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Collections are a way for you to organize kata so that you can create your own training routines. Every collection you create is public and automatically sharable with other warriors. After you have added a few kata to a collection you and others can train on the kata contained within the collection.
Get started now by creating a new collection.
If you google the ASCII table it might make more sense but basically the characters 0-9 are grouped together on the ascii table. Meaning if you take the character of 0 ('0') and add 1 to it, you'll get the character '1' and its basically how this coder is converting int to char and then throws it into the string(which is an array of chars, c++ strings are bit fancier than that tho).
namespace is not needed and on more complex programs delcaring a golbal namespace can lead to naming conflits.