I think you might be somewhat confused.  Dally does not ever slap Johnny.  In chapter three Johnny and Ponyboy were in the lot talking, and Johnny says he doesn't have anyone who cares about him.  Ponyboy tells him that he is wrong.  He says that the whole gang loves Johnny and the only reason Dally didn't slap him at the Drive-In was that he was a pet of the group.  Later Darry slaps Ponyboy because he comes in late and they get into an argument.  This is when Ponyboy runs away, joins Johnny and they go to the park.