Error: attempt to apply non-function in R

When programming in R, you may occasionally encounter the following error:

Error: attempt to apply non-function

This error can be confusing, especially for beginners, because it doesn’t always clearly indicate where the problem lies.

In most cases, this error occurs when R encounters parentheses () immediately after an object that is not a function. Since R interprets parentheses as a function call, it attempts to execute the object as a function and fails.

In this tutorial, you’ll learn:

• What causes the “attempt to apply non-function” error
• How to reproduce the error
• Multiple ways to fix it
• Common real-world scenarios that trigger this error
• Best practices for avoiding similar issues

What Does “Attempt to Apply Non-Function” Mean?

In R, functions are objects that can be called using parentheses.

For example:

sqrt(25)

Output:

[1] 5

Here, sqrt is a function, so R successfully executes it.

However, when parentheses follow something that is not a function, R attempts to execute it and returns:

Error: attempt to apply non-function

Example 1: Reproducing the Error

Suppose we write:

5(7)

Output:

Error: attempt to apply non-function

Why Does This Happen?

R interprets the code as:

Call the object 5 as a function and pass 7 as an argument

Since 5 is a numeric value and not a function, R cannot execute it.

Solution 1: Use a Mathematical Operator

If your intention was to perform arithmetic, include the appropriate operator.

5 + 7

Output:

[1] 12

Or:

5 * 7

Output:

[1] 35

The error disappears because R now understands the operation being performed.

Example 2: Missing Multiplication Operator

A common mistake among new R users is forgetting the multiplication operator.

Incorrect

10(2)

Output:

Error: attempt to apply non-function

Correct

10 * 2

Output:

[1] 20

Unlike some calculators, R requires an explicit multiplication operator.

Example 3: Accidentally Overwriting a Function

Another common cause occurs when you overwrite a built-in function.

Create an Object Named mean

mean <- 100

Now try:

mean(c(1,2,3,4,5))

Output:

Error: attempt to apply non-function

Why?

You replaced the built-in mean() function with a numeric object.

When R encounters:

mean(...)

it attempts to call the value 100 as a function.

Fixing Overwritten Functions

Remove the object:

rm(mean)

Now:

mean(c(1,2,3,4,5))

Output:

[1] 3

The original function is restored.

Example 4: Incorrect Data Frame Access

Suppose we have:

df <- data.frame(
  x = 1:5,
  y = 6:10
)

Incorrect syntax:

df$x()

Output:

Error: attempt to apply non-function

Why?

df$x is a vector, not a function.

Correct

df$x

Output:

[1] 1 2 3 4 5

Example 5: Function Returning a Value

Consider:

x <- sum(1:10)

Now:

x(5)

Output:

Error: attempt to apply non-function

Because:

x

contains:

[1] 55

which is not callable as a function.

How to Debug This Error

When you see:

Error: attempt to apply non-function

check the following:

1. Verify Object Type

class(object_name)

or

typeof(object_name)

Example:

class(mean)

2. Check Whether It’s a Function

is.function(object_name)

Example:

is.function(mean)

Output:

FALSE

indicates that the object is no longer a function.

3. Inspect the Object

str(object_name)

This helps determine what R is attempting to execute.

Common Causes of This Error

Best Practices

Avoid Naming Variables After Functions

Bad:

mean <- 100
sum <- 50
data <- 10

Better:

mean_value <- 100
total_sum <- 50
dataset <- 10

Use Explicit Operators

Always write:

5 * 7

instead of:

5(7)

Check Object Classes

Before calling an object:

is.function(my_object)

Use Descriptive Variable Names

Descriptive names reduce the risk of accidentally overwriting important functions.

Complete Example

# Incorrect code
5(7)

# Error:
# Error: attempt to apply non-function

# Correct code
5 + 7

# Output
[1] 12

Conclusion

The error:

Error: attempt to apply non-function

occurs when R tries to execute an object that is not actually a function.

The most common causes include:

• Missing mathematical operators
• Calling vectors as functions
• Overwriting built-in functions
• Syntax mistakes involving parentheses

When troubleshooting this error:

1. Check whether the object is a function.
2. Verify object classes.
3. Ensure operators are included where necessary.
4. Avoid using reserved function names as variable names.

By understanding how R interprets function calls, you can quickly identify and resolve this error, making your code more reliable and easier to debug.