整數列之和是雙倍

Question

我不明白爲什麼以及如何防止兩個整數列的和爲numeric，類型爲double。任何想法？

這裏是一個小的工作示例

library(data.table) 

set.seed(123) 

A <- rnorm(20, 100, 5) 
B <- rnorm(20, 50, 20) 

NA.A <- which(A %in% sample(A, 5)) 
NA.B <- which(B %in% sample(B, 10)) 

zero.A<- which(A %in% sample(A, 3)) 
zero.B <- which(B %in% sample(B, 8)) 

A[NA.A] <- NA 
A[zero.A] <- 0 
B[NA.B] <- NA 
B[zero.B] <- 0 

mydt <- data.table(A = as.integer(A), B = as.integer(B)) 
sapply(mydt, class) 
# A   B 
# "integer" "integer" 

mydt[, C := rowSums(.SD, na.rm=T), .SDcols = c("A","B")] 
sapply(mydt, class) 
# A   B   C 
# "integer" "integer" "numeric" 

sapply(mydt, typeof) 
# A   B   C 
# "integer" "integer" "double" 

Answer 1

如rowSums()總是會返回double類型，你可以交替結合使用Reduce()與+操作來回報新列integer。

mydt[,C:=Reduce(`+`, lapply(.SD, function(x) ifelse(!is.na(x),x,0))),.SDcols = c("A","B")]