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并非for循环中的所有R对象类型更新都保留

for-loop r

stschn · 技术社区 · 1 年前

我遇到了以下问题,目前我还没有解释。

a <- array(seq(24), dim = c(4, 3, 2))

# The axis to be fixed and read completely
axis = c(3)

d <- dim(a)
ds <- lapply(d, seq)

# initialize the list for later slicing
ds[-axis] <- 1

# The axis to be iterated along
axis_iter <- seq_along(d)[-axis]

# Number of iterations
n_iter <- prod(d[-axis])
order <- rev(seq_along(d[-axis]))

# Only for demonstration purposes
tmp <- rep(NA, n_iter)

for (iter in seq(n_iter)) {
  tmp[iter] <- iter
  # This shows that the values within ds are updated
  print(unlist(ds))
  # slice array and calculate the minimum value
  # Note: That's also only for demonstration purposes
  print(min(do.call(`[`, c(list(a), ds, list(drop = TRUE)))))
  for (i in order) {
    ds[axis_iter][[i]] <- ds[axis_iter][[i]] + 1
    if (ds[axis_iter][[i]] <= d[i])
      break
    else
      ds[axis_iter][[i]] <- 1
  }
}

代码本身工作得很好。但我不知道为什么向量tmp的更新会在for循环之后保留,而对于列表ds则不会。运行代码后,列表对象的值与之前相同:

> ds
[[1]]
[1] 1

[[2]]
[1] 1

[[3]]
[1] 1 2

提前感谢您的帮助!

1 回复 | 直到 1 年前

kernelize 1 年前

for 正在应用循环。列表 ds 结果和循环逻辑的初始after相同。

[ 1 ] Before nested for-loop:    1 1 1 2 
[ 1 ] After nested for-loop:     1 2 1 2 
[ 2 ] Before nested for-loop:    1 2 1 2 
[ 2 ] After nested for-loop:     1 3 1 2 
[ 3 ] Before nested for-loop:    1 3 1 2 
[ 3 ] After nested for-loop:     2 1 1 2 
[ 4 ] Before nested for-loop:    2 1 1 2 
[ 4 ] After nested for-loop:     2 2 1 2 
[ 5 ] Before nested for-loop:    2 2 1 2 
[ 5 ] After nested for-loop:     2 3 1 2 
[ 6 ] Before nested for-loop:    2 3 1 2 
[ 6 ] After nested for-loop:     3 1 1 2 
[ 7 ] Before nested for-loop:    3 1 1 2 
[ 7 ] After nested for-loop:     3 2 1 2 
[ 8 ] Before nested for-loop:    3 2 1 2 
[ 8 ] After nested for-loop:     3 3 1 2 
[ 9 ] Before nested for-loop:    3 3 1 2 
[ 9 ] After nested for-loop:     4 1 1 2 
[ 10 ] Before nested for-loop:   4 1 1 2 
[ 10 ] After nested for-loop:    4 2 1 2 
[ 11 ] Before nested for-loop:   4 2 1 2 
[ 11 ] After nested for-loop:    4 3 1 2 
[ 12 ] Before nested for-loop:   4 3 1 2 
[ 12 ] After nested for-loop:    1 1 1 2

根据逻辑,上一次迭代中嵌套的for循环将把[[1]]和[[2]]更新为1。

cat("[",iter,"]","Before nested for-loop:\t", unlist(ds),"\n")
# ds: 4 3 1 2
for (i in order) {
    ds[axis_iter][[i]] <- ds[axis_iter][[i]] + 1 # at i=2 => ds: 4 4 1 2, at i=1 => ds: 5 1 1 2
    if (ds[axis_iter][[i]] <= d[i]) # condition not satisfied
      break
    else
      ds[axis_iter][[i]] <- 1  # at i=2 => ds: 4 1 1 2, at i=1 => ds: 1 1 1 2
}
cat("[",iter,"]","After nested for-loop:\t", unlist(ds),"\n")
# ds: 1 1 1 2

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