Difference between revisions of "Linux-AWK/C2/Basics-of-Single-Dimensional-Array-in-awk/English"

• Arrays in awk
• Assigning array elements
• How it is different from arrays in other programming languages and
• Refer the elements of an array

We will do this through some examples.

System requirement

• Ubuntu Linux 16.04 Operating System and
• gedit text editor 3.20.1

You can use any text editor of your choice.

pre-requisite

You should have some basic knowledge of any programming language like C or C++.

If not, then please go through the corresponding tutorials on our website.

Please download and extract them.

• awk supports arrays for storing related elements.
• Elements can be a number or a string.
• Arrays in awk are associative.
• This means that each array element is an index-value pair.

But there are some important differences.

• First, we do not need to declare an array before using it.
• Also there is no need to specify how many elements the array will contain.

• In programming languages, array index is generally a positive integer.
• Usually the index starts from 0, then 1, then 2 and so on.

array-name[index] = value

Array name can be any valid variable name.

Here the index can be an integer or a string.

Let’s understand this with an example.

This file is available in the Code Files link below the player.

Please download and use it.

Here, the name of the array is day.

I have set the index as 1 and the value as Sunday.

So for index first, the value is Sunday.

The entire array is constructed likewise.

day[4]="wednesday"

day["fourth"]="wednesday"

day[3]="tuesday"

day["third"]="tuesday"

I have declared day four before day three.

• In awk arrays, index need not to be in a sequential manner.
• Advantage of associative array is that new pairs can be added at any time.

day[6]="Friday"

day["sixth"]="Friday"

Place the cursor at the end of the last line and press Enter.

Then type the following

But how should we refer to the array element?

Write the arrayname and the index within square brackets to refer an element at a particular index.

Let us try this.

Type:

print day[6]

Place the cursor in front of the closing curly brace.

Press Enter and type print space day within square brackets 6

awk -f array_intro.awk

[Enter]

awk space hyphen small f space array_intro.awk

Press Enter

For this, we have to use the in operator.

Let me explain this with an example.

Type:

if (2 in day) [Enter]

print "Index 2 is present."

[Enter]

if (7 in day) [Enter]

print "Index7 is present."

Place the cursor at the end of the print statement and press Enter.

Then type as shown.

Save the code.

Highlight if (2 in day)

Highlight print "Index 2 is present."

Highlight

if (7 in day)

print "Index7 is present."

The first if condition checks whether the index two is present in day.

If yes, then the corresponding print statement will get executed.

Then the second condition checks whether the index seven is present in day.

It will execute the print statement if it is true.

As we can see, index two is in the array and seven is not.

Let’s execute this file to verify the output.

Press the Up arrow key to get back the previously executed command.

Press Enter to execute.

if (day[7] != "")

print "Index 7 is not null"

if (7 in day)

print "Index 7 is present after null comparison."

Update the code as shown here.

This will check whether the value of index seven is null or not.

If true, it will print Index 7 is not null

We already know that we don’t have any index with 7, so it will not print anything.

Let’s see what happens when we execute the code.

Press Up arrow key

Press Enter

Press the up arrow key to get the previous command.

Press Enter to execute.

The statement "Index 7 is present after null comparison."

is printed.

How is it possible?

Highlight

if (day[7] != "")

This access itself will first create an element at index 7 and initialize it with the value null.

if (7 in day)

As null element is already created, the output shows that Index 7 is present after null comparison.

day at index 7 not equal to null is a wrong way to check the presence of an element.

It will create a null element at index 7.

It will not create any extra element in the array.

This brings us to the end of this tutorial.

Summary

• Arrays in awk
• Assigning array elements
• How it is different from arrays in other programming languages
• Refer the elements of an array

Assignment

• Define an array flowerColor
• Index will be the names of the flowers
• Value will be the corresponding color of the flowers

Assignment(Cont.)

• Insert entries for any five flowers of your choice
• Print the color of the fourth flower
• Check if the flower “Lotus” is present in the array

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Acknowledgement

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And this is Praveen from IIT Bombay signing off.

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