data.table用法解析

R語言中的data.table是一種資料處理時的高效率工具。在瞭解data.table之前先簡單說明一下管道函式的使用方法。

管道函式的使用方法

常用的管道函式有以下三種：

%in%：表示包含於

%>%：表示向右傳遞

%$%：表示向右傳遞列，並可以直接按列操作

data.table使用方法

1. 首先將資料變成data.table格式

data = data%>%

as.data.table()

2. 將資料按照以下5個變數彙總

data = data%>%

as.data.table()%$%

.[,.(value = sum(value,na.rm = TRUE)),

by = c("period","index","area","id1","id2")]

3. 按照某類別列的變數值篩選

data = data%$%

.[index%in%c("UV","DB","XS"),]

4.改變類別列中變數名稱

data = data%$%

.[index_name == "DB",index_name := "DB_PV"]%$%

.[index_name == "GZ",index_name := "GZ_UV"]%$%

.[index_name == "XSLZ",index_name := "XSLZ_UV"]

5. 取某列全部值的前6個字母

data = data %$%

.[,":="(id1 = substr(id1,1,6),

id2 = substr(id2,1,6))]%$%

.[,.(value = sum(value,na.rm = TRUE)),

by = c("period","index","area","id1","id2")]

6. 篩選列

data = data %$%

.[,.(period,id1,id2,area,index,value)]

7. 篩選行列

N <- rank.P[ID2%in%target,.(ID2, Month, prov_ID, Province, Index_name,Value_adj_per)]

8. 變更列名稱

data = data%>%

setnames(c("period","car_id1","car_id2","area","value"), c("Month","ID1","ID2","Province","Value"))

9. 按照某列合併表格，並篩選某些列

data = data %>%

merge(data2, by.x="Province",by.y="Province",all.x = TRUE)%$%

.[,.(Month,Province,ID,index_name,ID2,Value)]

9.1 合併後出現列名重複現象

new_data <- data%>%

merge(data2,

by.x = c("ID1","ID2","Month","prov_ID","Province","Index_name"),

by.y = c("ID2","ID1","Month","prov_ID","Province","Index_name"),

suffixes = c("_P","_N"))

10. 定義新列

new_data <- new_data[, ":="(Score = (Value_adj_per_P+Value_adj_per_N)/2)]%$%

.[, Rank := row_number(-Score),

by = c("ID2","Month","prov_ID","Province","Index_name")]%>%

merge(phase, by = "Index_name")%$%

.[, Index_name := NULL]