根据条件创建新的数字变量

Question

我正在尝试创建一个新的变量，该变量由访问时1的患者体重（kg）组成，但我的逻辑有问题。我尝试过以下代码，但没有运气

WL_A <- function(visitnum, Weight_KG) {
  if (visitnum == 1)
    return(Weight_KG)
}

数据

structure(list(Sid = structure(1:5, .Label = c("A", "B", "C", 
"D", "E"), class = "factor"), Weight_KG = c(100L, 500L, 70L, 
90L, 82L), visitnum = c(1L, 2L, 1L, 1L, 12L), BMI = c(13L, 25L, 
20L, 18L, 7L)), .Names = c("Sid", "Weight_KG", "visitnum", "BMI"
), class = "data.frame", row.names = c("1", "2", "3", "4", "5"))