Develop Reference

Creating a data frame named after values from another data frame

I have a data frame containing three columns, whereas col_1 and col_2 are containing some arbitrary data:
data = {"Height": range(1, 20, 1), "Col_1": range(2, 40, 2), "Col_2": range(3, 60, 3)}
df = pd.DataFrame(data)
Height Col_1 Col_2
0 1 2 3
1 2 4 6
2 3 6 9
3 4 8 12
4 5 10 15
5 6 12 18
6 7 14 21
7 8 16 24
8 9 18 27
9 10 20 30
10 11 22 33
11 12 24 36
12 13 26 39
13 14 28 42
14 15 30 45
15 16 32 48
16 17 34 51
17 18 36 54
18 19 38 57
and another data frame containing height values, that should be used to segment the Height column from the df.
data_segments = {"Section Height" : [1, 10, 20]}
df_segments = pd.DataFrame(data_segments)
Section Height
0 1
1 10
2 20
I want to create two new data frames, df_segment_0 containing all columns of the initial df but only for Height rows within the first two indices in the df_segments. The same approach should be taken for the df_segment_1. They should look like:
df_segment_0
Height Col_1 Col_2
0 1 2 3
1 2 4 6
2 3 6 9
3 4 8 12
4 5 10 15
5 6 12 18
6 7 14 21
7 8 16 24
8 9 18 27
df_segment_1
Height Col_1 Col_2
9 10 20 30
10 11 22 33
11 12 24 36
12 13 26 39
13 14 28 42
14 15 30 45
15 16 32 48
16 17 34 51
17 18 36 54
18 19 38 57
I tried the following code using the .loc method and added the suggestion of C Hecht to create a list of data frames:
df_segment_list = []
try:
for index in df_segments.index:
df_segment = df[["Height", "Col_1", "Col_2"]].loc[(df["Height"] >= df_segments["Section Height"][index]) & (df["Height"] < df_segments["Section Height"][index + 1])]
df_segment_list.append(df_segment)
except KeyError:
pass
Try-except is used only to ignore the error for the last name entry since there is no height for index=2. The data frames in this list can be accessed as C Hecht:
df_segment_0 = df_segment_list[0]
Height Col_1 Col_2
0 1 2 3
1 2 4 6
2 3 6 9
3 4 8 12
4 5 10 15
5 6 12 18
6 7 14 21
7 8 16 24
8 9 18 27
However, I would like to automate the naming of the final data frames. I tried:
for i in range(0, len(df_segment_list)):
name = "df_segment_" + str(i)
name = df_segment_list[i]
I expect that this code to simply automate the df_segment_0 = df_segment_list[0], instead I receive an error name 'df_segment_0' is not defined.
The reason I need separate data frames is that I will perform many subsequent operations using Col_1 and Col_2, so I need row-wise access to each one of them, for example:
df_segment_0 = df_segment_0 .assign(col_3 = df_segment_0 ["Col_1"] / df_segment_0 ["Col_2"])
How do I achieve this?
EDIT 1: Clarified question with the suggestion from C Hecht.

If you want to get all entries that are smaller than the current segment height in your segmentation data frame, here you go :)
import pandas as pd
df1 = pd.DataFrame({"Height": range(1, 20, 1), "Col_1": range(2, 40, 2), "Col_2": range(3, 60, 3)})
df_segments = pd.DataFrame({"Section Height": [1, 10, 20]})
def segment_data_frame(data_frame: pd.DataFrame, segmentation_plan: pd.DataFrame):
df = data_frame.copy() # making a safety copy because we mutate the df !!!
for sh in segmentation_plan["Section Height"]: # sh is the new maximum "Height"
df_new = df[df["Height"] < sh] # select all entries that match the maximum "Height"
df.drop(df_new.index, inplace=True) # remove them from the original DataFrame
yield df_new
# ATTENTION: segment_data_frame() will calculate each segment at runtime!
# So if you don't want to iterate over it but rather have one list to contain
# them all, you must use list(segment_data_frame(...)) or [x for x in segment_data_frame(...)]
for segment in segment_data_frame(df1, df_segments):
print(segment)
print()
print(list(segment_data_frame(df1, df_segments)))
If you want to execute certain steps on those steps you can just use the defined list like so:
for segment in segment_data_frame(df1, df_segments):
do_stuff_with(segment)
If you want to keep track and name the individual frames, you can use a dictionary

