Print value of multiple equal keys once time C++ - c++

Question

Good morning,
I have a multiple map defined in this way
multimap<string, string> H_test;
with value insered from user(multiple book written by the same author in a random order)
H_test.insert(pair<string, string>(author, book));
I would like to have as output something like that :
Author I // one author
Book A // all book written by this author
Book B
Author II
Book K
Author III
Book C
Book D
Book E
Using this code
for (multimap<string,string>::iterator it = H_test.begin(); it != H_test.end(); it++)
{
//if ()
{
pair <multimap<string, string>::iterator, multimap<string, string>::iterator> ret;
ret = H_test.equal_range(it->first);
std::cout << it->first << endl;
for (multimap<string, string>::iterator sd = ret.first; sd != ret.second; sd++)
{
cout << "\t" << sd->second << endl;
}
}
}
I obtain this output
Author I
Book A
Book B
Author I
Book A
Book B
Author II
Book K
Author III
..
In my opinion, it prints the name of first key (and all the books which have the same author) and then it goes to the second nome (which has the same name), and re-write everything.
Do you know how to solve it?
Thanks

Answer 1

upper_bound is your friend. You'd want to use it something like this:
auto start = begin(H_test);
while (start != end(H_test)) {
auto finish = H_test.upper_bound(start->first);
cout << "Author: " << start->first << endl;
for_each(start, finish, [](const auto& i) { cout << "Book: " << i.second << endl; });
start = finish;
}
Live Example
Note: You can use cbegin and cend instead of if H_test is const.
EDIT:
It helps me to conceptualize a multimap as a sorted array of pairs, where the first element in the pair is the key and the second element is the value. Here's effectively what we'll do:
Iterate through all the pairs using start as our iterator, beginning at the first pair: auto start = begin(H_test) and going to the last pair in the multimap: while(start != end(H_test))
Next we want to find the range of pairs in the container that share the same key, for that we'll find an iterator pointing one past the end using the upper_bound function that I linked the details about above: auto finish = H_test.upper_bound(start->first)
We will print the current author who's books we just found in the range: cout << "Author: " << start->first << endl
Then we'll print each book in the range defined by start to finish using for_each and a lambda to only print the value of the pair then move to the next line: for_each(start, finish, [](const auto& i) { cout << "Book: " << i.second << endl; });
Finally we'll set start to the beginning of the next range, which will be printed next time through the loop: start = finish

Answer 2

Simply iterate through the multimap and since all the elements with the same author are grouped together, you just need to detect a change in author to know you have moved on to the next author at which time you print the name of the author. Preset the lastAuthor to something that won't potentially match a real author. This executes in O(n)
string lastAuthor = "**SENTINEL**";
for (const auto& elem : H_test)
{
if(lastAuthor != elem.first)
{
cout << "Author: " << elem.first << '\n';
lastAuthor = elem.first;
}
cout << "Book: " << elem.second << '\n';
}