BEGIN and END

Learning objectives

• be able to do something at the start of an AWK script
• be able to do something at the end of an AWK script

For teachers

Teaching goals are:

• Learners are able to do something at the start of an AWK script
• Learners are able to do something at the end of an AWK script

Lesson plan:

• 5 mins: prior knowledge
• 5 mins: presentation
• 15 mins: challenge
• 5 mins: feedback

Overview

Sometimes one needs to do things before the actual calculation starts. Also, sometimes one needs to do things after the actual calculation has run.

Here we do things at the start and end of an AWK script.

Exercises

See the exercise procedure here.

Exercise 1: confirming things are true

Learning objectives

• experience the BEGIN and END blocks

Download the data

In a terminal, do:

wget https://pmitev.github.io/to-awk-or-not/data/coins.txt

to download a file called coins.txt.

BEGIN

In a terminal, in the same folder as where the data is downloaded, do:

awk 'BEGIN {print "There I go!" }' coins.txt

In English, this is: 'At the start, print 'There I go!''.

Confirm that this is true.

BEGIN and a print

In a terminal, in the same folder as where the data is downloaded, do:

awk 'BEGIN {print "Coin minting years:" } { print $3 }' coins.txt

In English, this is: 'At the start, print 'Coin minting years:', then, for every line, print the third column.

Confirm that this is true.

END

In a terminal, in the same folder as where the data is downloaded, do:

awk 'END {print "Done!" }' coins.txt

In English, this is: 'In the end, print 'Done!''.

Confirm that this is true.

END and a print

In a terminal, in the same folder as where the data is downloaded, do:

awk '{ print $0 } END { print "Done!" }' coins.txt

In English, this is: 'For every line, print the line. In the end, print 'Done!''.

Confirm that this is true.

BEGIN and END and a print

In a terminal, in the same folder as where the data is downloaded, do:

awk 'BEGIN { print "Start!" } { print $0 } END { print "Done!" }' coins.txt

In English, this is: 'At the start, print 'Start!'. Then, for every line, print the line. In the end, print 'Done!''.

Confirm that this is true.

BEGIN and END and a condition and a print

In a terminal, in the same folder as where the data is downloaded, do:

awk 'BEGIN { print "Start!" } $1 == "gold" { print $0 } END { print "Done!" }' coins.txt

In English, this is: 'At the start, print 'Start!'. Then, for every line, if the first column matches gold, print the line. In the end, print 'Done!''.

Confirm that this is true.

Exercise 2: explore data

Learning objectives

• practice using the BEGIN and END blocks

Download the data

In a terminal, do:

wget https://raw.githubusercontent.com/richelbilderbeek/awk_course/master/data/diamonds.tsv

to download a file called diamonds.tsv.

This file is a tab-separated file about diamonds and is part of the ggplot2 R package.

• Raw data
• Data description

The dataset has the following columns:

index	name	description
1	carat	weight of the diamond (0.2--5.01)
2	cut	quality of the cut (Fair, Good, Very Good, Premium, Ideal)
3	color	diamond colour, from D (best) to J (worst)
4	clarity	a measurement of how clear the diamond is (I1 (worst), SI2, SI1, VS2, VS1, VVS2, VVS1, IF (best))
5	depth	total depth percentage = z / mean(x, y) = 2 * z / (x + y) (43--79)
6	table	width of top of diamond relative to widest point (43--95)
7	price	price in US dollars (\(326--\)18,823)
8	x	length in mm (0--10.74)
9	y	width in mm (0--58.9)
10	z	depth in mm (0--31.8)

Explore the data

Using awk only:

• show a text 'Number of lines:', then print the number of lines
• show a text 'Number of columns:', then print the number of columns
• show a text 'Header:', then print only the header
• show a text 'Prices of first five diamonds:', then print the prices of the first five diamonds, while skipping the header
• show the line number of the diamonds that have a price below $400 USD, while skipping the header. At the end, print "... those were he cheapest diamonds"
• show a text 'Prices (in USD) of diamonds with a fair and worst color:'. Then show the price of the diamonds that have a fair cut and the worst color, while skipping the header. Then show the text 'Done!'

Think:

• these practice questions are only about printing. Could you already guess where BEGIN and END become more useful?