swift guard关键字详解及使用

func buy( money: Int , price: Int , capacity: Int , volume: Int){  if money >= price{    if capacity >= volume{      print("I can buy it!")      print("/(money-price) Yuan left.")      print("/(capacity-volume) cubic meters left")    }    else{      print("No enough capacity")    }  }  else{    print("Not enough money")  }}

func buy2( money: Int , price: Int , capacity: Int , volume: Int){  guard money >= price else{    print("Not enough money")    return  }  guard capacity >= volume else{    print("Not enough capacity")    return  }  print("/(money-price) Yuan left.")  print("/(capacity-volume) cubic meters left")}