Converting strings in a dataframe column to an index value

Question

Using Python and Pandas, I have a

dataframe, df
with a column entitled 'letters'
a list, letlist = ['a','b','c','d']

I would like to create a new column, 'letters_index', where the generate value would be the index of the string in column letters, in the list letlist

I tried

df['letters_index'] = letlist.index(df['letters'])

However, this didn't work. Do you have any suggestions?

harpan · Accepted Answer · 2018-05-23 00:20:33Z

2

As far as I understand, You need:

letlist = ['a', 'b', 'c', 'd']
print(df)

Output:

letters
0   a
1   b
2   b
3   d
4   c

And then

df['new_col'] = df['letters'].apply(lambda x: letlist.index(x))

Output:

0    0
1    1
2    1
3    3
4    2
Name: letters, dtype: int64

Beware that if the value in the column is not present in the list it would throw a ValueError.

edited May 23, 2018 at 0:20

answered May 23, 2018 at 0:14

harpan

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1 Comment

Joseph Mann Over a year ago

This works great! is there anyway that we can add the output as a new column in the dataframe?

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