When you're initialising an array if you don't specify elements (after the first) they're zero initialised. So all we have to do is specify the first element of the outer array:
int arr[10][10] = {{0}};
and we're done.
Oh, and mandatory lecture about using arrays in C++: Is there any reason you're using an array rather than a vector of vectors? Efficiency of creation is a legitimate answer as creating vectors of vectors is more expensive but when they're built they're just as efficient in most cases.
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