Why & How to Set a Random Seed in R (Example) | set.seed Function Explained
In this R programming tutorial you’ll learn how to specify a random seed using the set.seed() function.
Table of contents:
Let’s start right away…
Reasons for Using a Random Seed in R
Pseudorandom numbers are not truly random, but they aim to be as patternless as possible to simulate randomness.
Why is this important when using the R programming language?
In R, we can set a random seed to make the output of our R code reproducible. By setting a specific seed, the random processes in our script always start at the same point and hence lead to the same result.
Let’s do this in practice…
Example: Setting Random Seed Using set.seed() Function in R
In this example, I’ll show what happens if you don’t use a random seed and how you can use the set.seed function to set a seed in R.
rpois(5, 3) # Generate random numbers without seed #  1 3 3 2 6
The output of the previous R syntax is a numeric vector with the elements 1, 3, 3, 2, and 6.
Let’s execute exactly the same R code again:
rpois(5, 3) # Generate random numbers again # 3 6 3 1 2
Now, the result is a numeric vector consisting of the vector elements 3, 6, 3, 1, and 2.
As you can see, the output is completely different even though we have used exactly the same R code.
This can make it impossible to replicate a specific output and that makes it difficult to retrace the steps the programmer of the R code has made.
For that reason, it is considered as best practice for researchers to set a random seed at the beginning of R scripts that involve random processes.
Let’s see how we can do that in R!
The basic installation of the R programming language provides the set.seed function. Within the set.seed function, we simply have to specify a numeric value to set a seed.
Have a look at the following R code:
set.seed(12345) # Set seed for reproducibility rpois(5, 3) # Generate random numbers with seed #  4 5 4 5 3
We generated the random sequence 4, 5, 4, 5, and 3.
Let’s do this again with the same seed as before:
set.seed(12345) # Set same random seed rpois(5, 3) # Generate random numbers with same seed #  4 5 4 5 3
The output is exactly the same – great!
Video & Further Resources
In case you need more information on the topics of this tutorial, you might want to watch the following video of my YouTube channel. I’m illustrating the R programming code of this tutorial in the video:
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Summary: At this point you should have learned how to apply the set.seed function in the R programming language. Let me know in the comments section below, if you have additional questions.
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