# How to Fix in R: dim(X) must have a positive length

One error you may encounter in R is:

```Error in apply(df\$var1, 2, mean) : dim(X) must have a positive length
```

This error occurs when you attempt to use the apply() function to calculate some metric for a column of a data frame or matrix, yet provide a vector as an argument instead of a data frame or matrix.

This tutorial shares exactly how to fix this error.

### How to Reproduce the Error

Suppose we have the following data frame in R:

```#create data frame
df <- data.frame(points=c(99, 97, 104, 79, 84, 88, 91, 99),
rebounds=c(34, 40, 41, 38, 29, 30, 22, 25),
blocks=c(12, 8, 8, 7, 8, 11, 6, 7))

#view data frame
df

points rebounds blocks
1     99       34     12
2     97       40      8
3    104       41      8
4     79       38      7
5     84       29      8
6     88       30     11
7     91       22      6
8     99       25      7```

Now suppose we attempt to use the apply() function to calculate the mean value in the ‘points’ column:

```#attempt to calculate mean of 'points' column
apply(df\$points, 2, mean)

Error in apply(df\$points, 2, mean) : dim(X) must have a positive length
```

An error occurs because the apply() function must be applied to a data frame or matrix, yet in this example we attempt to apply it to a specific column in the data frame.

### How to Fix the Error

The way to fix this error is to simply provide the name of the data frame to the apply() function as follows:

```#calculate mean of every column in data frame
apply(df, 2, mean)

points rebounds   blocks
92.625   32.375    8.375
```

From the output, we can see the  mean value of each column in the data frame. For example, the mean value of the ‘points’ column is 92.625.

We can also use this function to only find the mean of specific values in the data frame:

```#calculate mean of 'points' and 'blocks' column in data frame
apply(df[c('points', 'blocks')], 2, mean)

points blocks
92.625  8.375```

Lastly, if we’d like to find the mean of just one column then we can use the mean() function without using the apply() function at all:

```#calculate mean of 'points' column
mean(df\$points)

 92.625
```