Editing the value in an unordered map for a given key

Question

The following is C++ code to get a count of the words in magazine. I'm trying to add the word if its value does not exist and if it does, increment it.

unordered_map<string,int>hash;
vector<string> magazine(m);

for(int i = 0;i <m;i++)
{
   cin >> magazine[i];
   if(hash[magazine[i]]>0)
       hash[magazine[i]]++;
   else
    hash.emplace(magazine[i],1);
}

But when I try to output, all magazine key gives 0 as value. Any idea why?

Answer 1

Your version doesn't work because this if(hash[magazine[i]]>0) will insert an element into hash if it doesn't exist, this new element will have a mapped value of 0¹. Which means that hash.emplace(magazine[i],1); is pointless here because there will always be an element at magazine[i] now. Because its value will be 0 your hash[magazine[i]]++; will never run either because the if will never be true. Leaving you with a map of i elements, all with value 0.

operator[] returns a reference to the mapped value if there is one, if not, it inserts one and then returns that reference¹.

Which means that you can factor out the if and just change it to:

for(int i = 0;i <m;i++)
{
   cin >> magazine[i];
   ++hash[magazine[i]];
}

Which basically means : "Get a reference to the mapped value for key magazine[i], if none is found, insert one and give me that one. Increment this reference."

¹: _{If insertion occurs the element is value-initialized. Because your mapped value type is int this will result into the mapped value being 0 after insertion.}