Take previous row's value in dataframe when condition meets

Question

I have a datafarme let's say something like this:

| ColumnX        |
| -------------- |
| Hi             |
| + Open         |
| How are you    |
| NAN            |
| Something      |
| something      |
| HEY            |
| + Open         |

now I need to go through the rows checking their values. If the row value is "+ Open" then select the previous row value and put it into a list. so far this is what I've done but I couldn't figure out how to take the previous value-

ilist=[]
for i in df["ColumnX"]:
    if i == '+ Open':
        ilist.append(i)

ThePyGuy · Accepted Answer · 2022-09-26 12:55:23Z

3

Create the mask for values, shift the mask passing -1, then fill NA by False which is basically the last row, then use this mask to get the values and finally create list out of the values:

>>> df.loc[df['ColumnX'].eq('+ Open').shift(-1).fillna(False) ,'ColumnX'].to_list()

# Output:
['Hi', 'HEY']

answered Sep 26, 2022 at 12:55

ThePyGuy

18.6k5 gold badges24 silver badges55 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Rahul Sharma · Accepted Answer · 2022-09-26 12:57:01Z

0

Maybe something like this:

ilist=[]
for i in range(0,len(df["ColumnX"])):
    if df["ColumnX"][i] == '+ Open':
        ilist.append(df["ColumnX"][i-1])

answered Sep 26, 2022 at 12:57

Rahul Sharma

3284 silver badges9 bronze badges

Comments

Timo · Accepted Answer · 2022-09-26 13:07:17Z

0

You could use enumerate and acces the previous value by index:

import pandas as pd

df = pd.DataFrame(["Hi", "+ Open", "How are you", "NAN", "Something", "Something", "HEY","+ Open"], columns=["ColumnX"])

ilist = []

for i, value in enumerate(df["ColumnX"]):
    if value == "+ Open" and i>0: # check if i>0 to prevent out of index error
        ilist.append(df.at[i-1, "ColumnX"])
        
print(ilist)

Out:

['Hi', 'HEY']

answered Sep 26, 2022 at 13:07

Timo

3771 silver badge11 bronze badges

Collectives™ on Stack Overflow

Take previous row's value in dataframe when condition meets

3 Answers 3

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related