Numpy "where" with multiple conditions
Question:
I try to add a new column “energy_class” to a dataframe “df_energy” which it contains the string “high” if the “consumption_energy” value > 400, “medium” if the “consumption_energy” value is between 200 and 400, and “low” if the “consumption_energy” value is under 200.
I try to use np.where from numpy, but I see that numpy.where(condition[, x, y])
treat only two condition not 3 like in my case.
Any idea to help me please?
Thank you in advance
Answers:
You can use a ternary:
np.where(consumption_energy > 400, 'high',
(np.where(consumption_energy < 200, 'low', 'medium')))
I would use the cut() method here, which will generate very efficient and memorysaving category
dtype:
In [124]: df
Out[124]:
consumption_energy
0 459
1 416
2 186
3 250
4 411
5 210
6 343
7 328
8 208
9 223
In [125]: pd.cut(df.consumption_energy,
[0, 200, 400, np.inf],
labels=['low','medium','high']
)
Out[125]:
0 high
1 high
2 low
3 medium
4 high
5 medium
6 medium
7 medium
8 medium
9 medium
Name: consumption_energy, dtype: category
Categories (3, object): [low < medium < high]
Try this:
Using the setup from @Maxu
col = 'consumption_energy'
conditions = [ df2[col] >= 400, (df2[col] < 400) & (df2[col]> 200), df2[col] <= 200 ]
choices = [ "high", 'medium', 'low' ]
df2["energy_class"] = np.select(conditions, choices, default=np.nan)
consumption_energy energy_class
0 459 high
1 416 high
2 186 low
3 250 medium
4 411 high
5 210 medium
6 343 medium
7 328 medium
8 208 medium
9 223 medium
I like to keep the code clean. That’s why I prefer np.vectorize
for such tasks.
def conditions(x):
if x > 400: return "High"
elif x > 200: return "Medium"
else: return "Low"
func = np.vectorize(conditions)
energy_class = func(df_energy["consumption_energy"])
Then just add numpy array as a column in your dataframe using:
df_energy["energy_class"] = energy_class
The advantage in this approach is that if you wish to add more complicated constraints to a column, it can be done easily.
Hope it helps.
I second using np.vectorize. It is much faster than np.where and also cleaner code wise. You can definitely tell the speed up with larger data sets. You can use a dictionary format for your conditionals as well as the output of those conditions.
# Vectorizing with numpy
row_dic = {'Condition1':'high',
'Condition2':'medium',
'Condition3':'low',
'Condition4':'lowest'}
def Conditions(dfSeries_element,dictionary):
'''
dfSeries_element is an element from df_series
dictionary: is the dictionary of your conditions with their outcome
'''
if dfSeries_element in dictionary.keys():
return dictionary[dfSeries]
def VectorizeConditions():
func = np.vectorize(Conditions)
result_vector = func(df['Series'],row_dic)
df['new_Series'] = result_vector
# running the below function will apply multi conditional formatting to your df
VectorizeConditions()
WARNING: Be careful with NaNs
Always be careful that if your data has missing values np.where
may be tricky to use and may give you the wrong result inadvertently.
Consider this situation:
df['cons_ener_cat'] = np.where(df.consumption_energy > 400, 'high',
(np.where(df.consumption_energy < 200, 'low', 'medium')))
# if we do not use this second line, then
# if consumption energy is missing it would be shown medium, which is WRONG.
df.loc[df.consumption_energy.isnull(), 'cons_ener_cat'] = np.nan
Alternatively, you can use onemore nested np.where
for medium versus nan which would be ugly.
IMHO best way to go is pd.cut
. It deals with NaNs and easy to use.
Examples:
import numpy as np
import pandas as pd
import seaborn as sns
df = sns.load_dataset('titanic')
# pd.cut
df['age_cat'] = pd.cut(df.age, [0, 20, 60, np.inf], labels=['child','medium','old'])
# manually add another line for nans
df['age_cat2'] = np.where(df.age > 60, 'old', (np.where(df.age <20, 'child', 'medium')))
df.loc[df.age.isnull(), 'age_cat'] = np.nan
# multiple nested where
df['age_cat3'] = np.where(df.age > 60, 'old',
(np.where(df.age <20, 'child',
np.where(df.age.isnull(), np.nan, 'medium'))))
# outptus
print(df[['age','age_cat','age_cat2','age_cat3']].head(7))
age age_cat age_cat2 age_cat3
0 22.0 medium medium medium
1 38.0 medium medium medium
2 26.0 medium medium medium
3 35.0 medium medium medium
4 35.0 medium medium medium
5 NaN NaN medium nan
6 54.0 medium medium medium
myassign["assign3"]=np.where(myassign["points"]>90,"genius",(np.where((myassign["points"]>50) & (myassign["points"]<90),"good","bad"))
when you wanna use only "where" method but with multiple condition. we can add more condition by adding more (np.where) by the same method like we did above. and again the last two will be one you want.
Let’s start by creating a dataframe with 1000000
random numbers between 0
and 1000
to be used as test
df_energy = pd.DataFrame({'consumption_energy': np.random.randint(0, 1000, 1000000)})
[Out]:
consumption_energy
0 683
1 893
2 545
3 13
4 768
5 385
6 644
7 551
8 572
9 822
A bit of a description of the dataframe
print(df.energy.describe())
[Out]:
consumption_energy
count 1000000.000000
mean 499.648532
std 288.600140
min 0.000000
25% 250.000000
50% 499.000000
75% 750.000000
max 999.000000
There are various ways to achieve that, such as:

Using
numpy.where
df_energy['energy_class'] = np.where(df_energy['consumption_energy'] > 400, 'high', np.where(df_energy['consumption_energy'] > 200, 'medium', 'low'))

Using
numpy.select
df_energy['energy_class'] = np.select([df_energy['consumption_energy'] > 400, df_energy['consumption_energy'] > 200], ['high', 'medium'], default='low')

Using
numpy.vectorize
df_energy['energy_class'] = np.vectorize(lambda x: 'high' if x > 400 else ('medium' if x > 200 else 'low'))(df_energy['consumption_energy'])

Using
pandas.cut
df_energy['energy_class'] = pd.cut(df_energy['consumption_energy'], bins=[0, 200, 400, 1000], labels=['low', 'medium', 'high'])

Using Python’s built in modules
def energy_class(x): if x > 400: return 'high' elif x > 200: return 'medium' else: return 'low' df_energy['energy_class'] = df_energy['consumption_energy'].apply(energy_class)

Using a lambda function
df_energy['energy_class'] = df_energy['consumption_energy'].apply(lambda x: 'high' if x > 400 else ('medium' if x > 200 else 'low'))
Time Comparison
From all the tests that I’ve done, by measuring time with time.perf_counter()
(for other ways to measure time of execution see this), pandas.cut
was the fastest approach.
method time
0 np.where() 0.124139
1 np.select() 0.155879
2 numpy.vectorize() 0.452789
3 pandas.cut() 0.046143
4 Python's builtin functions 0.138021
5 lambda function 0.19081
Notes:
 For the difference between
pandas.cut
andpandas.qcut
see this: What is the difference between pandas.qcut and pandas.cut?
Try this : Even if consumption_energy
contains nulls don’t worry about it.
def egy_class(x):
'''
This function assigns classes as per the energy consumed.
'''
return ('high' if x>400 else
'low' if x<200 else 'medium')
chk = df_energy.consumption_energy.notnull()
df_energy['energy_class'] = df_energy.consumption_energy[chk].apply(egy_class)