根据互惠条件子集a dataframe

Question

I have the following data frame structure

date	latitude
1951-03-22	66.08106
1951-03-22	59.59117
1951-04-08	59.59117
1952-10-20	55.41972
1960-08-12	66.05653
1960-09-10	66.08106

What I would like to do is: select the rows for all unique latitude and if there are 2 (or more) exact same latitudes, I want to keep only the one that has the earliest date but for each year .

So, for my previous example, it would give the following subset, without only the 3rd row:

date	latitude
1951-03-22	66.08106
1951-03-22	59.59117
1952-10-20	55.41972
1960-08-12	66.05653
1960-09-10	66.08106

Many thanks for the help.

PS: maybe it is important to precise that class(df$date) is "Date" and class(df$latitude) is "numeric".

Answer 1

Grouped by 'latitude' and year extracted from 'date', use slice_max to extract the row with max date, and then remove the 'year' column

library(dplyr)
library(lubridate)
df1 %>%    
    # grouped by latitude, and year extracted from Date class 
    # year is from lubridate
    group_by(latitude, year = year(date)) %>% 
    # slice 1 row from each group, ordered by the 'date' column
    slice_max(n = 1, order_by = date) %>%
    # remove the grouping
    ungroup %>%
    # remove the year column
    select(-year) %>%
    arrange(date)

-output

# A tibble: 5 × 2
  date       latitude
  <chr>         <dbl>
1 1951-03-22     66.1
2 1951-04-08     59.6
3 1952-10-20     55.4
4 1960-08-12     66.1
5 1960-09-10     66.1