[Chapter 8] 8.4 Arrays
Author: Dale Dougherty & Arnold Robbins
Date: 1998-08-03T20:58:54Z

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| Chapter 8 Conditionals, Loops, and Arrays |

## 8.4 Arrays

An array is a variable that can be used to store a set of values. Usually the values are related in some way. Individual elements are accessed by their index in the array. Each index is enclosed in square brackets. The following statement assigns a value to an element of an array:

> `array`[`subscript`] = `value`

In awk, you don't have to declare the size of the array; you only have to use the identifier as an array. This is best done by assigning a value to an array element. For instance, the following example assigns the string `"`cherry`"` to an element of the array named `flavor`.

>     flavor[1] = "cherry"

The index or subscript of this element of the array is "1". The following statement prints the string "cherry":

>     print flavor[1]

Loops can be used to load and extract elements from arrays. For instance, if the array `flavor` has five elements, you can write a loop to print each element:

>     flavor_count = 5
>     for (x = 1; x <= flavor_count; ++x)
>     	print flavor[x]

One way that arrays are used in awk is to store a value from each record, using the record number as the index to the array. Let's suppose we wanted to keep track of the averages calculated for each student and come up with a class average. Each time a record is read we make the following assignment.

>     student_avg[NR] = avg

The system variable `NR` is used as the subscript for the array because it is incremented for each record. When the first record is read, the value of `avg` is placed in `student_avg[1]`; for the second record, the value is placed in `student_avg[2]`, and so on. After we have read all of the records, we have a list of averages in the array `student_avg`. In an `END` rule, we can average all of these grades by writing a loop to get the total of the grades and then dividing it by the value of `NR`. Then we can compare each student average to the class average to collect totals for the number of students at or above average and the number below.

>     END {
>     	for ( x = 1; x <= NR; x++ )
>     		class_avg_total += student_avg[x]

>     	class_average = class_avg_total / NR

>     	for ( x = 1; x <= NR; x++ )
>     		if (student_avg[x] >= class_average)
>     			++above_average
>     		else
>     			++below_average

>     	print "Class Average: ", class_average
>     	print "At or Above Average: ", above_average
>     	print "Below Average: ", below_average
>     }

There are two `for` loops for accessing the elements of the array. The first one totals the averages so that it can be divided by the number of student records. The next loop retrieves each student average so that it can be compared to the class average. If it is at or above average, we increment the variable `above_average`; otherwise, we increment `below_average`.

### 8.4.1 Associative Arrays

In awk, all arrays are associative arrays. What makes an associative array unique is that its index can be a string or a number.

In most programming languages, the indices of arrays are exclusively numeric. In these implementations, an array is a sequence of locations where values are stored. The indices of the array are derived from the order in which the values are stored. There is no need to keep track of indices. For instance, the index of the first element of an array is "1" or the first location in the array.

An associative array makes an "association" between the indices and the elements of an array. For each element of the array, a pair of values is maintained: the index of the element and the value of the element. The elements are not stored in any particular order as in a conventional array. Thus, even though you can use numeric subscripts in awk, the numbers do not have the same meaning that they do in other programming languages - they do not necessarily refer to sequential locations. However, with numeric indices, you can still access all the elements of an array in sequence, as we did in previous examples. You can create a loop to increment a counter that references the elements of the array in order.

Sometimes, the distinction between numeric and string indices is important. For instance, if you use "04" as the index to an element of the array, you cannot reference that element using "4" as its subscript. You'll see how to handle this problem in a sample program `date-month`, shown later in this chapter.

Associative arrays are a distinctive feature of awk, and a very powerful one that allows you to use a string as an index to another value. For instance, you could use a word as the index to its definition. If you know the word, you can retrieve the definition.

For example, you could use the first field of the input line as the index to the second field with the following assignment:

>     array[$1] = $2

Using this technique, we could take our list of acronyms and load it into an array named `acro`.

>     acro[$1] = $2

Each element of the array would be the description of an acronym and the subscript used to retrieve the element would be the acronym itself. The following expression:

>     acro["BASIC"]

produces:

>     Beginner's All-Purpose Symbolic Instruction Code

There is a special looping syntax for accessing all the elements of an associative array. It is a version of the `for` loop.

> `for` ( `variable` `in` `array` )
> `do something with array`[`variable`]

The array is the name of an array, as it was defined. The variable is any variable, which you can think of as a temporary variable similar to a counter that is incremented in a conventional `for` loop. This variable is set to a particular subscript each time through the loop. (Because `variable` is an arbitrary name, you often see `item` used, regardless of what variable name was used for the subscript when the array was loaded.) For example, the following `for` loop prints the name of the acronym item and the definition referenced by that name, acro[item].

