle_count[H[i]] += 1;
    }

Error Given:
error: no match for 'operator[]' (operand types are 'std::unordered_map<std::__cxx11::basic_string<char>, int>' and '__gnu_cxx::__alloc_traits<std::allocator<char>, char>::value_type' {aka 'char'})|

[iron silo]

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[old light]

unordered map not working

[river quail]

@old light You've created an unordered map of string keys to integer values

But you're emplacing single characters (which might have weird implications with std::string's constructor) and then indexing with single characters

That's what the error message means, there's no operator[] for std::unordered_map<std::__cxx11::basic_string<char>, int> that takes a single char as an argument

[old light]

So I should just use char instead of string?

[river quail]

Try it and see

[old light]

ok

[river quail]

After you play around with it a little bit, here's my suggestion for how to do this: Just do:

    unordered_map<char, int> letter_count, needle_count;
    for(char c : H) {
        letter_count[H[i]] += 1;
        needle_count[H[i]] += 1;
    }
```When `std::unordered_map::operator[]` is called with a key that doesn't exist it inserts a default-constructed value into the map, so this should just work

[old light]

Nice Thanks! I think that works. btw is there a way to reset all the values back to zero as well or do I just have to loop through and set it individually?

[river quail]

You can do

for(auto& entry : letter_count) {
    entry->second = 0;
}

Now, since your map has a relatively small set of keys, less than 128 in practice, an std::unordered_map is probably overkill here

A normal array could be used. But you're welcome to use the map if you want, it's probably more expressive

[old light]

Maybe. I was just trying to translate my python code as best as I could

[river quail]

That's as good a reason as any 🙂

[old light]

Anyways, Thanks for all the help!

[river quail]

My pleasure, let us know if you have any more questions

[iron silo]

@old light Has your question been resolved? If so, run !solved :)

[old light]

!solved