使用字符串中的变量名称访问变量值，R

问题描述

数据集具有大量的 age.year 变量: age.1990，age.1991等.我有一个字符串值数组> length(age.years)表示那些变量，以使 age.years [1] 返回"age.1990" ，等等.

A data set has a large number of age.year variables: age.1990, age.1991, etc. I have an array of string values of length(age.years) representing those variables such that age.years[1] returns "age.1990", etc.

我想搜索每个记录的 age.year 变量以找到值 60 .最终，如果 age.1991 [1] 等于60，则新变量 Y.60 [1] 将采用值 Y.1991 [1] .

I want to search the age.year variables for each record to find the value, 60. Ultimately, if age.1991[1] equals 60, then a new variable Y.60[1] will take the value Y.1991[1].

如何使用数组中的字符串作为变量名称，以避免手工编码每个 var.year 变量? Get()似乎不起作用.

How do I use strings from arrays as variable names to avoid coding each var.year variable by hand? Get() does not seem to work.

# EXAMPLE CODE
big.data= data.frame(ID= c(1,2), age.1990= c(60, NA),
  age.1991= c(61, 60), age.1992= c(62, 61), Y.1990= c(100, 120),
  Y.1991= c(NA, 125), Y.1992= c(115, 130), Y.60= c(NA, NA) )

big.data
#   ID age.1990 age.1991 age.1992 Y.1990 Y.1991 Y.1992 Y.60
# 1  1       60       61       62    100     NA    115   NA
# 2  2       NA       60       61    120    125    130   NA

age.years = names(big.data)[2:4]
Y.years = names(big.data)[5:7]
age.years[1]= paste0("big.data$", age.years[1])
age.years[1]
# [1] "big.data$age.1990"
summary(age.years[1])
# Length     Class      Mode
#    1 character character
summary(get(age.years[1]))
# Error in get(age.years[1]) : object 'big.data$age.1990' not found
# Why not found??

推荐答案

您可以像使用$一样使用括号中的内容来访问列.

You can just use paste in brackets to access the column as you would otherwise do with $.

big.data[paste0(age.years[1])]

此外，您可以仅使用数字来访问像这样的列.

Additionally, you can use just the numbers to access the columns like this.

years <- c(1990:1992)
big.data[paste0("age.",years[1])]

然后循环将像这样工作.

And the loop will work like this.

for (iy in 1:length(years)){
  big.data$Y.60 <- NA
  big.data$Y.60  <- ifelse(big.data[paste0("age.",years[iy])] == 60,     +
     paste0("Y.",years[iy]),big.data$Y.60 )
 }

如果我正确理解了您的目的.

If I understood your purpose correctly.

更新:

或者是作者@jtd的替代答案，使用方括号代替粘贴.

Or an alternative answer from the author @jtd with brackets instead of paste.

for (iy in 1:length(age.years)) {
   big.data$Y = ifelse(big.data[[age.years[iy]]] == 60,
              big.data[[Y.years[iy]]],
              big.data$Y
              )
}

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