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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.
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str[i] = str[i]*str[i]; looks incorrect as well. You are squaring a character with ASCII values between 48 and 58. Since 48 * 48 > 255, the answer placed into that character spot in the string would be unusable. Likewise, changing the characters to their integer value wouldn't be sufficient since the square of numbers greater then 3 would need two characters to be correctly represented in the string.