How can I add a column to an existing filled dataframe

Question

I tried to add value to a new column for an existing data frame:

val_df["perplexity"][10]=14

There is no column named perplexity and I thought this will create the column. I guessed this answer says the same, but I got the following error:

KeyError: 'perplexity'

So if you want to add new column called perplexity, you need to specify the values. What are those values? — Dani Mesejo
– Dani Mesejo, Commented Mar 6, 2019 at 19:20
@DanielMesejo they must be set in a loop, however I can set a default value of 0 — Ahmad
– Ahmad, Commented Mar 6, 2019 at 19:21
Then I guess you are just find doing val_df["perplexity"] = 0 and then setting the other values with the for loop. — Dani Mesejo
– Dani Mesejo, Commented Mar 6, 2019 at 19:22

Supratim Haldar · Accepted Answer · 2019-03-06 19:59:34Z

2

Since 'perplexity' column does not exist yet, a specific index (row) cannot be accessed. So, create the column and assign a default value, then update specific row's value.

val_df["perplexity"] = ''
val_df["perplexity"][10] = 14

EDIT: As suggested by @ALollz below, chained indexing should be avoided to avoid the SettingWithCopy warning. The following code is recommended.

val_df.loc[10, 'perplexity'] = 14

edited Mar 6, 2019 at 19:59

answered Mar 6, 2019 at 19:29

Supratim Haldar

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2 Comments

ALollz Over a year ago

chained assignment should be avoided. Use .loc: val_df.loc[10, 'perplexity'] = 14

Supratim Haldar Over a year ago

You are right, thanks for pointing it out. I have added this as an edit.

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