pandas 如何将值添加到熊猫数据框中的新列？
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How to add values to a new column in pandas dataframe?
提问by barciewicz
I want to create a new named column in a Pandas dataframe, insert first value into it, and then add another values to the same column:
我想在 Pandas 数据框中创建一个新的命名列，将第一个值插入其中，然后向同一列添加另一个值：
Something like:
就像是：
import pandas
df = pandas.DataFrame()
df['New column'].append('a')
df['New column'].append('b')
df['New column'].append('c')
etc.
How do I do that?
我怎么做？
采纳答案by jezrael
Dont do it, because slow:
不要这样做，因为很慢：
6) updating an empty frame a-single-row-at-a-time. I have seen this method used WAY too much. It is by far the slowest. It is probably common place (and reasonably fast for some python structures), but a DataFrame does a fair number of checks on indexing, so this will always be very slow to update a row at a time. Much better to create new structures and concat.
6) 一次更新一个空帧 a-single-row-at-a-time。我已经看到这种方法使用太多了。它是迄今为止最慢的。这可能是常见的地方（并且对于某些 python 结构来说相当快），但是 DataFrame 对索引进行了大量检查，因此一次更新一行总是很慢。创建新结构和连接要好得多。
Better is create list of data and create DataFrameby contructor:
更好的是创建数据列表DataFrame并由构造函数创建：
vals = ['a','b','c']
df = pandas.DataFrame({'New column':vals})
回答by amo3tasem
If I understand correctly you want to append value to an existing column in a pandas dataframe, the thing is with DFs you need to maintain a matrix-like shape so number of rows is equal for each columns what you can do is add a column with a default value then update this value with
如果我理解正确，您想将值附加到 Pandas 数据框中的现有列，问题是使用 DF，您需要保持类似矩阵的形状，因此每列的行数相等，您可以做的是添加一列一个默认值然后更新这个值
for index, row in df.iterrows():
df.at[index, 'new_column'] = new_value
for index, row in df.iterrows():
df.at[index, 'new_column'] = new_value
