如何根据多个条件提取 data.table 的值？

Question

如何根据多个条件提取data.table的值？

我需要一个函数，它根据其他两个列值返回列 data.table 的值：

require(data.table)

dt <- data.table(
    "base" = c("of", "of", "of", "lead and background vocals", "save thou me from", "silent in the face"),
    "prediction" = c("the", "set", "course", "from", "the", "of"),
    "count" = c(258586, 246646, 137533, 4, 4, 4)
)

> dt
#                         base prediction  count
#1:                         of        the 258586
#2:                         of        set 246646
#3:                         of     course 137533
#4: lead and background vocals       from      4
#5:          save thou me from        the      4
#6:         silent in the face         of      4

# the function needs to return the "prediction" value based on the max "count" value for the input "base" value.
# giving the input "of" to function:
> prediction("of")
# the desired output is:
> "the"
# or:
> prediction("save thou me from")
> "the"

Answer 1

我们可以指定i，根据'count'中的max值提取'prediction'值

dt[base == 'of', prediction[which.max(count)]]
#[1] "the"
dt[base == 'save thou me from', prediction[which.max(count)]]
#[1] "the"

可以封装成函数

f1 <- function(val) dt[base == val, prediction[which.max(count)]]
f1("of")
#[1] "the"
f1("save thou me from")
#[1] "the"

注意：最好将数据集标识符和列名也作为参数