Apparently your a variable was initialized with 0.
%udisplays an unsigned integer, and you pass the memory address as an argument%pusually displays the value of a pointer, so in case of 0 it shows it as(nil)%pdoes the same, but you now pass the address ofa, which is displayed in hex.%fpis%f(float formatting) and a literalp. I’m pretty sure this one causes undefined behavior since printf expects a float and you pass an integer (pointers are long/integer values).
What do we learn from it? Don’t write nonsense code and don’t pass arguments to printf-style functions unless you have a format string that expects exactly those arguments.