如何在不跟踪索引的情况下将元素附加到列表? [英] how to append an element to a list without keeping track of the index?

问题描述

我正在寻找 r 相当于python

mylist = []
for this in that:
  df = 1
  mylist.append(df)

基本上只是创建一个空列表，然后将在循环中创建的对象添加到其中.

basically just creating an empty list, and then adding the objects created within the loop to it.

我只看到 R 解决方案必须指定新元素的索引(比如 mylist[[i]] <- df)，因此需要创建索引 i 在循环中.

I only saw R solutions where one has to specify the index of the new element (say mylist[[i]] <- df), thus requiring to create an index i in the loop.

有没有比在最后一个元素后面追加更简单的方法.

Is there any simpler way than that to just append after the last element.

推荐答案

有个函数叫append:

ans <- list()
for (i in 1992:1994){
n <- 1 #whatever the function is
ans <- append(ans, n)
}

  ans
## [[1]]
## [1] 1
## 
## [[2]]
## [1] 1
## 
## [[3]]
## [1] 1
## 

注意:使用 apply 函数而不是 for 循环更好(不一定更快)，但这取决于您的实际目的循环.

Note: Using apply functions instead of a for loop is better (not necessarily faster) but it depends on the actual purpose of your loop.

回答 OP 的评论:关于使用 ggplot2 并将绘图保存到列表中，这样的事情会更有效:

Answering OP's comment: About using ggplot2 and saving plots to a list, something like this would be more efficient:

plotlist <- lapply(seq(2,4), function(i) {
            require(ggplot2)
            dat <- mtcars[mtcars$cyl == 2 * i,]
            ggplot() + geom_point(data = dat ,aes(x=cyl,y=mpg))
})

感谢 @Wen 分享c()的比较code> 和 append() 函数:

Thanks to @Wen for sharing Comparison of c() and append() functions:

连接 (c) 非常快，但 append 更快，因此在仅连接两个向量时更可取.

Concatenation (c) is pretty fast, but append is even faster and therefor preferable when concatenating just two vectors.

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