Using a dictionary to map specific values in python

Question

I am iterating through a dataframe and pulling out specific lines and then enriching those lines with some other elements. I have a dictionary that has the following definition mapping:

testdir = {0: 'zero', 40: 'forty', 60: 'sixty', 80: 'eighty'}

When i pull out a specific line from the original dataframe which looks like this

a   b   c      x   str
0  0   0   0  100.0  aaaa

i want the str cell to now be set to the string value of column c which is 0 so

output should be

a   b   c      x   str
0  0   0   0  100.0  zero

and then after meeting some other conditions a new line is pulled out from the original dataframe and the output should be

a   b   c      x   str
0  0   0   0  100.0  zero
3  4  30  60  100.0  sixty

i tried to use the map() method so something like'

df['str'][-1] = df['c'][-1].map(testdir)

but i'm erroring all over the place!

Does this answer your question? [Adding a new pandas column with mapped value from a dictionary](https://stackoverflow.com/questions/24216425/adding-a-new-pandas-column-with-mapped-value-from-a-dictionary) — David Erickson, Jun 15 '20 at 07:53

yatu · Answer 1 · 2020-06-15T07:56:15.807

map is intended for pd.Series, so if you can, just map the entire column when it is fully populated, that way you avoid the oevrhead of multiple calls to map:

df['str'] = df.c.map(testdir)

print(df)

   a   b   c      x    str
0  0   0   0  100.0   zero
3  4  30  60  100.0  sixty

Note that, to correctly index the dataframe on a single cell, and map with the dictionary, you need something like:

testdir[df.iat[-1, 2]]

Chained indexing as df['c'][-1] is discouraged in the docs, and has a negative effect when assigning, which is that your not updating the original dataframe, since it returns a copy.

Using a dictionary to map specific values in python

1 Answers1