Data Wrangling in Python – How to Applying Operations Over pandas Dataframes

Applying Operations Over pandas Dataframes

Import Modules

import pandas as pd
import numpy as np

Create a dataframe

data = {'name': ['Jason', 'Molly', 'Tina', 'Jake', 'Amy'], 
        'year': [2012, 2012, 2013, 2014, 2014], 
        'reports': [4, 24, 31, 2, 3],
        'coverage': [25, 94, 57, 62, 70]}
df = pd.DataFrame(data, index = ['Cochice', 'Pima', 'Santa Cruz', 'Maricopa', 'Yuma'])
coverage name reports year
Cochice 25 Jason 4 2012
Pima 94 Molly 24 2012
Santa Cruz 57 Tina 31 2013
Maricopa 62 Jake 2 2014
Yuma 70 Amy 3 2014

Create a capitalization lambda function

capitalizer = lambda x: x.upper()

Apply the capitalizer function over the column ‘name’

apply() can apply a function along any axis of the dataframe

Cochice       JASON
Pima          MOLLY
Santa Cruz     TINA
Maricopa       JAKE
Yuma            AMY
Name: name, dtype: object

Map the capitalizer lambda function over each element in the series ‘name’

map() applies an operation over each element of a series

Cochice       JASON
Pima          MOLLY
Santa Cruz     TINA
Maricopa       JAKE
Yuma            AMY
Name: name, dtype: object

Apply a square root function to every single cell in the whole data frame

applymap() applies a function to every single element in the entire dataframe.

/* Drop the string variable so that applymap() can run */
df = df.drop('name', axis=1)

/* Return the square root of every cell in the dataframe */
coverage reports year
Cochice 5.000000 2.000000 44.855323
Pima 9.695360 4.898979 44.855323
Santa Cruz 7.549834 5.567764 44.866469
Maricopa 7.874008 1.414214 44.877611
Yuma 8.366600 1.732051 44.877611

Applying A Function Over A Dataframe

Create a function that multiplies all non-strings by 100

# create a function called times100
def times100(x):
    /* that, if x is a string, */
    if type(x) is str:
        /* just returns it untouched */
        return x
    /* but, if not, return it multiplied by 100 */
    elif x:
        return 100 * x
    /* and leave everything else */

Apply the times100 over every cell in the dataframe

coverage reports year
Cochice 2500 400 201200
Pima 9400 2400 201200
Santa Cruz 5700 3100 201300
Maricopa 6200 200 201400
Yuma 7000 300 201400

Python Example for Beginners

Two Machine Learning Fields

There are two sides to machine learning:

  • Practical Machine Learning:This is about querying databases, cleaning data, writing scripts to transform data and gluing algorithm and libraries together and writing custom code to squeeze reliable answers from data to satisfy difficult and ill defined questions. It’s the mess of reality.
  • Theoretical Machine Learning: This is about math and abstraction and idealized scenarios and limits and beauty and informing what is possible. It is a whole lot neater and cleaner and removed from the mess of reality.

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