Unfortunately I don't 100% understand what you have in mind, but I hope that the following should help you in finding the answer:
import pandas as pd
import numpy as np
df = pd.DataFrame({'Section Height': [20, 90, 111, 232, 252, 3383, 3768, 3826, 3947, 4100], 'df_names': [f'df_section_{i}' for i in range(10)]})
df['shifted'] = df['Section Height'].shift(-1)
new_dfs = []
for index, row in df.iterrows():
if np.isnan(row['shifted']):
# Don't know what you want to do here
pass
else:
new_df = pd.DataFrame({'heights': [i for i in range(int(row['Section Height']), int(row['shifted']))]})
new_df.name = row['df_names']
new_dfs.append(new_df)
The content of new_dfs are dataframes that look like this:
heights
0 20
1 21
2 22
3 23
4 24
.. ...
65 85
66 86
67 87
68 88
69 89
[70 rows x 1 columns]
If you clarify your questions given this input, we could help you all the way, but this should hopefully point you in the right direction.
Edit: A small comment on using df.name: This is not really stable and if you do stuff like dropping a column, pickling/unpickling, etc. the name will likely be lost. But you can surely find a good solution to maintain the name depending on your needs.

Sample dataframe by value in column and keep all rows

I want to sample a Pandas dataframe using values in a certain column, but I want to keep all rows with values that are in the sample.
For example, in the dataframe below I want to randomly sample some fraction of the values in b, but keep all corresponding rows in a and c.
d = pd.DataFrame({'a': range(1, 101, 1),'b': list(range(0, 100, 4))*4, 'c' :list(range(0, 100, 2))*2} )
Desired example output from a 16% sample:
Out[66]:
a b c
0 1 0 0
1 26 0 50
2 51 0 0
3 76 0 50
4 4 12 6
5 29 12 56
6 54 12 6
7 79 12 56
8 18 68 34
9 43 68 84
10 68 68 34
11 93 68 84
12 19 72 36
13 44 72 86
14 69 72 36
15 94 72 86
I've tried sampling the series and merging back to the main data, like this:
In [66]: pd.merge(d, d.b.sample(int(.16 * d.b.nunique())))
This creates the desired output, but it seems inefficient. My real dataset has millions of values in b and hundreds of millions of rows. I know I could also use some version of ``isin```, but that also is slow.
Is there a more efficient way to do this?

I really doubt that isin is slow:
uniques = df.b.unique()
# this maybe the bottle neck
samples = np.random.choice(uniques, replace=False, size=int(0.16*len(uniques)) )
# sampling here
df[df.b.isin(samples)]
You can profile the steps above. In case samples=... is slow, you can try:
idx = np.random.rand(len(uniques))
samples = uniques[idx<0.16]
Those took about 100 ms on my system on 10 million rows.
Note: d.b.sample(int(.16 * d.b.nunique())) does not sample 0.16 of the unique values in b.

