Read a single lowercase letter and print its uppercase form, using arithmetic on the character’s numeric value –
not a library helper like toupper.
This is the payoff of “a char is just a small integer”: 'a', 'b', … 'z' are consecutive integer values in
ASCII, and so are 'A' … 'Z', with a fixed gap between the two blocks. 'a' - 'A' computes that gap as a
number (32, though you shouldn’t need to know that – write it in terms of the character constants, not the
literal 32, so the code documents why it works). Subtracting the gap from any lowercase letter lands you on the
matching uppercase letter.