I have a matrix of data 29523 rows x 503 cols of which 3 cols are indices (below is a subset for example).
IDX1| IDX2 | IDX3 | 1983 Q4 | X | Y | Z |1984 Q1 | X | Y | Z
---------------------------------------------------------------------------
A | A1 | Q | 10 | A | F | NaN | 110 | A | F | NaN
A | A2 | Q | 20 | B | C | 40 | 120 | B | C | 240
A | A3 | Q | 30 | A | F | NaN | 130 | A | F | NaN
A | A4 | Q | 40 | B | C | 80 | 140 | B | C | 280
A | A5 | Q | 50 | A | F | NaN | 150 | A | F | NaN
A | A6 | Q | 60 | B | F | 120 | 160 | B | F | 320
I read this into a DataFrame
with:
>>> df = pd.read_csv(C:\filename.csv, low_memory=False, mangle_dupe_cols=False)
and then use pandas.melt()
to pivot the data:
df1 = pd.melt(df, id_vars=['IDX1', 'IDX2', 'IDX3'], var_name='ValueType',
value_name = 'Value')
I have also tried stack()
but melt()
proved better here.
IDX1 | IDX2 | IDX3 | ValueType | Value
---------------------------------------------------------------
A | A1 | Q | 1983 Q4 | 10
A | A1 | Q | X | A
A | A1 | Q | Y | F
A | A1 | Q | Z | NaN
A | A1 | Q | 1984 Q1 | 110
A | A1 | Q | X | A
A | A1 | Q | Y | F
A | A1 | Q | Z | NaN
A | A2 | Q | 1983 Q4 | 20
A | A2 | Q | X | B
A | A2 | Q | Y | C
A | A2 | Q | Z | 40
The option mangle_dupe_cols
on the read_csv
if True
will place a .int
suffix against all ValueType
s that are duplicated. This is not ideal, but without it there is no way of linking the values for the variables to the correct period.
What I would prefer to do is instead of having the Period
(1984 Q1)
as a ValueType
, give the Period
s corresponding Value
a variable 'W'
and have each period form part of the IDX
as below:
IDX1 | IDX2 | IDX3 | IDX4 | ValueType | Value
---------------------------------------------------------------
A | A1 | Q | 1983 Q4| W | 10
A | A1 | Q | 1983 Q4| X | A
A | A1 | Q | 1983 Q4| Y | F
A | A1 | Q | 1983 Q4| Z | NaN
A | A1 | Q | 1984 Q1| W | 110
A | A1 | Q | 1984 Q1| X | A
A | A1 | Q | 1984 Q1| Y | F
A | A1 | Q | 1984 Q1| Z | NaN
A | A2 | Q | 1983 Q4| W | 20
A | A2 | Q | 1983 Q4| X | B
A | A2 | Q | 1983 Q4| Y | C
A | A2 | Q | 1983 Q4| Z | 40
Is the above possible with pandas or numpy?
My final DataFrame
is going to be 14,761,500 rows x 6 cols.
Given
In [189]: df
Out[189]:
IDX1 IDX2 IDX3 1983 Q4 X Y Z 1984 Q1 X.1 Y.1 Z.1
0 A A1 Q 10 A F NaN 110 A F NaN
1 A A2 Q 20 B C 40 120 B C 240
2 A A3 Q 30 A F NaN 130 A F NaN
3 A A4 Q 40 B C 80 140 B C 280
4 A A5 Q 50 A F NaN 150 A F NaN
5 A A6 Q 60 B F 120 160 B F 320
Let us first set ['IDX1', 'IDX2', 'IDX3']
as the index.
df = df.set_index(['IDX1', 'IDX2', 'IDX3'])
The other columns have a periodic quality to them; we want to handle every 4 columns as a group . This idea of "handling as a group" leads naturally to assigning a new index level to the column index; some value which is the same for every 4 columns. This would be ideal:
1983 Q4 1984 Q1
W X Y Z W X Y Z
IDX1 IDX2 IDX3
A A1 Q 10 A F NaN 110 A F NaN
A2 Q 20 B C 240 120 B C 240
A3 Q 30 A F NaN 130 A F NaN
A4 Q 40 B C 280 140 B C 280
A5 Q 50 A F NaN 150 A F NaN
A6 Q 60 B F 320 160 B F 320
We can achieve this by building a MultiIndex and assigning it to df.columns
:
columns = [col for col in df.columns if col[0] not in set(list('XYZ'))]
df.columns = pd.MultiIndex.from_product([columns, list('WXYZ')])
Now the desired long-format DataFrame can be obtained by calling df.stack
to move the column levels into the row index:
df.columns.names = ['IDX4', 'ValueType']
series = df.stack(['IDX4', 'ValueType'], dropna=False)
Note also that when mangle_dupe_cols=False
, the duplicate columns, X
, Y
, Z
, get overwritten . So you lose data with mangle_dupe_cols=False
. For example, when you use mangle_dupe_cols=False
the last row's Z
value gets assigns to every Z
column regardless of the period.
So we must use mangle_dupe_cols=True
, (or just leave it out since that is the default) and adjust the code accordingly. That, fortunately, is not hard to do since we are reassigning df.columns
to a custom-build MultiIndex anyway.
Putting it all together:
import numpy as np
import pandas as pd
df = pd.read_table('data', sep=r'\s*[|]\s*')
df = df.set_index(['IDX1', 'IDX2', 'IDX3'])
columns = [col for col in df.columns if col[0] not in set(list('XYZ'))]
df.columns = pd.MultiIndex.from_product([columns, list('WXYZ')])
df.columns.names = ['IDX4', 'ValueType']
series = df.stack(['IDX4', 'ValueType'], dropna=False)
print(series.head())
yields
IDX1 IDX2 IDX3 IDX4 ValueType
A A1 Q 1983 Q4 W 10
X A
Y F
Z NaN
1984 Q1 W 110
dtype: object
Note that since we've removed all the column levels, the result is a Series. If you want a DataFrame with 6 columns, then we should follow it up with:
series.name = 'Value'
df = series.reset_index()
print(df.head())
which yields
IDX1 IDX2 IDX3 IDX4 ValueType Value
0 A A1 Q 1983 Q4 W 10
1 A A1 Q 1983 Q4 X A
2 A A1 Q 1983 Q4 Y F
3 A A1 Q 1983 Q4 Z NaN
4 A A1 Q 1984 Q1 W 110
...
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