Using df.apply on a function with multiple inputs to generate multiple outputs

I have a dataframe that looks like this
initial year0 year1
0 0 12
1 1 13
2 2 14
3 3 15
Note that the number of year columns year0, year1... (year_count) is completely variable but will be constant throughout this code
I first wanted to apply a function to each of the 'year' columns to generate 'mod' columns like so
def mod(year, scalar):
return (year * scalar)
s = 5
year_count = 2
# Generate new columns
df[[f"mod{y}" for y in range (year_count)]] = df[[f"year{y}" for y in range(year_count)]].apply(mod, scalar=s)
initial year0 year1 mod0 mod1
0 0 12 0 60
1 1 13 5 65
2 2 14 10 70
3 3 15 15 75
All good so far. The problem is that I now want to apply another function to both the year column and its corresponding mod column to generate another set of val columns, so something like
def sum_and_scale(year_col, mod_col, scale):
return (year_col + mod_col) * scale
Then I apply this to each of the columns (year0, mod0), (year1, mod1) etc to generate the next tranche of columns.
With scale = 10 I should end up with
initial year0 year1 mod0 mod1 val0 val1
0 0 12 0 60 0 720
1 1 13 5 65 60 780
2 2 14 10 70 120 840
3 3 15 15 75 180 900
This is where I'm stuck - I don't know how to put two existing df columns together in a function with the same structure as in the first example, and if I do something like
df[['val0', 'val1']] = df['col1', 'col2'].apply(lambda x: sum_and_scale('mod0', 'mod1', scale=10))
I don't know how to generalise this to have arbitrary inputs and outputs and also apply the constant scale parameter. (I know the last piece of won't work but it's the other avenue to a solution I've seen)
The reason I'm asking is because I believe the loop that I currently have working is creating performance issues with the number of columns and the length of each column.
Thanks

IMHO, it's better with a simple for loop:
for i in range(2):
df[f'val{i}'] = sum_and_scale(df[f'year{i}'], df[f'mod{i}'], scale=10)

How to extract mean and fluctuation by equal index?

I have a CSV file like the below (after sorted the dataframe by iy):
iy,u
1,80
1,90
1,70
1,50
1,60
2,20
2,30
2,35
2,15
2,25
I'm trying to compute the mean and the fluctuation when iy are equal. For example, for the CSV above, what I want is something like this:
iy,u,U,u'
1,80,70,10
1,90,70,20
1,70,70,0
1,50,70,-20
1,60,70,-10
2,20,25,-5
2,30,25,5
2,35,25,10
2,15,25,-10
2,25,25,0
Where U is the average of u when iy are equal, and u' is simply u-U, the fluctuation. I know that there's a function called groupby.mean() in pandas, but I don't want to group the dataframe, just take the mean, put the values in a new column, and then calculate the fluctuation.
How can I proceed?

Use groupby with transform to calculate a mean for each group and assign that value to a new column 'U', then pandas to subtract two columns:
df['U'] = df.groupby('iy').transform('mean')
df["u'"] = df['u'] - df['U']
df
Output:
iy u U u'
0 1 80 70 10
1 1 90 70 20
2 1 70 70 0
3 1 50 70 -20
4 1 60 70 -10
5 2 20 25 -5
6 2 30 25 5
7 2 35 25 10
8 2 15 25 -10
9 2 25 25 0
You could get fancy and do it in one line:
df.assign(U=df.groupby('iy').transform('mean')).eval("u_prime = u-U")

