First Transition Value of DataFrame Column without Temporary Variables

Question

I am trying to find the first transition value of a dataframe column as efficiently as possible. I would prefer not to have temporary variables. Say I have a dataframe (df) with a column of:

Column1  
0  
0
0
-1
1

In this case, the value which I'm looking for is -1, which is the first time the value changes. I want to use this in an if statement for whether the value is first transitioning to 1 or -1. The pseudocode being:

if (first transition value == 1):
    # Something
elif: (first transition value == -1):
    # Something else

pythonic833 · Accepted Answer · 2019-08-15 18:45:45Z

1

General case

You can compare the values in the dataframe to the first one, take only the differing values and use the first value of these.

df[df.Column1 != df.Column1.iloc[0]].Column1.values[0]

Special case

If you always want to find the first differing element from 0 you could just do it like this:

df[df.Column1 != 0].Column1.values[0]

answered Aug 15, 2019 at 18:45

pythonic833

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1 Comment

Andrew Drake Over a year ago

I had to add an extra check using stackoverflow.com/questions/36543606/…, but besides that it works! Clean too, thanks

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