Comparing value with previous row in Pandas DataFrame

Question

import pandas as pd
data={'col1':[1,3,3,1,2,3,2,2]}
df=pd.DataFrame(data,columns=['col1'])
print df


     col1  
0     1          
1     3          
2     3          
3     1          
4     2          
5     3          
6     2          
7     2

I have the following Pandas DataFrame and I want to create another column that compares the previous row of col1 to see if the value of the row is greater than that of the previous row. It should come out like the following:

    col1  match  
0     1   False     
1     3   False     
2     3   True     
3     1   False     
4     2   False     
5     3   True     
6     2   False     
7     2   True

Thank you.

jezrael · Accepted Answer · 2020-11-16 09:25:15Z

1

Compare shifted values by Series.gt and Series.shift,last missing value is replaced to -1 for True, working, if all values are positive:

df['match'] = df['col1'].gt(df['col1'].shift(-1, fill_value=-1))
print (df)

   col1  match
0     1  False
1     3  False
2     3   True
3     1  False
4     2  False
5     3   True
6     2  False
7     2   True

If need last value set to True for any Dataframe:

df['match'] = df['col1'].gt(df['col1'].shift(-1))
df.loc[df.index[-1], 'match'] = True

answered Nov 16, 2020 at 9:25

jezrael

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5 Comments

adir abargil Over a year ago

Is it only work with gt? or the operator >= would work too?

NovaPoi Over a year ago

Thank you for your prompt reply. Another quick question here: what if I want to compare row values with custom operations? Say print "True" if the value of a row is greater than that of the previous row by 20%?

jezrael Over a year ago

@adirabargil - gt is like >, for => is used ge

jezrael Over a year ago

@NovaPoi - 20% Do you need 20% from values and compare? Not understand, I think it should be new question.

jezrael Over a year ago

@NovaPoi - or need df['match'] = df['col1'].gt(df['col1'].mul(0.2).shift(-1, fill_value=-1)) ?

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