I have a data frame with entries running from 0 to 5 where 0 means a missing entry. I want to repalce the missing entries with the mean of each row. The mean should be calculated only on the not missing values. How do I do that?

For example, the row:

```
[1] 1 2 4 0 3

```

Will become:

```
[1] 1.0 2.0 4.0 2.5 3.0

```