How to look up values between dates in r - r

Question

The table below is a reference table. Column a (far left column) represents start dates. Column b (middle column) represents end dates. Column d (far right column) represents a "unique value" that corresponds to each of the time periods on the left.
a b d
1/1/07 1/1/08 a
1/1/08 1/1/09 b
1/1/09 1/1/10 c
1/1/10 1/1/11 d
1/1/11 1/1/12 e
Using the table above I have a list of dates (shown below). I would like to populate the "unique values" that correspond with the dates below. if the date below falls between two of the dates in the reference table above, the "unique value" is identified and populated below. Column e is the input. Column f is the output
e f
2/2/09 c
8/8/07 a
8/7/10 d
1/1/11 e
I am able to do the calculation in excel using vlookups, min and the array function. But I have no clue as to how to do it in r.
I tried using the merge function but it seems to require an exact match. I also tried the following code without success
Ifelse ( e >= x$a & e < x$b, d, "")
x is the name of the dataframe with columns a,b,d. FYI the dates were formatted for use in r and converted to numeric.
Thank you

Answer 1

Using sqldf package:
library(sqldf)
#reference data
df1 <- read.table(text="
a b d
1/1/07 1/1/08 a
1/1/08 1/1/09 b
1/1/09 1/1/10 c
1/1/10 1/1/11 d
1/1/11 1/1/12 e", header=TRUE, as.is=TRUE)
#data
df2 <- read.table(text="
e
2/2/09
8/8/07
8/7/10
1/1/11", header=TRUE, as.is=TRUE)
#convert to numeric
df1$a <- as.numeric(as.Date(df1$a,format="%d/%m/%y"))
df1$b <- as.numeric(as.Date(df1$b,format="%d/%m/%y"))
df2$e <- as.numeric(as.Date(df2$e,format="%d/%m/%y"))
#data
df1
# a b d
# 1 13514 13879 a
# 2 13879 14245 b
# 3 14245 14610 c
# 4 14610 14975 d
# 5 14975 15340 e
df2
# e
# 1 14277
# 2 13733
# 3 14798
# 4 14975
#output
sqldf("select e,d
from df1, df2
where df2.e >= df1.a and df2.e < df1.b")
# e d
# 1 13733 a
# 2 14277 c
# 3 14798 d
# 4 14975 e

Answer 2

Here is an answer with looping (as the guys pointed out you should get this part right first) hence I used loops for this example. Here I generated dates in months d1 and d2 and the corresponding dates you're interested in by weeks as e. Then created some random numbers in f and checked which ones fit the critera.
d1 <- seq(from=as.Date('2013-01-01'), to=as.Date('2013-11-12'), by='months')
d2 <- seq(from=as.Date('2013-02-01'), to=as.Date('2013-12-12'), by='months')
e <- seq(from=as.Date('2013-01-01'), to=as.Date('2013-12-13'), by='weeks')
f <- runif(length(e), 1, 10)
output <- NULL
i <- 1
j <- 1
while (i <= length(e) & j <= length(d1))
{
if (e[i] >= d1[j] & e[i] <= d2[j])
{
output[i] <- f[i]
i <- i + 1
}
else
{
j <- j + 1
}
}
output