我有以下資料框：
df <- data.frame(Business_Date = c("01/01/2018", "01/01/2019", "01/01/2020"), Business_Date_2019 = c("01/01/2019","01/01/2019","01/01/2019"), Television = c("A","B","C"))
我想創建第四列，其中包含 2019 年特定日期的電視：
desired_output <- data.frame(Business_Date = c("01/01/2018", "01/01/2019", "01/01/2020"), Business_Date_2019 = c("01/01/2019","01/01/2019","01/01/2019"), Television = c("A","B","C"),
Television_2019 = c("B","B","B"))
但是，當我寫：
df$Television_2019 <- case_when(
df$Business_Date == df$Business_Date_2019 ~ df$Television,
TRUE ~ NA_character)
它不回傳所需的輸出：
Business_Date Business_Date_2019 Television Television_2019
1 01/01/2018 01/01/2019 A <NA>
2 01/01/2019 01/01/2019 B B
3 01/01/2020 01/01/2019 C <NA>
由于第 1 行和第 3 行在第二列中也有"01/01/2019"，我也想"B"在第四列中回傳這些行。
如何撰寫一個從df回傳desired_output的腳本？
uj5u.com熱心網友回復：
您可以通過創建一個命名向量來用作查找表來完成此操作：
tv_lookup <- setNames(df$Television, df$Business_Date)
df$Television_2019 <- tv_lookup[df$Business_Date_2019]
df
# Business_Date Business_Date_2019 Television Television_2019
# 1 01/01/2018 01/01/2019 A B
# 2 01/01/2019 01/01/2019 B B
# 3 01/01/2020 01/01/2019 C B
原因case_when(df$Business_Date == df$Business_Date_2019)沒有給你你想要的，因為它會將 in 中的每個元素與in 中相同位置的Business_Date_2019元素進行比較。使用您的示例資料，這將導致：Business_Date
"01/01/2018" == "01/01/2019" # FALSE, therefore <NA>
"01/01/2019" == "01/01/2019" # TRUE, therefore "B"
"01/01/2020" == "01/01/2019" # FALSE, therefore <NA>
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