Question

I have created a multimap as I have repeating keys. But I want do an efficient manipulation so that I can generate a new multimap with subsequent higher keys aligned. This is what I mean:

This is what I have:

key        values
11          qwer
11          mfiri
21          iernr
21          ghfnfjf
43          dnvfrf

This is what I want to achive

key        values
11          qwer,iernr
11          mfiri,iernr
21         iernr,dnvfrf
21          ghfnfjf,dnvfrf
43          dnvfrf

I have about 10 million entries so I am looking for something efficient.

In above value “qwer,iernr” is one string.

Answer 1

Looks like straight-forward way would work fine. Map elements will be laid out in ascending order (assuming compare operator suits you). So just going through the equal ranges and modifying them with value of the element just after the range will do what you want.

Clone map (if you need the original), take first element, get equal_range() for its key, modify values with value of second iterator in the range (unless it is the last one). Get equal_range() for the key of second iterator. Repeat.