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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.
I believe it is because you have not split the string into separate letters with str.split(''). Therefore when you call str[i] without it being an array you get an error message.