Creating a for Loop to Choose Random Years Without Replacement in R

In this article, we will explore the process of creating random groups of years without replacement using a for loop in R. We will delve into the details of how the sample() function works, and we’ll also discuss some best practices for generating random samples.

Understanding the Problem

The problem at hand involves selecting 8 groups of 4 years each and two additional groups with 5 years without replacement from a given vector of years.

Choosing Random Years Without Replacement

R’s sample() function is used to select random elements from a vector or list. When choosing randomly from a set, we want to ensure that the selected elements are drawn without replacement. This means that once an element is chosen, it cannot be selected again.

The general syntax for the sample() function is:

sample(x, size = NULL, replace = FALSE)

In this context, x represents the vector or list of elements from which to draw the sample, and size specifies the number of elements to choose. The replace parameter determines whether replacement is allowed (i.e., if an element can be selected multiple times).

Creating a Vector of Years

To start generating random groups of years without replacement, we first need to create a vector of years.

g <- c(1979:2020)

This creates a vector g containing all the years from 1979 to 2020.

Splitting the Sample

We can use R’s built-in functions split() and gl() (generalized linear) to split our sample into groups. However, this approach is not suitable for creating groups with different sizes without replacement.

Using a for Loop to Create Groups

A more suitable approach involves using a for loop to create the desired groups of years. Here’s an example implementation:

g <- c(1979:2020)
l <- vector(10, mode = "list")

for (i in seq(1, 10)) {
  
  if(i %in% seq(8)) {
    l[[i]] <- sample(g, size = 4)
    g <- g[!(g %in% unlist(l))]
  } else if (i %in% 9:10) {
    l[[i]] <- sample(g, size = 5)
    g <- g[!(g %in% unlist(l))]
  }
}

# Print the groups
l

This code creates a vector l to store our generated groups of years. We then use a for loop to iterate over the range from 1 to 10 (inclusive). For each iteration, we check whether the current index is within the first 8 elements or between 9 and 10.

Handling Different Group Sizes

If the index falls within the first 8 elements, we generate a group of 4 years using sample(g, size = 4). We then update the original vector g by removing all the chosen years from it. This ensures that no year is selected multiple times.

For indices between 9 and 10, we generate a group of 5 years using sample(g, size = 5).

Printing the Groups

Finally, we print the groups stored in our vector l by calling print(l).

Last modified on 2025-05-01