How to make groups in a data.frame equal length?

dataframe, r

Solution

One way using `data.table`

df <- structure(list(V1 = structure(c(1L, 1L, 2L, 2L, 2L, 3L), 
          .Label = c("A  ", "B  ", "C  "), class = "factor"), 
          V2 = c(45, 66, 99, 34, 71, 22)), 
          .Names = c("V1", "V2"), 
          class = "data.frame", row.names = c(NA, -6L))

require(data.table)
dt <- data.table(df, key="V1")

# get maximum index
idx <- max(dt[, .N, by=V1]$N)

# get final result
dt[, list(V2 = c(V2, rep(0, idx-length(V2)))), by=V1]

#     V1 V2
# 1: A   45
# 2: A   66
# 3: A    0
# 4: B   99
# 5: B   34
# 6: B   71
# 7: C   22
# 8: C    0
# 9: C    0

Problem

I have this data.frame: ``` df <- data.frame(id=c('A','A','B','B','B','C'), amount=c(45,66,99,34,71,22)) id | amount ----------- A | 45 A | 66 B | 99 B | 34 B | 71 C | 22 ``` which I need to expand so that each `by` group in the data.frame is of equal length (filling it out with zeroes), like so: ``` id | amount ----------- A | 45 A | 66 A | 0 <- added B | 99 B | 34 B | 71 C | 22 C | 0 <- added C | 0 <- added ``` What is the most efficient way of doing this? NOTE Benchmarking the some of the solutions provided with my actual 1 million row data.frame I got: ``` plyr | data.table | unstack ----------------------------------- Elapsed: 139.87s | 0.09s | 2.00s ```

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