How can i create a text file from a pandas dataframe mixing two columns in an alternate form?

Question

Lets say i have two columns on a same huge dataframe (more than 1G of data) in pandas something like this (simplified):

df

A B
C D

and i want to create a text file with all data in the form below:

A
B
C
D

how would you do this?

Answer 1

There are several ways. You could use .stack() or .melt() or .unstack() or pd.concat():

input:

0 1
A B
C D

#1:

pd.DataFrame(df.stack().reset_index(drop=True))

#2:

pd.DataFrame(df.melt().iloc[:,-1])

#3:

pd.DataFrame(df.unstack().reset_index(drop=True)) #keeps order as A, C, B, D

#4:

pd.DataFrame(pd.concat([df['0'],df['1']]).reset_index(drop=True)) #keeps order as A, C, B, D

output:

0
A
B
C
D