英文:
Appending to lists within loops in Python3 (again)
问题
我在逐步向列表中添加元素时遇到了困难。
这是一个最小化工作示例（MWE）：
# 给定一个嵌套的值列表或集合sets = [[1, 2, 3], [1, 2, 4], [1, 2, 5]]# 将一个值添加到每个子列表，表示该集合在列表中的编号。n_sets = len(sets)for s in range(n_sets):(sets展开收缩).insert(0, s)# 现在重复这些集合reps次reps = 4expanded_sets = [item for item in sets for i in range(reps)]# 然后为每个集合的每次出现分配一个重复编号。rep_list = list(range(reps)) * n_setsfor i in range(n_sets * reps):(expanded_sets[i]).insert(0, rep_list[i])expanded_sets
这段代码返回的结果是：
[[3, 2, 1, 0, 0, 1, 2, 3],[3, 2, 1, 0, 0, 1, 2, 3],[3, 2, 1, 0, 0, 1, 2, 3],[3, 2, 1, 0, 0, 1, 2, 3],[3, 2, 1, 0, 1, 1, 2, 4],[3, 2, 1, 0, 1, 1, 2, 4],[3, 2, 1, 0, 1, 1, 2, 4],[3, 2, 1, 0, 1, 1, 2, 4],[3, 2, 1, 0, 2, 1, 2, 5],[3, 2, 1, 0, 2, 1, 2, 5],[3, 2, 1, 0, 2, 1, 2, 5],[3, 2, 1, 0, 2, 1, 2, 5]]
而不是期望的结果：
[[0, 0, 1, 2, 3],[1, 0, 1, 2, 3],[2, 0, 1, 2, 3],[3, 0, 1, 2, 3],[0, 1, 1, 2, 4],[1, 1, 1, 2, 4],[2, 1, 1, 2, 4],[3, 1, 1, 2, 4],[0, 2, 1, 2, 5],[1, 2, 1, 2, 5],[2, 2, 1, 2, 5],[3, 2, 1, 2, 5]]
希望这次的回答对你有所帮助。
英文:
I'm having difficulty adding to a list iteratively.
Here's a MWE:
# Given a nested list of values, or setssets = [[1, 2, 3], [1, 2, 4], [1, 2, 5]]# add a value to each sublist giving the number of that set in the list.n_sets = len(sets)for s in range(n_sets):(sets展开收缩).insert(0, s)# Now repeat those sets reps timesreps = 4expanded_sets = [item for item in sets for i in range(reps)]# then assign a repetition number to each occurance of a set.rep_list = list(range(reps)) * n_setsfor i in range(n_sets * reps):(expanded_sets[i]).insert(0, rep_list[i])expanded_sets
which returns
[[3, 2, 1, 0, 0, 1, 2, 3],[3, 2, 1, 0, 0, 1, 2, 3],[3, 2, 1, 0, 0, 1, 2, 3],[3, 2, 1, 0, 0, 1, 2, 3],[3, 2, 1, 0, 1, 1, 2, 4],[3, 2, 1, 0, 1, 1, 2, 4],[3, 2, 1, 0, 1, 1, 2, 4],[3, 2, 1, 0, 1, 1, 2, 4],[3, 2, 1, 0, 2, 1, 2, 5],[3, 2, 1, 0, 2, 1, 2, 5],[3, 2, 1, 0, 2, 1, 2, 5],[3, 2, 1, 0, 2, 1, 2, 5]]
instead of the desired
[[0, 0, 1, 2, 3],[1, 0, 1, 2, 3],[2, 0, 1, 2, 3],[3, 0, 1, 2, 3],[0, 1, 1, 2, 4],[1, 1, 1, 2, 4],[2, 1, 1, 2, 4],[3, 1, 1, 2, 4],[0, 2, 1, 2, 5],[1, 2, 1, 2, 5],[2, 2, 1, 2, 5],[3, 2, 1, 2, 5]]
Just for fun, the first loop returns an expected value of sets
[[0, 1, 2, 3], [1, 1, 2, 4], [2, 1, 2, 5]]
but after the second loop sets changed to
[[3, 2, 1, 0, 0, 1, 2, 3], [3, 2, 1, 0, 1, 1, 2, 4], [3, 2, 1, 0, 2, 1, 2, 5]]
I suspect the issue has something to do with copies and references. I've tried adding .copy() and slices in various places, but with the indexed sublists I haven't come across a combo that works. I'm running Python 3.10.6.
Thanks for looking!
Per suggested solution, [list(range(reps)) for _ in range(n_sets)] doesn't correctly replace the list(range(reps)) * n_sets, since it gives [[0, 1, 2, 3], [0, 1, 2, 3], [0, 1, 2, 3]] instead of
the desired [0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3]. Do I need to flatten, or is there a syntax with the _ notation that gives me a single list?
Further update . . .
replacing
rep_list = list(range(reps)) * n_sets
with
rep_list_nest = [list(range(reps)) for _ in range(n_sets)]rep_list = [i for sublist in rep_list_nest for i in sublist]
gives the same undesired result for expanded_sets.
答案1
得分: 1
问题出在这里：
expanded_sets = [itemfor item in setsfor i in range(reps)]
现在，这个列表中连续包含了sets中的每个元素四次，然后是下一个元素重复四次，以此类推。
创建item的副本可以修复这个问题：
expanded_sets = [item.copy()for item in setsfor i in range(reps)]
如果你想要一个更符合Python风格的方法，那就认识到结果是两个范围的乘积，并且是你原始的sets列表中所有元素的连接：
from itertools import productsets = [[1, 2, 3], [1, 2, 4], [1, 2, 5]]expanded_sets = [[inner_counter, outer_counter] + sets_elemfor sets_elem, outer_counter, inner_counter in product(sets, range(len(sets)), range(4))]
英文:
The problem is here:
expanded_sets = [itemfor item in setsfor i in range(reps)]
This list now contains the same element of sets four times in a row, followed by the next element repeated four times, and so on.
Creating copies of item fixes the issue:
expanded_sets = [item.copy()for item in setsfor i in range(reps)]
If you want a more pythonic approach, then recognize that the result is a product of two ranges, and your original sets all concatenated together:
from itertools import productsets = [[1, 2, 3], [1, 2, 4], [1, 2, 5]]expanded_sets = [[inner_counter, outer_counter] + sets_elemfor sets_elem, outer_counter, inner_counter in product(sets, range(len(sets)), range(4))]
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