Lecture 6

Rows

DATA 8

Summer 2017

Slides created by John DeNero (denero@berkeley.edu), Ani Adhikari (adhikari@berkeley.edu), and Sam Lau (samlau95@berkeley.edu).

Tables Review

Table Structure

• A Table is a sequence of labeled columns
• Labels are strings
• Columns are arrays, all with the same length

Table Methods

• Creating and extending tables:
• Table().with_columns and Table.read_table
• Finding the size: num_rows and num_columns
• Referring to columns: labels, relabeling, and indices
• labels and relabeled; column indices start at 0
• Accessing data in a column
• column takes a label or index and returns an array
• Using array methods to work with data in columns
• item, sum, min, max, and so on
• Creating new tables containing some of the original columns:
• select, drop

Quick Check

The table students has columns Name, ID, and Score.

Write one line of code that evaluates to:

1. A table consisting of only the column labeled Name

students.select('Name')

b) The largest score

students.column('Score').max()

students.select(0)

max(students.column('Score'))

Sort

Sorting Tables

Tables are ordered collections of rows

The sort method creates a new table with the same rows in a different order (the original table is unaffected)

The show method displays the first rows of a table

(Demo)

Lists

Lists are Generic Sequences

A list is a sequence of values (just like an array), but the values can all have different types

​

[2+3, 'four', Table().with_column('K', [3, 4])]

If you create a table column from a list, it will be converted to an array automatically

​

(Demo)

Take

Take Rows, Select Columns

The select method returns a table with only some columns

The take method returns a table with only some rows

• Rows are numbered, starting at 0
• Taking a single number returns a one-row table
• Taking a list of numbers returns a table as well

(Demo)

Where

The Where Method

The where method specifies a column and a condition

It returns a new table with all rows satisfying the condition

(Demo)

https://www.inferentialthinking.com/chapters/05/2/selecting-rows.html#Some-More-Conditions

Manipulating Rows

• t.sort(column) sorts the rows in increasing order
• t.take(row_numbers) keeps the numbered rows
• Each row has an index, starting at 0
• t.where(column, are.condition) keeps all rows for which a column's value satisfies a condition
• t.where(column, value) keeps all rows containing a certain value in a column