Sort Data by decade in R - r

Question

So I am fairly new to R and I am having a bit of trouble getting the hang of it. What I am trying to do is to sort my data into decades so that I can analyze the mean value for each decade. So far this is what I have tried:
fred$decade = cut(as.numeric(format(fred$DATE, "%Y")),breaks=seq(1940, 2020, 10))
Error in format.default(structure(as.character(x), names = names(x),
dim = dim(x), :
invalid 'trim' argument
Here is part of the data I am using: I am looking at CPI data since 1948 for every month until 9/1/2016. I want to get the mean CPI of each decade since then:
DATE CPI
8/1/49 23.7
9/1/49 23.75
10/1/49 23.67
11/1/49 23.7
12/1/49 23.61
1/1/50 23.51
2/1/50 23.61
3/1/50 23.64
4/1/50 23.65
5/1/50 23.77
6/1/50 23.88
7/1/50 24.07
8/1/50 24.2
When I use this I always get an error message. I cannot seem to figure out what I am doing wrong. I went through my data to make sure it was fine. Thanks for your help!

Answer 1

Considering dput(stsample) as
structure(list(Date = structure(c(8L, 10L, 11L, 12L, 13L, 1L,
2L, 3L, 4L, 5L, 6L, 7L, 9L), .Label = c("01-01-1950", "02-01-1950",
"03-01-1950", "04-01-1950", "05-01-1950", "06-01-1950", "07-01-1950",
"08-01-1949", "08-01-1950", "09-01-1949", "10-01-1949", "11-01-1949",
"12-01-1949"), class = "factor"), CPI = c(23.7, 23.75, 23.67,
23.7, 23.61, 23.51, 23.61, 23.64, 23.65, 23.77, 23.88, 24.07,
24.2)), .Names = c("Date", "CPI"), class = "data.frame", row.names = c(NA,
-13L))
you can try something like
stsample$Date <- as.Date(stsample$Date, "%d-%m-%Y")
stsample$year<-as.numeric(format(stsample$Date, "%Y"))
stsample$decade = cut(stsample$year, seq(from = 1940, to = 2020, by = 10))
Note that the breaks work only on the year part of the date and not the whole object. If you have datetime objects, it might be worth looking into
cut.POSIXt

Answer 2

You can try this too (output shown with some randomly generated data):
# assuming 40-49 is the decade 40s
fred$DECADE <- 10*as.integer(as.numeric(substring(as.character(fred$DATE), 7, 8)) / 10)
head(fred)
DATE CPI DECADE
1 08/01/49 23.41955 40
2 09/01/49 26.99772 40
3 10/02/49 29.53724 40
4 11/02/49 19.84247 40
5 12/03/49 26.75672 40
6 01/03/50 30.97788 50
# mean value for each DECADE
aggregate(CPI~DECADE, data=fred, FUN=mean)
DECADE CPI
1 40 25.31074
2 50 25.27004
3 60 24.72269