extract values based on first 4 integers

Suppose, I have this dataframe (integers).

df <- data.frame(date = c(20010101:20010103, 20050304, 20050506, 20220113, 20220216))

And I have the target date like this:

target_date = 2001

I want to extract only those values from df of which first 4 integers match with the target date.
Result like this:

20010101 20010102 20010103

>Solution :

We may use substr to extract the first four character and then compare (==) with target_date to extract the subset of rows and pull the ‘date’ column

subset(df, substr(date, 1, 4) == target_date)$date
[1] 20010101 20010102 20010103

Or another option is to convert to Date class, extract the year and then filter

library(dplyr)
library(lubridate)
df %>%
  filter(year(ymd(date)) == target_date)
      date
1 20010101
2 20010102
3 20010103