Input, Subset and Output External Data Files in using Pandas in Python

The Input, Subset and Output External Data Files using Pandas in Python can be manipulated in different ways. Learn more about it here.

This section shows basic code for reading, sub-setting and writing external data files using pandas.

Using Pandas in Python: Basic Code to Import, Subset and Write External Data Files Using Pandas

Print the working directory import os
print os.getcwd()
C:\Python27\Scripts
Set the working directory
os.chdir('C:/Users/general1/Documents/simple Python files')
print os.getcwd()
C:\Users\general1\Documents\simple Python files
load pandas
import pandas as pd
read a csv data file named 'small_dataset.csv' containing 4 lines and 3 variables my_data = pd.read_csv("small_dataset.csv")
my_data
x y z
0 1 2 3
1 4 5 6
2 7 8 9
3 10 11 12
my_data.shape # number of rows and columns in data set
(4, 3)
my_data.shape[0] # number of rows in data set
4
my_data.shape[1] # number of columns in data set
3
Python uses 0-based indexing. The first row or column in a data set is located
at position 0. In R the first row or column in a data set is located
at position 1.
Select the first two rows
my_data[0:2]
x y z
0 1 2 3
1 4 5 6
Select the second and third rows my_data[1:3]
x y z
1 4 5 6
2 7 8 9
Select the third row my_data[2:3]
x y z
2 7 8 9
Select the first two elements of the first column my_data.iloc[0:2, 0:1]
x
0 1
1 4
Select the first element of the variables y and z my_data.loc[0, ['y', 'z']]
y 2
z 3
Select the first three elements of the variables y and z my_data.loc[0:2, ['y', 'z']]
y z
0 2 3
1 5 6
2 8 9

Write the first three elements of the variables y and z
to an external file. Here index = 0 means do not write row names.
my_data2 = my_data.loc[0:2, [‘y’, ‘z’]]
my_data2.to_csv(‘my.output.csv’, index = 0)

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