Understanding lapply and Looping through a List in R

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In this article, we will delve into the world of R programming language and explore how to use the lapply function to loop through a list. We will examine why using the dollar operator ($) on a fixed string instead of a variable name can lead to unexpected results.

Introduction to lapply

The lapply function in R is used to apply a function to each element of a list. It returns an object of class “list” where each component is the result of applying the function to the corresponding element of the input list.

Basic Syntax

lapply(x, func)

• x: The input list.
• func: The function to be applied to each element of the list.

Subsetting a List in R

When working with lists in R, it’s essential to understand how to subset them. There are several ways to do this, and we will explore some of them in this section.

Using the Dollar Operator

The dollar operator ($) is used to subset a list or data frame. However, when using the dollar operator on a fixed string instead of a variable name can lead to unexpected results.

df$fixed_string

Is equivalent to:

df["fixed_string"]

or,

df[["fixed_string"]]

As we will see later in this article, using the dollar operator on a fixed string instead of a variable name can lead to unexpected results.

The Issue with Subsetting a List Using the Dollar Operator

In your question, you mentioned that if you just do xts(l.df$T[,-1:-5],order.by=as.POSIXct(rownames(l.df$T))), you get the desired output. However, this is because you are using a variable name (i) instead of a fixed string.

lapply(list(function(i) xts(l.df$i[,-1:-5],order.by=as.POSIXct(rownames(l.df$i))))

In this code, i is the variable name that holds the value "T". Therefore, l.df$i subsets the data frame l.df using the variable name i, which is equivalent to l.df["T"].

On the other hand, if you use the dollar operator on a fixed string, it will not subset the data frame correctly.

lapply(list(function(i) xts(l.df$fixed_string[,-1:-5],order.by=as.POSIXct(rownames(l.df$fixed_string))))

In this code, l.df$fixed_string attempts to subset the data frame using a fixed string "fixed_string". However, since there is no column with that name in the data frame, it will return an error.

Subsetting a List Using Variable Names

To avoid unexpected results when subsetting a list, it’s essential to use variable names instead of fixed strings. In your example, you can achieve the desired output by using a variable name like this:

list(function(i) xts(l.df$i[,-1:-5],order.by=as.POSIXct(rownames(l.df$i))))

In this code, i is a variable that holds the value "T". Therefore, l.df$i subsets the data frame l.df using the correct column name.

Using Subsetting Functions

If you need to subset a list multiple times, you can use the following functions:

[[ and [[]

These functions allow you to subset a list without quotes.

df[["column_name"]]

or,

df["column_name"]

Both of these will return the specified column from the data frame.

$

The dollar operator ($) is used to subset a list or data frame. However, it only works if you use variable names instead of fixed strings.

df.variable_name

is equivalent to:

df[variable_name]

or,

df["variable_name"]

[ and [[

These functions are used to subset a list or data frame. The double square brackets [[ ]] will return an error if the key does not exist, while the single square brackets [ ] will return NA.

df[ "column_name" ]

is equivalent to:

df["column_name"]

However, if you use the following function:

df[["column_name"]]

It will return an error if the key does not exist.

Conclusion

In this article, we have explored how to loop through a list in R using lapply. We have examined why using the dollar operator on a fixed string instead of a variable name can lead to unexpected results. Finally, we have discussed some subsetting functions that you can use to subset a list in R.

Best Practices

• Always use variable names instead of fixed strings when subsetting a list or data frame.
• Use the following subsetting functions:
  • [ and [[]
  • $
  • [[ (double square brackets)
• Avoid using the dollar operator ($) on a fixed string.

By following these best practices, you can avoid unexpected results when working with lists in R.

Last modified on 2023-06-15