How to update the value of a key in std::map
after using the find
method?
I have a map and iterator declaration like this:
map <char, int> m1;
map <char, int>::iterator m1_it;
typedef pair <char, int> count_pair;
I'm using the map to store the number of occurrences of a character.
I'm using Visual C++ 2010.
I would use the operator[].
map <char, int> m1;
m1['G'] ++; // If the element 'G' does not exist then it is created and
// initialized to zero. A reference to the internal value
// is returned. so that the ++ operator can be applied.
// If 'G' did not exist it now exist and is 1.
// If 'G' had a value of 'n' it now has a value of 'n+1'
So using this technique it becomes really easy to read all the character from a stream and count them:
map <char, int> m1;
std::ifstream file("Plop");
std::istreambuf_iterator<char> end;
for(std::istreambuf_iterator<char> loop(file); loop != end; ++loop)
{
++m1[*loop]; // prefer prefix increment out of habbit
}
You can use std::map::at
member function, it returns a reference to the mapped value of the element identified with key k.
std::map<char,int> mymap = {
{ 'a', 0 },
{ 'b', 0 },
};
mymap.at('a') = 10;
mymap.at('b') = 20;
You can update the value like following
auto itr = m.find('ch');
if (itr != m.end()){
(*itr).second = 98;
}
You can also do like this-
std::map<char, int>::iterator it = m.find('c');
if (it != m.end())
(*it).second = 42;
If you already know the key, you can directly update the value at that key using m[key] = new_value
Here is a sample code that might help:
map<int, int> m;
for(int i=0; i<5; i++)
m[i] = i;
for(auto it=m.begin(); it!=m.end(); it++)
cout<<it->second<<" ";
//Output: 0 1 2 3 4
m[4] = 7; //updating value at key 4 here
cout<<"\n"; //Change line
for(auto it=m.begin(); it!=m.end(); it++)
cout<<it->second<<" ";
// Output: 0 1 2 3 7
Source: Stackoverflow.com