Geting new data into old data frame in R

library(sqldf)

sqldf('select a.rowid
       from "df.B" b 
       left join "df.A" a on obs_min >= bin_min and obs_max <= bin_max')

   rowid
1      4
2     NA
3      5
4      2
5     NA
6      4
7      3
8      4
9      5
10     2

m <- merge(df.B, df.A)
stack(by(m, as.numeric(sub(".*_", "", m$obs_ID)), 
      with, c(which(obs_min >= bin_min & obs_max <= bin_max), NA)[1]))

   values ind
1       4   1
2      NA   2
3       5   3
4       2   4
5      NA   5
6       4   6
7       3   7
8       4   8
9       5   9
10      2  10

sapply(1:nrow(df.B), function(i)
  c(which(df.A$bin_max >= df.B$obs_max[i] & df.A$bin_min <= df.B$obs_min[i]), NA)[1]) 

[1]  4 NA  5  2 NA  4  3  4  5  2

mapply(function(x, y) c(which(x >= df.A$bin_min & y <= df.A$bin_max), NA)[1], 
       df.B$obs_min, 
       df.B$obs_max)

seq_len(nrow(df.A)) %*% 
  (outer(df.A$bin_max, df.B$obs_max, ">=") & outer(df.A$bin_min, df.B$obs_min, "<="))

     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9] [,10]
[1,]    4    0    5    2    0    4    3    4    5     2

df <- data.frame(
    month = rep(c("Jan","Mar","Apr"), 4)
    , somevalue = runif(12)
)
groups <- unique(df$month)


newdf_list <- split(df, f = df$month)
list2env(setNames(newdf_list, groups), .GlobalEnv)