我正在嘗試根據指定資料框中列的物件將資料呼叫到新資料框。
這是一個可重現的示例，表明我無法達到目標結果：
dataframe1 <- data.frame('first' = seq(1:5),
'second' = letters[seq(1:5)])
specifcdata1 <- 'dataframe1[first]'
specifcdata2 <- 'dataframe1$second'
dataframe2 <- data.frame('Specific 1' = tail(specifcdata1, 1),
'Specific 2' = tail(specifcdata2, 1))
導致：
> dataframe2
Specific.1 Specific.2
1 dataframe1[first] dataframe1$second
目標結果是：
> dataframe2
Specific.1 Specific.2
1 5 e
考慮到dataframe1，
> dataframe1
first second
1 1 a
2 2 b
3 3 c
4 4 d
5 5 e
問題：我們如何呼叫物件（資料，而不是存盤的字符）？
uj5u.com熱心網友回復：
You are looking for the combination of parse() and eval(). I also added quotation marks in 'dataframe1["first"]' or else in would not have worked.
dataframe1 <- data.frame('first' = seq(1:5),
'second' = letters[seq(1:5)])
specifcdata1 <- 'dataframe1["first"]'
specifcdata2 <- 'dataframe1$second'
dataframe2 <- data.frame('Specific 1' = tail(eval(parse(text = specifcdata1)), 1),
'Specific 2' = tail(eval(parse(text = specifcdata2)), 1))
dataframe2
#> first Specific.2
#> 5 5 e
Created on 2022-03-30 by the reprex package (v2.0.1)
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