Aggregate a data frame on variance - r

Question

Say I have this data frame, df,
Day value
1 2012-06-10 552
2 2012-06-10 4850
3 2012-06-11 4642
4 2012-06-11 4132
5 2012-06-11 4190
6 2012-06-12 4186
7 2012-06-13 1139
8 2012-06-13 490
9 2012-06-13 5156
10 2012-06-13 4430
11 2012-06-13 4447
12 2012-06-14 4256
13 2012-06-14 3856
14 2012-06-14 1163
15 2012-06-17 564
16 2012-06-17 4866
17 2012-06-17 4421
18 2012-06-19 4206
19 2012-06-20 4272
20 2012-06-20 3993
21 2012-06-20 1211
22 2012-07-21 698
23 2012-07-21 5770
24 2012-07-21 5103
25 2012-07-21 775
26 2012-07-21 5140
27 2012-07-22 4868
I would like a to create a data.frame, dfvar, that would contain the daily variance: something like:
Day Variance
1 2012-06-10 9236402
2 2012-06-11 X
3 2012-06-12 4186
4 2012-06-13 1139
5 2012-06-14 4256
6 2012-06-17 564
7 2012-06-19 4206
8 2012-06-20 4272
9 2012-07-21 698
10 2012-07-22 4868
So for example, I computed it, the entry
dfvar$Variance[1] = var(c(552, 4850))
I tried to do
dfvar <- aggregate(df, by = list(Day), FUN = var)
but this isn't the input I expected. I really want to have the variance of the values of the same day, without the other days...
Any ideas about that?

Answer 1

Is this what you want ?
library(dplyr)
df%>%group_by(Day)%>%dplyr::summarise(Variance=var(value))#return NA if only one value within the group
Day Variance
<fctr> <dbl>
1 2012-06-10 9236402.00
2 2012-06-11 77961.33
3 2012-06-12 NA
4 2012-06-13 4615704.30
5 2012-06-14 2829816.33
6 2012-06-17 5596946.33
7 2012-06-19 NA
8 2012-06-20 2864514.33
9 2012-07-21 6422224.70
10 2012-07-22 NA