When you access the value of a variable that's got just one value, such as 73 or "Learn more about R at Computerworld.com," you'll also see this in your console before the value:

That's telling you that your screen printout is starting at vector item number one.

If you've got a vector with lots of values so the printout runs across multiple lines, each line will start with a number in brackets, telling you which vector item number that particular line is starting with.

And by the way, R assumes that 3 is the same class as 3.0 -- numeric (i.e., with a decimal point).

A matrix has rows and columns; you can find a matrix dimension with dim() such as

A matrix needs to have all the same data type in every column, such as numbers everywhere.

The text above is a summary, you can read full article here.