I am new to R and rather flumoxed by the following problem. I have two vectors of dates (the vectors are not necessarily aligned, nor of the same length).

I want to find for each date in the first vector the next date in the second vector.

```
vecA <- as.Date(c('1951-07-01', '1953-01-01', '1957-04-01', '1958-12-01',
'1963-06-01', '1965-05-01'))
vecB <- as.Date(c('1952-01-12', '1952-02-01', '1954-03-01', '1958-08-01',
'1959-03-01', '1964-03-01', '1966-05-01'))
```

In SQL I would write something like this, but I cannot find any tips in SO as to how to do this in R.

```
select vecA.Date, min(vecB.Date)
from vecA inner join vecB
on vecA.Date < vecB.Date
group by vecA.Date
```

The output should look like this:

```
Start End
1951-07-01 1952-01-12
1953-01-01 1954-03-01
1957-04-01 1958-08-01
1958-12-01 1959-03-01
1963-06-01 1964-03-01
1965-05-01 1966-05-01
```

Answer:

Here's a possible solution using `data.table`

rolling joins

```
library(data.table)
dt1 <- as.data.table(vecA) ## convert to `data.table` object
dt2 <- as.data.table(vecB) ## convert to `data.table` object
setkey(dt2) # key in order to perform a binary join
res <- dt2[dt1, vecB, roll = -Inf, by = .EACHI] # run the inner join while selecting closest date
setnames(res, c("Start", "End"))
res
# Start End
# 1: 1951-07-01 1952-01-12
# 2: 1953-01-01 1954-03-01
# 3: 1957-04-01 1958-08-01
# 4: 1958-12-01 1959-03-01
# 5: 1963-06-01 1964-03-01
# 6: 1965-05-01 1966-05-01
```

Alternatively, we can also do:

```
data.table(vecA=vecB, vecB, key="vecA")[dt1, roll=-Inf]
```

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