如何迭代保存结果格式的列表？ [英] How to iterate over a list preserving the format of the results?

问题描述

我有一个简单的函数来创建列表中的光栅文件列表及其名称（以.grd格式，在下面的例子中称为list_names）：

<$ （）函数（list_names）{

raster_list< - list（）＃初始化栅格列表

for（ i in 1：（length（list_names）））{
grd_name< - list_names [i]＃list_names包含.grd格式的所有图像名称
raster_file< - raster（grd_name）


raster_list< - append（raster_list，raster_file）＃每次迭代更新raster_list
}

＃应用函数
raster_list < -lapply（list_names，FUN = ListRasters）

期望的结果应该是格式：

  [[1]] 
 class：RasterLayer 
＃etc 
 
 [ [2]] 
 class：RasterLayer 
＃etc 
  

他们的格式为：

  [[ 1]] 
 [[1]] [[1]] 
 class：RasterLayer 
＃etc 
 $ b [[2]] 
 [[2 ]] [[1]] 
 class：RasterLayer 
＃etc 
  

是一个问题，因为以后我不能访问栅格列表上的项目。
我找不到解决方案，因为我不明白为什么迭代会给出这种格式的结果。你可以给我一些解释或建议如何解决这个功能，或者你可以看到我犯了什么错误？

任何帮助都是值得欢迎的！ / p>

谢谢!!

解决方案

循环。不过， lapply 会返回一个列表，也许你应该试试 sapply 。

  raster_list< -sapply（list_names，FUN = ListRasters）
  

$ b $
$ b

编辑：

我认为这应该会给你想要的东西。如果你把一个最小的可重复的例子，因为我们不知道数据是怎么样的，什么是 raster 函数会更好，但下面是一个玩具的例子 lapply 和 sapply 。

的输出差异

PS ：看来你的公式迭代是不正确的，但你将不得不提供更多的信息，所以我们可以修复公式。

  list_names <-c（a，b，c，d）#defining list_names 
 
 #defining the function 
 ListRasters<  -  function list_names）{
 
 raster_list<  -  list（）＃初始化栅格列表
 
 for（i in 1： st_names）））{
 grd_name<  -  list_names [i]＃list_names包含.grd格式的所有图像名称
 raster_file<  -  paste（file，grd_name，sep =。 ）
 
 
 raster_list<  -  append（raster_list，raster_file）＃每次迭代更新raster_list 
} 
  

使用 lapply ：

  raster_list< -lapply（list_names，FUN = ListRasters）
  

输出结果是：

  [[1]] 
 [[1]] [[1]] 
 [1] file.a
 
 
 [[2]] 
 [[2]] [[1]] 
 [1]file.b
 
 $ b [[3]] 
 [[3]] [[1]] 
 [1]file.c
 
 
 [[4]] 
 [[4]] [[1]] 
 [1]file.d
  

使用 sapply ：

 <$ c 
 
 
 $ b 
输出结果是： raster_list <-sapply（list_names，FUN = ListRasters）
  
  $ a 
 [1]file.a
 
 
 $ b [1]file.b 
 
 
 [1]file.c
 
 $ d 
 [1]file.d
  
 
I have a simple function to create a list of raster files from a list with their names (in .grd format, and called list_names in the example below):
ListRasters <- function(list_names) {

  raster_list <- list() # initialise the list of rasters

  for (i in 1:(length(list_names))){ 
    grd_name <- list_names[i] # list_names contains all the names of the images in .grd format
    raster_file <- raster(grd_name)
  }

  raster_list <- append(raster_list, raster_file) # update raster_list at each iteration
}

# Apply the function 
raster_list <-lapply(list_names, FUN = ListRasters)
The desired result should be in the format:
[[1]]
class : RasterLayer
# etc

[[2]]
class : RasterLayer
# etc
But instead I get them in the format:
[[1]]
[[1]][[1]]
class : RasterLayer 
# etc

[[2]]
[[2]][[1]]
class : RasterLayer
# etc 
That is a problem because later on I can not access the items on the raster list.
I can't find a solution because I don't understand why the iteration gives this format of result. Can you give me some explanation or some advice on how to fix the function, or can you see where I am making a mistake?


Any help is more than welcome!

Thankyou!!
解决方案
It seems that something is odd with your loop. Nevertheless, the lapply is going to return a list, maybe you should try sapply.
raster_list <-sapply(list_names, FUN = ListRasters)
I think this should give you what you want.

EDIT:

It would be better if you had put a minimal reproducible example, for we do not know how the data is and what is the raster function, but below follows a toy example with the differences of output of lapply and sapply.

PS: and it does seem that your formula iteration is not correct, but you would have to provide more information so we can fix the formula.
list_names <- c("a", "b","c", "d") #defining the list_names

#defining the function
ListRasters <- function(list_names) {

  raster_list <- list() # initialise the list of rasters

  for (i in 1:(length(list_names))){ 
    grd_name <- list_names[i] # list_names contains all the names of the images in .grd format
    raster_file <- paste("file", grd_name, sep=".")
  }

  raster_list <- append(raster_list, raster_file) # update raster_list at each iteration
}
Using lapply:
raster_list <-lapply(list_names, FUN = ListRasters)
The output is:
  [[1]]
[[1]][[1]]
[1] "file.a"


[[2]]
[[2]][[1]]
[1] "file.b"


[[3]]
[[3]][[1]]
[1] "file.c"


[[4]]
[[4]][[1]]
[1] "file.d"
Using sapply:
raster_list <-sapply(list_names, FUN = ListRasters)
The output is:
$a
[1] "file.a"

$b
[1] "file.b"

$c
[1] "file.c"

$d
[1] "file.d"


                        
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