How to do SUMIFS in R

Question

Data:

 set.seed(42)

df1 = data.frame(
  Date = seq.Date(as.Date("2018-01-01"),as.Date("2018-01-30"),1),
  value = sample(1:30),
  Y = sample(c("yes", "no"), 30, replace = TRUE)
)

df2 = data.frame(
  Date = seq.Date(as.Date("2018-01-01"),as.Date("2018-01-30"),7)
)

I want for each date in df2$Date calculate the sum of df1$Value if date in df1$Date falls within df2$Date and df2$Date+6

Inshort I need to calculate weekly sums

Answer 1

Using data.table, create a range start/end, then merge on overlap, then get sum over group:

library(data.table)

df1$start <- df1$Date
df1$end <- df1$Date

df2$start <- df2$Date
df2$end <- df2$Date + 6

setDT(df1, key = c("start", "end"))
setDT(df2, key = c("start", "end"))

foverlaps(df1, df2)[, list(mySum = sum(value)), by = Date ]
#          Date mySum
# 1: 2018-01-01   138
# 2: 2018-01-08    96
# 3: 2018-01-15    83
# 4: 2018-01-22   109
# 5: 2018-01-29    39