combine data in depending on the value of one column - r

Question

I have a data frame in R
year group sales
1 2000 1 20
2 2001 1 25
3 2002 1 23
4 2003 1 30
5 2001 2 50
6 2002 2 55
And I want to group the data by groups or create some kind of object. I want to create one array for each group that will store the year and the sales. And the I will try to save it as a json file with this structure:
[{"group": 1, "sales":[[2000,20],[2001, 25], [2002,23], [2003, 30]]},
{"group": 2, "sales":[[2001, 50], [2002,55]]}]
Is it possible to do it automatically?
Thanks a lot

Answer 1

We can use data.table to paste the 'year' and 'sales' column grouped by 'group. We convert the 'data.frame' to 'data.table' (setDT(df1)). Group by 'group', we use sprintf to paste the 'year', 'sales' along with the parentheses ([]), then collapse the output to a single string with toString (it is a wrapper for paste(..., collapse=', ')), paste the [], and use toJSON.
library(jsonlite)
library(data.table)
toJSON(setDT(df1)[, list(sales= paste0('[',toString(sprintf('[%d,%d]',
year, sales)),']')), by = group])
#[{"group":1,"sales":"[[2000,20], [2001,25], [2002,23], [2003,30]]"},
#{"group":2,"sales":"[[2001,50], [2002,55]]"}]
The paste by group can be done using base R. We split the dataset by the 'group' column to create a list. Loop through the list with lapply, paste, the 'year', 'sales' column as mentioned above. Create a data.frame with the first element of 'group' and the string from the paste step, rbind the list elements to create a single data.frame and then use toJSON.
toJSON(
do.call(rbind,
lapply(
split(df1, df1$group),
function(x) data.frame(group=x$group[1L],
sales=paste0('[',
toString(sprintf('[%d,%d]', x$year, x$sales)),
']')))))
data
df1 <- structure(list(year = c(2000L, 2001L, 2002L, 2003L, 2001L, 2002L
), group = c(1L, 1L, 1L, 1L, 2L, 2L), sales = c(20L, 25L, 23L,
30L, 50L, 55L)), .Names = c("year", "group", "sales"),
class = "data.frame", row.names = c(NA, -6L))

Answer 2

Since the other answer uses data.table, I thought it would be a interesting exercise to try to do this in dplyr. This is not the optimal way but illustrates do which I'm not convinced is well enough documented. I have also shown the more appropriate summarise solution.
df <-read.table(textConnection('
year group sales expenses
2000 1 20 19
2001 1 25 19
2002 1 23 20
2003 1 30 15
2001 2 50 27
2002 2 55 30
'),header=TRUE)
library(dplyr)
library(jsonlite)
df %>%
group_by( group ) %>%
do(
sales = group_by(.,year) %>% select(sales) %>% apply(MARGIN=2,identity),
expenses = group_by(.,year) %>% select(expenses) %>% apply(MARGIN=2,identity)
)
df %>%
group_by( group ) %>%
summarise(
sales = list(apply( data.frame(year,sales), MARGIN=2, identity ))
,expenses = list(apply( data.frame(year,sales), MARGIN=2, identity ))
) %>% jsonlite::toJSON()