Pandas make new column from string slice of another column

Question

I want to create a new column in Pandas using a string sliced for another column in the dataframe.

For example.

Sample  Value  New_sample
AAB     23     A
BAB     25     B

Where New_sample is a new column formed from a simple [:1] slice of Sample

I've tried a number of things to no avail - I feel I'm missing something simple.

What's the most efficient way of doing this?

Answer 1

You can call the str method and apply a slice, this will be much quicker than the other method as this is vectorised (thanks @unutbu):

df['New_Sample'] = df.Sample.str[:1]

You can also call a lambda function on the df but this will be slower on larger dataframes:

In [187]:

df['New_Sample'] = df.Sample.apply(lambda x: x[:1])
df
Out[187]:
  Sample  Value New_Sample
0    AAB     23          A
1    BAB     25          B

Answer 2

You can also use slice() to slice string of Series as following:

df['New_sample'] = df['Sample'].str.slice(0,1)

From pandas documentation :

Series.str.slice(start=None, stop=None, step=None)

Slice substrings from each element in the Series/Index

For slicing index ( if index is of type string ), you can try:

df.index = df.index.str.slice(0,1)

Answer 3

Adding solution to a common variation when the slice width varies across DataFrame Rows:

#--Here i am extracting the ID part from the Email (i.e. the part before @)

#--First finding the position of @ in Email
d['pos'] = d['Email'].str.find('@')

#--Using position to slice Email using a lambda function
d['new_var'] = d.apply(lambda x: x['Email'][0:x['pos']],axis=1)

#--Imagine x['Email'] as a string on which, slicing is applied

Hope this Helps !