r - Reasons for using the set.seed function

Question

Many times I have seen the set.seed function in R, before starting the program. I know it's basically used for the random number generation. Is there any specific need to set this?

Answer 1

The need is the possible desire for reproducible results, which may for example come from trying to debug your program, or of course from trying to redo what it does:

These two results we will "never" reproduce as I just asked for something "random":

R> sample(LETTERS, 5)
[1] "K" "N" "R" "Z" "G"
R> sample(LETTERS, 5)
[1] "L" "P" "J" "E" "D"

These two, however, are identical because I set the seed:

R> set.seed(42); sample(LETTERS, 5)
[1] "X" "Z" "G" "T" "O"
R> set.seed(42); sample(LETTERS, 5)
[1] "X" "Z" "G" "T" "O"
R> 

There is vast literature on all that; Wikipedia is a good start. In essence, these RNGs are called Pseudo Random Number Generators because they are in fact fully algorithmic: given the same seed, you get the same sequence. And that is a feature and not a bug.