a valid value for that component of
dimnames (x) . For a matrix or array this is either
NULL or a character vector of non-zero length equal to the
Unfortunately only for first item is actually true. The data.frame() and as.data.frame() methods try to do some conversions to make more of the items in the above list are usually true. We know data.frame is implemented as a list of columns, but the idea is the class data.frame overrides a lot of operators and should be able to maintain some useful invariants for us.
I think that travelling is one of the most ancient occupations of people. In ancient time travelling was slow, tiring and even dangerous at times. Nowadays it is a miracle of speed and comfort. It goes without saying that horse carriages and steamboats of the past can’t be compared to our modern fast trains and ocean-liners, to say nothing of planes. But still there is something which unites modern and ancient travellers - it’s the spirit of adventure in their souls, the desire to see and expierence everything themselves.
No one can choose between various means of transportation. Travelling by air is surely the fastest and the most convenient way of travelling.
Travelling by sea or river is really cool if you are not seasick. Modern sea-liners look like real floating cities with all modern conveniences and a wide choice of sport and entertainment facilities. Travelling by train affords a great comfort for the people. They needn’t trouble about foodstuffs as there’s usually a dining- car in each train.
If you are a great lover of nature, there’s nothing for you like travelling on foot. You can’t but observe many interesting things and admire the beautiful landscape around.
I think that at present I prefer to travel by fast trains with dining cars. I love making myself comfortable in the compartment, getting acquainted with people, watching landscapes and cities fly by. I enjoy the noise of the wheels which always brings me sweet dreams. I travelled a lot by train and visited: Moscow, Petersbug, Odessa. I was greatly impressed by my trips.
I hope that I’ll have more opportunities to travel in future. I’m looking forward to it with great pleasure.