I have the Dataframe:

df <- tibble(value_2024 = runif(4), 
value_2023 = runif(4), 
value_2022 = runif(4), 
selection = c(2023, 2022, 2024, 2022))

I want to rowwise calculate the sum of the columns value_2022 : value_n, where n is the value in column selection.

I did:

df %>% mutate(value_sum = rowSums(
across(value_2022:ends_with(as.character(df$selection)))
))

This of course gives the warning:

numerical expression has 4 elements: only the first used.

I do understand the warning and also know why it occurs but I don't know how I can solve it.

Update

It is also possible to make it without rowwise, but you probably need more efforts, e.g.,

df %>%
    mutate(rid = 1:n()) %>%
    pivot_longer(-c(rid, selection)) %>%
    reframe(
        value_sum = sum(value[1:n() %in% which(name == "value_2022"):which(name == str_c("value_", selection))]),
        .by = c(selection, rid)
    ) %>%
    arrange(rid) %>%
    {
        bind_cols(df, select(., value_sum))
    }

which gives

# A tibble: 4 × 5
  value_2024 value_2023 value_2022 selection value_sum
       <dbl>      <dbl>      <dbl>     <dbl>     <dbl>
1      0.897      0.908     0.661       2023     1.57
2      0.266      0.202     0.629       2022     0.629
3      0.372      0.898     0.0618      2024     1.33
4      0.573      0.945     0.206       2022     0.206

Maybe you can try rowwise

df %>%
    rowwise() %>%
    mutate(value_sum = rowSums(across(value_2022:ends_with(as.character(.$selection)[cur_group_id()]))))

which gives

# A tibble: 4 × 5
# Rowwise: 
  value_2024 value_2023 value_2022 selection value_sum
       <dbl>      <dbl>      <dbl>     <dbl>     <dbl>
1      0.897      0.908     0.661       2023     1.57
2      0.266      0.202     0.629       2022     0.629
3      0.372      0.898     0.0618      2024     1.33
4      0.573      0.945     0.206       2022     0.206

data

set.seed(0)
df <- tibble(value_2024 = runif(4), value_2023 = runif(4), value_2022 = runif(4), selection = c(2023, 2022, 2024, 2022))