CODEWITHSUNDEEP
c++visual-c++c++11
Sriram Subramanian
Sriram Subramanian

Reputation: 2753

How to modify value in unorderedmap?

I would like to try inserting an element into the map with key k and value v. If the key is already present, I want to increment the value of that key.

Example,

typedef std::unordered_map<std::string,int> MYMAP;

MYMAP mymap;
std::pair<MYMAP::iterator, bool> pa=  
    mymap.insert(MYMAP::value_type("a", 1));
if (!pa.second)
{
    pa.first->second++;
}

This does not work. How can I do this?

Upvotes: 1

Views: 4847

Answers (2)

Manohar Reddy Poreddy
Manohar Reddy Poreddy

Reputation: 27525

unordered_map:

Some beautiful code (variable names simplified):
From here http://en.cppreference.com/w/cpp/container/unordered_map/operator_at

std::unordered_map<char, int> mu1 {{'a', 27}, {'b', 3}, {'c', 1}}; 
mu1['b'] = 42;  // update an existing value
mu1['x'] = 9;   // insert a new value
for (const auto &pair: mu1) {
    std::cout << pair.first << ": " << pair.second << '\n';
}

// count the number of occurrences of each word
std::unordered_map<std::string, int> mu2;
for (const auto &w : { "this", "sentence", "is", "not", "a", "sentence", "this", "sentence", "is", "a", "hoax"}) {
    ++mu2[w];   // the first call to operator[] initialized the counter with zero
}
for (const auto &pair: mu2) {
    std::cout << pair.second << " occurrences of word '" << pair.first << "'\n";
}

Upvotes: 1

Puppy
Puppy

Reputation: 147054

You don't need iterators to achieve this goal. Because your v is V() + 1, then you can simply increment without needing to know whether or not the key was already present in the map.

mymap["a"]++;

This will do fine in your given example.

Upvotes: 4

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