将项目附加到python中的列表列表[重复]

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List of lists changes reflected across sublists unexpectedly 13 answers
列表更改反映在子列表中的意外13个答案

I'm getting mad with list indexes, and can't explain what I'm doing wrong.

我对列表索引很生气，无法解释我做错了什么。

I have this piece of code in which I want to create a list of lists, each one containing values of the same circuit parameter (voltage, current etc..) that I'm reading from a csv file that looks like this:

我有这段代码，我想在其中创建一个列表列表，每个列表包含相同电路参数（电压，电流等）的值，我正在从一个csv文件中读取，如下所示：

Sample, V1, I1, V2, I2
0, 3, 0.01, 3, 0.02
1, 3, 0.01, 3, 0.03

And so on. What I want is to create a list that for example contains V1 and I1 (but I want to chose interactively) in the form [[V1], [I1]], so:

等等。我想要的是以[[V1]，[I1]]的形式创建一个列表，例如包含V1和I1（但我想以交互方式选择），所以：

[[3,3], [0.01, 0.01]]

The code that I'm using is this:

我正在使用的代码是这样的：

plot_data = [[]]*len(positions)    
for row in reader:
    for place in range(len(positions)):
        value = float(row[positions[place]])
        plot_data[place].append(value)

plot_data is the list that contains all the values, while positions is a list with the indexes of the columns that I want to copy from the .csv file. The problem is that if I try the commands in the shell, seems to work, but if I run the script instead of appending each value to the proper sub-list, it appends all values to all lists, so I obtain 2 (or more) identical lists.

plot_data是包含所有值的列表，而position是一个列表，其中包含我要从.csv文件中复制的列的索引。问题是，如果我在shell中尝试命令，似乎工作，但如果我运行脚本而不是将每个值附加到正确的子列表，它会将所有值附加到所有列表，所以我获得2（或更多））相同的清单。

2 个解决方案

#1

Python lists are mutable objects and here:

Python列表是可变对象，在这里：

plot_data = [[]] * len(positions)

you are repeating the same list len(positions) times.

你重复相同的列表len（位置）时间。

>>> plot_data = [[]] * 3
>>> plot_data
[[], [], []]
>>> plot_data[0].append(1)
>>> plot_data
[[1], [1], [1]]
>>>

Each list in your list is a reference to the same object. You modify one, you see the modification in all of them.

列表中的每个列表都是对同一对象的引用。你修改一个，你看到所有这些修改。

If you want different lists, you can do this way:

如果您想要不同的列表，可以这样做：

plot_data = [[] for _ in positions]

for example:

例如：

>>> pd = [[] for _ in range(3)]
>>> pd
[[], [], []]
>>> pd[0].append(1)
>>> pd
[[1], [], []]

#2

import csv
cols = [' V1', ' I1'] # define your columns here, check the spaces!
data = [[] for col in cols] # this creates a list of **different** lists, not a list of pointers to the same list like you did in [[]]*len(positions) 
with open('data.csv', 'r') as f:
    for rec in csv.DictReader(f):
        for l, col in zip(data, cols):
            l.append(float(rec[col]))
print data

# [[3.0, 3.0], [0.01, 0.01]]

#1