Confusion about array initialization in C

Question

In C language, if initialize an array like this:

int a[5] = {1,2};

then all the elements of the array that are not initialized explicitly w

Answer 1

I don't understand, why does a[0] print 1 instead of 0?

Presumably a[2]=1 initializes a[2] first, and the result of the expression is used to initialize a[0].

From N2176 (C17 draft):

6.7.9 Initialization

23. The evaluations of the initialization list expressions are indeterminately sequenced with respect to one another and thus the order in which any side effects occur is unspecified. ¹⁵⁴⁾

So it would seem that output 1 0 0 0 0 would also have been possible.

Conclusion: Don't write initializers that modifies the initialized variable on the fly.