In Pandas, how to operate between columns in max perfornace

I have the following df:
usersidid clienthostid LoginDaysSumLastMonth LoginDaysSumLast7Days LoginDaysSum
0 9 1 50 7 1728
1 3 1 43 3 1331
2 6 1 98 9 216
3 4 1 10 6 64
4 9 2 64 32 343
5 12 3 45 43 1000
6 8 3 87 76 512
7 9 3 16 3 1200
What I'm trying to do is:
For every 'clienthostid' look for the 'usersidid' with the highest 'LoginDaysSum', I check if there is a usersidid which is the highest LoginDaysSum in two different clienthostid (for instance, usersidid = 9 ia the highest LoginDaysSum in both clienthostid 1, 2 and 3 in rows 0, 4 and 7 accordingly).
In this case, I want to choose the higher LoginDaysSum (in the example it would be the row with 1728), lets call it maxRT.
I want to calculate the ratio of LoginDaysSumLast7Days between maxRT and each of the other rows (in example, it would be rows index 7 and 4).
If the ratio is below 0.8 than I want to drop the row:
index 4- LoginDaysSumLast7Days_ratio = 7/32 < 0.8 //row will drop!
index 7- LoginDaysSumLast7Days_ratio = 7/3 > 0.8 //row will stay!
The same condition also will be applied of LoginDaysSumLastMonth.
So for the example the result will be:
usersidid clienthostid LoginDaysSumLastMonth LoginDaysSumLast7Days LoginDaysSum
0 9 1 50 7 1728
1 3 1 43 3 1331
2 6 1 98 9 216
3 4 1 10 6 64
5 12 3 45 43 1000
6 8 3 87 76 512
7 9 3 16 3 1200
Now here's the snag- performance is critical.
I tried to implement it using .apply but not only i couldn't make it to work right, it also ran way too slow :(
My code so far (forgive me of it's written terribly wrong, I only started working for the first time with SQL, Pandas and Python last week and everything I learned is from examples I found here ^_^):
df_client_Logindayssum_pairs = df.merge(df.groupby(['clienthostid'], as_index=False, sort=False)['LoginDaysSum'].max(),df, how='inner', on=['clienthostid', 'LoginDaysSum'])
UsersWithMoreThan1client = df_client_Logindayssum_pairs.groupby(['usersidid'], as_index=False, sort=False)['LoginDaysSum'].count().rename(columns={'LoginDaysSum': 'NumOfClientsPerUesr'})
UsersWithMoreThan1client = UsersWithMoreThan1client[UsersWithMoreThan1client.NumOfClientsPerUesr >= 2]
UsersWithMoreThan1client = df_client_Logindayssum_pairs[df_client_Logindayssum_pairs.usersidid.isin(UsersWithMoreThan1Device.loc[:, 'usersidid'])].reset_index(drop=True)
UsersWithMoreThan1client = UsersWithMoreThan1client.sort_values(['clienthostid', 'LoginDaysSum'], ascending=[True, False], inplace=True)
UsersWithMoreThan1client = ttm.groupby(['clienthostid'], sort=False)['LoginDaysSumLast7Days'].apply(lambda x: x.iloc[0] / x.iloc[1]).reset_index(name='ratio')
UsersWithMoreThan1client = UsersWithMoreThan1client[UsersWithMoreThan1client.ratio > 0.8]
UsersWithMoreThan1client = ttm.groupby(['clienthostid'], sort=False)['LoginDaysSumLastMonth'].apply(lambda x: x.iloc[0] / x.iloc[1]).reset_index(name='ratio2')
UsersWithMoreThan1client = UsersWithMoreThan1client[UsersWithMoreThan1client.ratio2 > 0.8]
Would very much appreciate any suggestions on how to do it
Thank you

I believe this is what you need:
# Put the index as a regular column
data = data.reset_index()
# Find greates LoginDaysSum for each clienthostid
agg1 = data.sort_values(by='LoginDaysSum', ascending=False).groupby(['clienthostid']).first()
# Collect greates LoginDaysSum for each usersidid
agg2 = agg1.sort_values(by='LoginDaysSum', ascending=False).groupby('usersidid').first()
# Join both previous aggregations
joined = agg1.set_index('usersidid').join(agg2, rsuffix='_max')
# Compute ratios
joined['LoginDaysSumLast7Days_ratio'] = joined['LoginDaysSumLast7Days_max'] / joined['LoginDaysSumLast7Days']
joined['LoginDaysSumLastMonth_ratio'] = joined['LoginDaysSumLastMonth_max'] / joined['LoginDaysSumLastMonth']
# Select index values that do not meet the required criteria
rem_idx = joined[(joined['LoginDaysSumLast7Days_ratio'] < 0.8) | (joined['LoginDaysSumLastMonth_ratio'] < 0.8)]['index']
# Restore index and remove the selected rows
data = data.set_index('index').drop(rem_idx)
The result in data is:
usersidid clienthostid LoginDaysSumLastMonth LoginDaysSumLast7Days LoginDaysSum
index
0 9 1 50 7 1728
1 3 1 43 3 1331
2 6 1 98 9 216
3 4 1 10 6 64
5 12 3 45 43 1000
6 8 3 87 76 512
7 9 3 16 3 1200