依群組計算

R 的 data.table 資料操作

Matt Dowle, Arun Srinivasan

Instructors, DataCamp

by 參數

by 參數可針對 by {{1}} 中指定的（分組）欄位，每個唯一值分別計算。

# 每個 start_station 發生了多少次行程？
ans <- batrips[, .N, by = "start_station"]
head(ans, 3)

          start_station        N
San Francisco City Hall     2145
 Embarcadero at Sansome    12879
      Steuart at Market    11579

by 參數可接受欄名的 character 向量，或變數／運算式的 list

# 等同於 batrips[, .N, by = "start_station"]
ans <- batrips[, .N, by = .(start_station)]
head(ans, 3)

          start_station        N
San Francisco City Hall     2145
 Embarcadero at Sansome    12879
      Steuart at Market    11579

可即時重新命名分組欄位

ans <- batrips[, .(no_trips = .N), by = .(start = start_station)]
head(ans, 3)

                  start   no_trips
San Francisco City Hall       2145
 Embarcadero at Sansome      12879
      Steuart at Market      11579

by 中的 list() 或 .() 運算式可即時計算分組變數

# 取得每月、每個 start_station 的行程數
ans <- batrips[ , .N, by = .(start_station, mon = month(start_date))]
head(ans, 3)

          start_station mon    N
San Francisco City Hall   1  193
 Embarcadero at Sansome   1  985
      Steuart at Market   1  813

R 的 data.table 資料操作