What is the name and reason for the [1] at the output prompt?

Question

What's the name for the [1] below.

What is its significance?

Is it always only [1]? If not, then under what conditions is it something else? (example please)

> bb <- c(5,6,7)
> bb
[1] 5 6 7

Answer 1

It shows the count of the variables. In your case, it shows

bb <- c(5,6,7)
> bb
# [1] 5 6 7

Try,

c(1:50)
#[1]  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
#[35] 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

You can also avoid that being displayed by using cat

cat(c(1:50))

#1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50