Finding a row from a dataframe - r

Question

I am trying to write a function get_value <- function(gyear, gmonth) that selects a row from already existing dataframe or a value if the month is between two time points. The dataframe is e.g.
df <- read.table(header = TRUE, text = "
year month var1 var2
2022 1 123 987
2021 4 234 876
2021 1 345 765
2020 7 456 654
2020 3 567 543
2020 1 678 432
2019 1 789 321")
For example in year 2021 the same row
year month var1 var2
2021 1 345 765
is valid for months 1,2,3 and then comes a change and the next row
year month var1 var2
2021 4 234 876
is valid for months 4,5,6,7,8,9,10,11,12.
If the year & month are already in the dataframe, then I can have the row like
get_value <- function(gyear, gmonth){
library(tidyverse)
temp <- df %>% filter(year == gyear & month == gmonth)
}
get_value(gyear = 2020, gmonth = 1)
but what I want is also to be able to have rows (months) that are between the months that are included in the dataframe. For example I would like to be able to call
get_value(gyear = 2021, gmonth = 5)
that returns row
year month var1 var2
2021 4 234 876
because in year 2021 the month is between 4-12.
Thanks in advance for your help!

Answer 1

You could first create a new column month2 to indicate the ending months.
library(dplyr)
df2 <- df %>%
group_by(year) %>%
arrange(month, .by_group = TRUE) %>%
mutate(month2 = lead(month-1, default = 12), .after = month) %>%
ungroup()
# # A tibble: 7 × 5
# year month month2 variable1 variable2
# <int> <int> <dbl> <int> <int>
# 1 2019 1 12 789 321
# 2 2020 1 2 678 432
# 3 2020 3 6 567 543
# 4 2020 7 12 456 654
# 5 2021 1 3 345 765
# 6 2021 4 12 234 876
# 7 2022 1 12 123 987
Then customize a filter function that takes a dataframe as input and extract those rows where gmonth >= month & gmonth <= month2.
get_value <- function(data, gyear, gmonth){
data %>%
filter(year == gyear & gmonth >= month & gmonth <= month2)
}
I don't encourage you to put the code where I manipulate df to get month2 into this function. If you do that, the data will be repeatedly manipulated whenever get_value() is run. It's out of efficiency.
Output
get_value(df2, gyear = 2020, gmonth = 1)
# # A tibble: 1 × 5
# year month month2 variable1 variable2
# <int> <int> <dbl> <int> <int>
# 1 2020 1 2 678 432
get_value(df2, gyear = 2021, gmonth = 5)
# # A tibble: 1 × 5
# year month month2 variable1 variable2
# <int> <int> <dbl> <int> <int>
# 1 2021 4 12 234 876