How to Deal with Error in eval in R

The eval() function in R allows us to execute expressions dynamically. But there can be errors during this execution. This article discusses common errors in eval() and how to effectively handle them.

Common Errors in eval() and Their Solutions

1. Undefined Variable

An error occurs when a variable used in the expression is not defined.

expr <- parse(text = "undefined_var + 5")

tryCatch(
  eval(expr),
  error = function(e) {
    print(paste("Error:", e$message))
  }
)

Output:

[1] "Error: object 'undefined_var' not found"

Solution: Define the variable before using it

undefined_var <- 10
expr <- parse(text = "undefined_var + 5")

tryCatch(
  eval(expr),
  error = function(e) {
    print(paste("Error:", e$message))
  }
)

Output:

[1] 15

• Construct an expression (expr) with parse(), which is the addition of undefined_var and 5.
• Utilize tryCatch() to execute the expression with eval().
• If there is an error (e.g., undefined_var is not declared), the error message is printed.
• If there is no error, the expression is executed, and the result (15) is printed.

2. Syntax Errors

expr <- parse(text = "sum(1, 2, 3,)")

tryCatch(
  eval(expr),
  error = function(e) {
    print(paste("Error:", e$message))
  }
)

Output:

[1] "Error: argument 4 is empty"

The expression attempts to calculate the sum of numbers, but there's a syntax error due to the extra comma after the last element in the sequence. The error message highlights the syntax issue.

Solution: Correct the syntax error in the expression.

# Code Example
expr <- parse(text = "sum(1, 2, 3)")

tryCatch(
  eval(expr),
  error = function(e) {
    print(paste("Error:", e$message))
  }
)

Output:

[1] 6

An expression (expr) is created using the parse() function, representing the sum of 1, 2, and 3.

• The tryCatch() block is used to dynamically evaluate the expression (eval(expr)).
• If an error occurs during evaluation (e.g., due to syntax issues), the error function within tryCatch is triggered.

3. Runtime Errors

expr <- parse(text = "1 / 0")

tryCatch(
  eval(expr),
  error = function(e) {
    print(paste("Error:", e$message))
  }
)

Output:

[1] Inf

This example involves a runtime error by attempting to divide by zero. The error message indicates that a numeric calculation encountered an infinite result.

Solution: Add checks to prevent runtime errors, such as division by zero.

divisor <- 2
expr <- parse(text = paste("1 /", divisor))

tryCatch(
  eval(expr),
  error = function(e) {
    print(paste("Error:", e$message))
  }
)

Output:

[1] 0.5

divisor is defined and assigned a value of 2.

• An expression (expr) is created using the parse() function, representing the division of 1 by the value of divisor.
• The tryCatch() block is used to dynamically evaluate the expression (eval(expr)).

4. Invalid Function Call

expr <- parse(text = "1 / 0")

tryCatch(
  eval(expr),
  error = function(e) {
    print(paste("Error:", e$message))
  }
)

Output:

[1] Inf

The expression calls a function (`nonexistent_function`) that is not defined or loaded in the R environment, resulting in an error.

Solution: Ensure the function is defined or loaded before calling it.

nonexistent_function <- function(x) {
  x * 2
}

expr <- parse(text = "nonexistent_function(3)")

tryCatch(
  eval(expr),
  error = function(e) {
    print(paste("Error:", e$message))
  }
)

Output:

[1] 6

nonexistent_function is defined to take a parameter x and return its double.

• An expression (expr) is created using the parse() function, representing a call to nonexistent_function with the argument 3.
• The tryCatch() block is used to dynamically evaluate the expression (eval(expr)).