Object not found

In exercise 2.5.1 we ask to store the column names of the murders dataframe in question1. Submitting the following code where we mistype murders

### loading data ###
library(dslabs)
data(murders)

### question 1 ###
question1 <- names(muders)

Will return the following error

Error while evaluating context: object `muders` not found

Meaning Dodona can not find the object muders as it does not exist.

This message can also be returned when you are trying to access columns of a dataframe inside a tidyverse function such as mutate() or filter(). Take for example the example code from 4.2.1-3: The correct column name is population, but here we wrote populations.

library(dplyr)
library(dslabs)
data(murders)

murders <- mutate(murders, population_in_millions = populations / 10^6)

Inside tidyverse functions like mutate R will look for a column named populations, as it does not exist it cannot find it. It will then look if there is an object with that name, there is none. Therefore Dodona returns the following error

Error while evaluating context: Problem with `mutate()` column `population_in_millions`. ℹ `population_in_millions = populations/10^6`. ✖ object 'populations' not found

Unexpected symbol

A common mistake is to write a certain character more than needed, for example a ) too much after a function. Or forgetting to close a for loop. Submitting the following code:

group_iq <- function(n){
    mean(rnorm(n, 100, 15))
}}

Will return the error

Error while evaluating context: <text>:3:2: unexpected '}' 2: mean(rnorm(n, 100, 15)) 3: }} ^

This actually tells you all you need. After it explains that at line 3, character 2 there is an unexpected bracket. After which it shows you the line before and the line with the mistake.

It might not always be as obvious where the error is. If we submit the following code, where we didn’t close our mean and rnorm function.

group_iq <- function(n){
    mean(rnorm(n, 100, 15
}

We get the same error.

Error while evaluating context: <text>:3:1: unexpected '}' 2: mean(rnorm(n, 100, 15 3: } ^

It still tells us that there is an unexpected } at line 3 character 1. However this is not what our mistake is. Because we did not close the round brackets from mean and rnorm R will continue reading the code. Only when R reads } on line 3 it returns the error.

https://dodona.ugent.be/en/submissions/8439221/¹

non-numeric argument to binary operator