python - interpolation on dataframe values

Question

My code:

import pandas as pd
import numpy as np
from scipy.interpolate import interp1d

def interpolate_iv_my(x,y,newX_Value):
    y_interp =  interp1d(x,y)
    iv = y_interp(newX_Value)
    return iv

import pandas as pd
df = pd.DataFrame({'30': [-23, 12, -12, 10, -23, 12, -32, 15, -20, 10],
                   '40': [-30, 20, -21, 15, -33, 22, -40, 25, -22, 12],
                   '50': [-40, 25, -26, 19, -39, 32, -45, 35, -32, 18],
                   '60': [-45, 34, -29, 25, -53, 67, -55, 45, -42, 19],
})

x = [30,40,50,60]
df['x_'] = np.random.choice([35,33,42,52],10).tolist()

Picture of the dataframe:

I am trying to get a new column with interpolated value corresponding to the value in the 'x_' column. for instance for the first row for the x value of 52

The interpolated value will be

y = [-23,-30,-40,-45]

interpolate_iv_my(x,y,52)

which gives me -41.0

What's the best way to do this for all the rows (my dataframe has over 50k rows)?

Edited

ok came up with this not sure if this is the best solution

iter_list = []
for l,r in df.iterrows():
    y = []
    y.append(r['30'])
    y.append(r['40'])
    y.append(r['50'])
    y.append(r['60'])
    iter_list.append(interpolate_iv_my(x,y,r['x_']).tolist())

df ['interpolated'] = iter_list

Does this answer your question? [Applying function with multiple arguments to create a new pandas column](https://stackoverflow.com/questions/19914937/applying-function-with-multiple-arguments-to-create-a-new-pandas-column) — Jakub Szlaur, Oct 11 '21 at 15:16

score 0 · Answer 1 · answered Oct 11 '21 at 09:41

0

I think your interpolate_iv_my function isn't working properly.

But the correct code for applying function to all rows in a dataframe and creating new column based on those results is this:

df['interpolate'] = df.apply(lambda row: interpolate_iv_my(row, x, 52), axis=1)

answered Oct 11 '21 at 09:41

Jakub Szlaur

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no I think the function looks fine, I just manually ran interpolation manually on excel for the first three rows and matched the python values – user13412850 Oct 11 '21 at 10:09
coming back to the solution you proposed how do I pass the row in my code thats where I am struggling, and I think it should be (x, row, 52) in your code not (row, x, 52) – user13412850 Oct 11 '21 at 10:11
Did it work when you swaped it? – Jakub Szlaur Oct 11 '21 at 12:53
your code doesnt tell me how am I passing the row to the apply function. – user13412850 Oct 11 '21 at 12:56
I am sorry but yes it does - it uses lambda function to which it passes the `row` argument. Similar answer here: https://stackoverflow.com/a/33518725/11101156 – Jakub Szlaur Oct 11 '21 at 15:13

python - interpolation on dataframe values

Edited

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