如何访问频率表中的值

Question

我有一个频率表,计算向量中元素的频率

a = table(c(0,1,1,1,0,2,2,4,1,2,3,2,1,2,3,1,1,1,2,3,4,1,1,0))
a
#  0  1  2  3  4 
#  3 10  6  3  2

我知道我可以通过名字(a)访问该名称.但是当我试图访问SECOND行的值时

a[1, "0"]
# Error in a[1, "0"] : incorrect number of dimensions

a[1, 1]
# Error in a[1, 1] : incorrect number of dimensions

Answer 1

这个表实际上是一个数组.

x <- c(0,1,1,1,0,2,2,4,1,2,3,2,1,2,3,1,1,1,2,3,4,1,1,0)
(a <- table(x))
#
# 0  1  2  3  4 
# 3 10  6  3  2 
class(unclass(a))
# [1] "array"

它的名字在顶部,而值在底部.

names(a)
[1] "0" "1" "2" "3" "4"

您可以通过多种方式访问​​其元素.

a[1]                     ## access named element by index
# 0 
# 3
a[[1]]                   ## access unnamed element by index
# [1] 3
a["0"]                   ## access named element by name
# 0 
# 3 
a[["0"]]                 ## access unnamed element by name
# [1] 3
as.vector(a)             ## as.vector() drops table down to unnamed vector
# [1]  3 10  6  3  2
c(a)[2:4]                ## c() drops table down to named vector
#  1  2  3 
# 10  6  3 
class(a[2:4])
# [1] "array"
class(c(a)[2:4])
# [1] "integer"

它还有一个nrow和dim属性,它们在最后几行中设置table.

y <- array(tabulate(bin, pd), dims, dimnames = dn)
class(y) <- "table"

虽然我真的不清楚为什么nrow(a)是5然后会a[1,]返回错误.