Writing to Non-Local Variables with assign()
In R, the assign() function can be used to write to variables in non-local environments. Unlike the superassignment operator (<<-), assign() allows more control over the environment where a variable is being modified. Here’s a detailed explanation:
Understanding assign()
The assign() function allows you to assign a value to a variable in a specified environment, which can be useful for manipulating variables outside the current function’s local scope.
Syntax:
assign(x, value, envir = .GlobalEnv, inherits = FALSE)
- x: The name of the variable to be assigned (as a string).
- value: The value to be assigned to the variable.
- envir: The environment in which to assign the variable (default is .GlobalEnv for the global environment).
- inherits: Logical. If TRUE, it searches parent environments for the variable (default is FALSE).
Basic Example
Here’s a simple example of using assign() to modify a variable in the global environment:
# Define a variable in the global environment
my_var <- 10
# Function to modify 'my_var' in the global environment
modify_global_var <- function() {
assign("my_var", 20, envir = .GlobalEnv)
}
modify_global_var()
print(my_var) # Prints 20
In this example, assign() changes the value of my_var in the global environment.
Using assign() with Environments
assign() can also be used to assign values in specific environments other than the global environment.
# Create a new environment
my_env <- new.env()
# Define a variable in this environment
my_env$var <- 5
# Function to modify 'var' in 'my_env'
modify_env_var <- function(env) {
assign("var", 10, envir = env)
}
modify_env_var(my_env)
print(my_env$var) # Prints 10
Here, assign() modifies var within my_env rather than the global environment.
Using assign() for Dynamic Variable Names
assign() is useful for dynamically generating variable names.
# Function to create multiple variables dynamically
create_vars <- function(n) {
for (i in 1:n) {
assign(paste0("var_", i), i, envir = .GlobalEnv)
}
}
create_vars(3)
print(var_1) # Prints 1
print(var_2) # Prints 2
print(var_3) # Prints 3
In this example, assign() creates multiple variables with names constructed dynamically using paste0().
Differences Between assign() and <<-
- Scope Control: assign() allows specifying the environment explicitly, whereas <<- affects the first environment it finds up the scope chain.
- Flexibility: assign() provides more control over which environment is affected and can be used to manipulate variables in non-standard environments.
Summary
- assign() Function: Allows assigning values to variables in a specified environment.
- Syntax: assign(x, value, envir = .GlobalEnv, inherits = FALSE).
- Basic Example: Modifies a global variable.
- Environments: Can modify variables in specific environments other than the global one.
- Dynamic Variables: Useful for creating variables with names generated dynamically.