Python: How to multiply selection of a colomn of a dataframe (without using a for loop)

Question

I am working with a pandas dataframe, and want to multiply a selection of a specific column. I want to select all the values of col2 less than 5, and multiply them by 10. How do I do this (without using a for loop)?

df = pandas.DataFrame({'col1':[2,3,5,7], 'col2':[1,2,30,4]})
    col1   col2
0   2      1  
1   3      2
2   5      30
3   7      4

I tried the following code, but this did not result in the dataframe I want:

df['col2'] = df.col2[df['col2']<5]*10
    col1   col2
0   2      10.0 
1   3      20.0
2   5      NaN
3   7      40.0

With a for loop, I was able to multiply each value smaller than 5 by 10, but the data set i am working on is quite large, so I would rather not use a for loop.

count=0
for value in df.col2:
    if value < 5:
        df.col2[count]=df.col2[count]*10
        count+=1

Does someone know how to do this?

use `np.where` - `np.where(df['col2'].le(5),df['col2'] * 10, df['col2'])` — Umar.H, Mar 23 '20 at 13:37

score 1 · Answer 1 · answered Mar 23 '20 at 13:37

1

I managed to do this with Lambda expression:

df["col2"] = df["col2"].apply(lambda x:x*10 if x < 5 else x)

I assumed that numbers larger than 10 stay unchanged

answered Mar 23 '20 at 13:37

Guy_g23

316
2
7

Python: How to multiply selection of a colomn of a dataframe (without using a for loop)

1 Answers1