有条件地替换列中的值

Question

I have a pandas dataframe, where the 2nd, 3rd and 6th columns look like so:

start	end	strand
108286	108361	+
734546	734621	-
761233	761309	+

I'm trying to implement a conditional where, if strand is +, then the value in end becomes the equivalent value in start + 1, and if strand is -, then the value in start becomes the value in end, so the output should look like this:

start	end	strand
108286	108287	+
734620	734621	-
761233	761234	+

And where the pseudocode may look like this:

if df["strand"] == "+": 
        df["end"] = df["start"] + 1
        
else:
        df["start"] = df["end"] - 1

I imagine this might be best done with loc/iloc or numpy.where ? but I can't seem to get it to work, as always, any help is appreciated!

Answer 1

You are correct, loc is the operator you are looking for

df.loc[df.strand=='+','end'] = df.loc[df.strand=='+','start']+1
df.loc[df.strand=='-','start'] = df.loc[df.strand=='-','end']-1

Answer 2

You could also use numpy.where :

import numpy as np
df[['start', 'end']] = np.where(df[['strand']]=='-', df[['end','end']]-[1,0], df[['start','start']]+[0,1])

Note that this assumes strand can have one of two values: + or - . If it can have any other values, we can use numpy.select instead.

Output:

    start     end strand
0  108286  108287      +
1  734620  734621      -
2  761233  761234      +