How to update std map after using the find method

Question

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  lt char  int gt  m1  map  lt char  int gt   iterator m1 it  typedef pair  lt char  int gt  count pair    I m using the map to store the number of occurrences of a character    I m using Visual C   2010

User · Answer

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 lt char int gt  mymap                                       a   0                                      b   0                                     mymap at  a     10    mymap at  b     20

User · Answer

I would use the operator     map  lt char  int gt  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  lt char  int gt                 m1  std  ifstream                  file  Plop    std  istreambuf iterator lt char gt  end   for std  istreambuf iterator lt char gt  loop file   loop    end    loop          m1  loop      prefer prefix increment out of habbit

User · Answer

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 lt int  int gt  m   for int i 0  i lt 5  i        m i    i   for auto it m begin    it  m end    it        cout lt  lt it- gt second lt  lt        Output  0 1 2 3 4  m 4    7     updating value at key 4 here  cout lt  lt   n     Change line  for auto it m begin    it  m end    it        cout lt  lt it- gt second lt  lt         Output  0 1 2 3 7

User · Answer

std  map  find returns an iterator to the found element  or to the end   if the element was not found    So long as the map is not const  you can modify the element pointed to by the iterator   std  map lt char  int gt  m  m insert std  make pair  c   0        c is for cookie  std  map lt char  int gt   iterator it   m find  c     if  it    m end        it- gt second   42

User · Answer

You can update the value like following     auto itr   m find  ch          if  itr    m end                  itr  second   98

User · Answer

You can also do like this-   std  map lt char  int gt   iterator it   m find  c      if  it    m end       it  second   42

[c++] How to update std::map after using the find method?

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