R：使用for循环和过滤数据表 [英] R: Using for-loop and filter data.table

问题描述

我想循环遍历一个列表，用于子集data.table并通过几行代码运行结果。使用变量进行子操作在没有for循环的情况下工作正常，但是当循环包含时，它不能按预期工作。我想这个问题是由数据类型或范围引起的。

I'm trying to loop through a list which is used to subset a data.table and run the results through few lines of code. Subseting with a variable is working perfectly without the for loop but when the loop is included it does not work as expected. I guess the problem is either caused by data type or scope.

以下代码完美地工作：

dt <- data.table(mpg)
list <- levels(dt$manufacturer)

dt[manufacturer==list[3]]

如果我尝试使用for循环遍历列表：[updated code - 已解决]

If I then try to loop through the list with a for loop: [updated code - solved]

for (var in list) {
  subs <- data.table(melt(dt[manufacturer==var, list(model, hwy, cty)], id.vars='model'))

  png(paste(var, 'png', sep='.'))
  print(ggplot(subs, aes(model, value, col=variable)) + geom_point())
  dev.off()
}

我没有任何东西。

应该平等地工作。有没有人有一个建议，什么可能导致我没有得到任何东西与第二次过滤？

For me the code parts look identical and should work equally. Does anyone have a suggestion what might cause that I do not get anything with the second filtering ? Any help is appreciated.

推荐答案

这是一个很常见的问题。检查区别：

This is a very common question. Check the difference:

for (i in 1:5) {i}
# no output
for (i in 1:5) {print(i)}
[1] 1
[1] 2
[1] 3
[1] 4
[1] 5

但是，代码执行：

for (i in 1:5) {j <- i^2}
j
[1] 25

只要使用全局变量来存储结果，如果你在循环中运行一些代码。

Just use a global variable to store results in if you are running some code in a loop.

UPD：

此讨论：

在循环中，自动打印关闭，因为它在函数内。如果您想要查看输出，您需要在这两种情况下显式打印。

In a loop, automatic printing is turned off, as it is inside a function. You need to explicitly print something in both cases if you want to see the output.

缺少打印只是一个约定，从偶尔垃圾邮件。

The absence of printing is just a convention, which is preventing your console from being occasionally spammed.

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