Pandas - add value at specific iloc into new dataframe column

Question

I have a large dataframe containing lots of columns.

For each row/index in the dataframe I do some operations, read in some ancilliary ata, etc and get a new value. Is there a way to add that new value into a new column at the correct row/index?

I can use .assign to add a new column but as I'm looping over the rows and only generating the data to add for one value at a time (generating it is quite involved). When it's generated I'd like to immediately add it to the dataframe rather than waiting until I've generated the entire series.

This doesn't work and gives a key error:

df['new_column_name'].iloc[this_row]=value 

Do I need to initialise the column first or something?

Answer 1

There are two steps to created & populate a new column using only a row number... (in this approach iloc is not used)

First, get the row index value by using the row number

rowIndex = df.index[someRowNumber] 

Then, use row index with the loc function to reference the specific row and add the new column / value

df.loc[rowIndex, 'New Column Title'] = "some value" 

These two steps can be combine into one line as follows

df.loc[df.index[someRowNumber], 'New Column Title'] = "some value"