subset() a factor by its number of observation

Question

I have a problem with subset()function. How can I subset a factor of my dataframe by its number of observation? ``` NAME CLASS COLOR VALUE antonio B YELLOW 5 antonio B BLUE 8 antonio B BLUE 7 antonio B BLUE 12 luca C YELLOW 99 luca B YELLOW 87 luca B YELLOW 98 giovanni A BLUE 48 ``` I would like to obtain data where the three factors "NAME","CLASS" and "COLOR" compare at least three times in order to make a mean of VALUE. in this case I'll obtain: ``` NAME CLASS COLOR VALUE antonio B BLUE mean ``` because antonio is the only with three observations for each factor thank you so much Nik

Accepted Answer

You can use the `table` function as follows:

```
subset(df, table(FACTOR)[FACTOR] >= 3)
#    FACTOR VALUE
# 1 ANTONIO     5
# 2 ANTONIO     8
# 3 ANTONIO     7
```

To help you understand, see what these return:

```
table(df$FACTOR)
table(df$FACTOR)[df$FACTOR]
table(df$FACTOR)[df$FACTOR] >= 3
```

You could also use the `ave` function to compute the number of observations:

```
subset(df, ave(VALUE, FACTOR, FUN = length) >= 3)
```

This last method may be a little more flexible if you have multiple factors like you asked in your comment and updated question. You can do:

```
subset(df, ave(VALUE, NAME, CLASS, COLOR, FUN = length) >= 3)
```

subset() a factor by its number of observation

Solution

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