Examples:

char s[6]   = {'h', 'e', 'l', 'l', 'o', '\0'};

char t[6]   = "hello";

int fib[20] = {1, 1};

int vec[]   = {5, 4, 3, 2, 1};

In the third case, fib[0] == fib[1] == 1 while the initial values fib[2] .. fib[19] are undefined.

In the last case, C infers the array length   (as if we declared vec[5]).