Concat columns in a for loop

Question

I have the following data set:

1   8   15  22
2   9   16  23
3   10  17  24
4   11  18  25
5   12  19  26
6   13  20  27
7   14  21  28

I want to get the following result:

So I want to loop over all columns of my dataset and concat each column to the first one.

import pandas as pd

df = pd.read_csv("data.csv", delimiter=";", header=-1)

number_of_columns= len(df.columns)
print(number_of_columns)


for i in range (1,number_of_columns):
  df1 = df.iloc[:,i]
  df2 = pd.concat([df,df1], ignore_index=True)


print(df2)

With this only the last column is concatenated in the final dataframe. I get it that df2 is overwritten in each iteration of the for-loop.

So how can I "save" df2 after each for loop so that every column is concantenated?

Thanks a lot!

Use df.unstack()

Zero
– Zero

2018-07-28 13:03:28 +00:00
Commented Jul 28, 2018 at 13:03 — Zero
– Zero, Commented Jul 28, 2018 at 13:03

rafaelc · Accepted Answer · 2018-07-28 13:05:21Z

3

For column-wise

`stack`+`tolist`

df.stack().tolist()

[1,
 8,
 15,
 22,
 2,
 9,
 16,
 23,
 3,
 10,
 17,
 24,
 4,
 11,
 18,
 25,
 5,
 12,
 19,
 26,
 6,
 13,
 20,
 27,
 7,
 14,
 21,
 28]

For row-wise

`melt`

df.melt().value.tolist()

[1,
 2,
 3,
 4,
 5,
 6,
 7,
 8,
 9,
 10,
 11,
 12,
 13,
 14,
 15,
 16,
 17,
 18,
 19,
 20,
 21,
 22,
 23,
 24,
 25,
 26,
 27,
 28]

`unstack`+`tolist`

df.unstack().tolist()

#outputs same as above

answered Jul 28, 2018 at 13:05

rafaelc

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Comments

Pyd · Accepted Answer · 2018-07-28 16:59:04Z

2

simply np.flatten(),

 pd.Series(df.values.flatten())
 (or)
 pd.Series(df.unstack().values)

answered Jul 28, 2018 at 16:59

Pyd

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Comments

Bal Krishna Jha · Accepted Answer · 2018-07-28 13:47:54Z

1

You can also do it:

txt = '''1   8   15  22
2   9   16  23
3   10  17  24
4   11  18  25
5   12  19  26
6   13  20  27
7   14  21  28'''
arr1 = np.fromstring(txt, dtype=int, sep=' ')
arr1.reshape(7,-1).flatten(order = 'F') # for column wise, 'C' can be used for row wise.

answered Jul 28, 2018 at 13:47

Bal Krishna Jha

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Comments

jpp · Accepted Answer · 2018-07-28 14:57:55Z

1

You may not require a 3rd party library. You can use csv and itertools modules from the standard library to return a list of numbers:

from io import StringIO
from itertools import chain
import csv

mystr = StringIO("""1   8   15  22
2   9   16  23
3   10  17  24
4   11  18  25
5   12  19  26
6   13  20  27
7   14  21  28""")

with mystr as fin:
    reader = csv.reader(mystr, skipinitialspace=True, delimiter=' ')
    res = list(map(int, chain.from_iterable(zip(*reader))))

print(res)

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10,
 11, 12, 13, 14, 15, 16, 17, 18, 19, 20,
 21, 22, 23, 24, 25, 26, 27, 28]

answered Jul 28, 2018 at 14:57

jpp

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Concat columns in a for loop

4 Answers 4

`stack`+`tolist`

`melt`

`unstack`+`tolist`

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4 Answers 4

stack+tolist

melt

unstack+tolist

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`stack`+`tolist`

`melt`

`unstack`+`tolist`