How do I get count of number of items in selection?

Question

I want to list in array format how many in each Diet group (there are four) have Time > 21.

I have tried to solve this in RStudio.

data(ChickWeight)
newdata <- subset(ChickWeight, Time >= 21, select=Diet)

In order to find how many observations are in newdata, I used nrow(newdata), but I would like to find out how many observations meet the criteria just by making it a part of this expression:

newdata <- subset(ChickWeight, Time >= 21, select=Diet) 

so that when I display newdata the table will also contain the number of observations that meet the criteria in a new column.

Desire output:

Diet   Number Observations
1      200 (I just created the numbers for this column as examples)
2       75
3      150
4      100 

Is there a way to do that?

Answer 1

It can be done in base:

transform(table(Diet=subset(ChickWeight, Time >= 21, select=Diet)))

#>   Diet Freq
#> 1    1   16
#> 2    2   10
#> 3    3   10
#> 4    4    9

Answer 2

Consider a straightforward aggregate after the subset call:

newdata <- subset(ChickWeight, Time >= 21, select=Diet)

aggregate(cbind(Obs=Diet) ~ Diet, newdata, FUN=length)

#   Diet Obs
# 1    1  16
# 2    2  10
# 3    3  10
# 4    4   9

Answer 3

We can do this with summarize from dplyr:

library(dplyr)

newdata %>%
  group_by(Diet) %>%
  summarize(Num_Obs = n())

We can even combine the subset to a single dplyr workflow:

ChickWeight %>%
  filter(Time >= 21) %>%
  group_by(Diet) %>%
  summarize(Num_Obs = n())

Output:

# A tibble: 4 x 2
  Diet  Num_Obs
  <fct>   <int>
1 1          16
2 2          10
3 3          10
4 4           9