>     for ( item in acro )
>     	print item, acro[item]

In this example, the print statement prints the current subscript ("BASIC," for instance) followed by the element of the `acro` array referenced by the subscript ("Beginner's All-Purpose Symbolic Instruction Code").

This syntax can be applied to arrays with numeric subscripts. However, the order in which the items are retrieved is somewhat random.[2] The order is very likely to vary among awk implementations; be careful to write your programs so that they don't depend on any one version of awk.

> [2] The technical term used in The AWK Programming Language is "implementation dependent."

It is important to remember that all array indices in awk are strings. Even when you use a number as an index, awk automatically converts it to a string first. You don't have to worry about this when you use integer indices, since they get converted to strings as integers, no matter what the value may be of `OFMT` (original awk and earlier versions of new awk) or `CONVFMT` (POSIX awk). But if you use a real number as an index, the number to string conversion might affect you. For instance:

>     $ gawk 'BEGIN { data[1.23] = "3.21"; CONVFMT = "%d"
>     > printf "<%s>\n", data[1.23] }'
>     <>

Here, nothing was printed between the angle brackets, since the second time, `1.23` was converted to just `1`, and `data["1"]` has the empty string as its value.

> NOTE: Not all implementations of awk get the number to string conversion right when `CONVFMT` has changed between one use of a number and the next. Test the above example with your awk to be sure it works correctly.

Now let's return to our student grade program for an example. Let's say that we wanted to report how many students got an "A," how many got a "B," and so on. Once we determine the grade, we could increment a counter for that grade. We could set up individual variables for each letter grade and then test which one to increment.

>     if ( grade == "A" )
>     	++gradeA
>     else if (grade == "B" )
>     	++gradeB
>     .
>     .
>     .

However, an array makes this task much easier. We can define an array called `class_grade`, and simply use the letter grade (A through F) as the index to the array.

>     ++class_grade[grade]

Thus, if the grade is an "A" then the value of `class_grade["A"]` is incremented by one. At the end of the program, we can print out these values in the `END` rule using the special `for` loop:

>     for (letter_grade in class_grade)
>          print letter_grade ":", class_grade[letter_grade] | "sort"

The variable `letter_grade` references a single subscript of the array `class_grade` each time through the loop. The output is piped to `sort`, to make sure the grades come out in the proper order. (Piping output to programs is discussed in Chapter 10, The Bottom Drawer.) Since this is the last addition we make to the `grades.awk` script, we can look at the full listing.

>     # grades.awk -- average student grades and determine
>     # letter grade as well as class averages.
>     # $1 = student name; $2 - $NF = test scores.

>     # set output field separator to tab.
>     BEGIN { OFS = "\t" }

>     # action applied to all input lines
>     {
>       # add up grades
>     	total = 0
>     	for (i = 2; i <= NF; ++i)
>     		total += $i
>       # calculate average
>     	avg = total / (NF - 1)
>       # assign student's average to element of array
>     	student_avg[NR] = avg
>       # determine letter grade
>     	if (avg >= 90)  grade = "A"
>     	else if (avg >= 80) grade = "B"
>     	else if (avg >= 70) grade = "C"
>     	else if (avg >= 60) grade = "D"
>     	else grade = "F"
>       # increment counter for letter grade array
>     	++class_grade[grade]
>       # print student name, average and letter grade
>     	print $1, avg, grade
>     }
>     # print out class statistics
>     END {
>       # calculate class average
>     	for (x = 1; x <= NR; x++)
>     		class_avg_total += student_avg[x]
>     	class_average = class_avg_total / NR
>       # determine how many above/below average
>     	for (x = 1; x <= NR; x++)
>     		if (student_avg[x] >= class_average)
>     			++above_average
>     		else
>     			++below_average
>       # print results
>     	print ""
>     	print "Class Average: ", class_average
>     	print "At or Above Average: ", above_average
>     	print "Below Average: ", below_average
>       # print number of students per letter grade
>     	for (letter_grade in class_grade)
>     		print letter_grade ":", class_grade[letter_grade] | "sort"
>     }

Here's a sample run:

>     $ cat grades.test
>     mona 70 77 85 83 70 89
>     john 85 92 78 94 88 91
>     andrea 89 90 85 94 90 95
>     jasper 84 88 80 92 84 82
>     dunce 64 80 60 60 61 62
>     ellis 90 98 89 96 96 92
>     $ awk -f grades.awk grades.test
>     mona    79      C
>     john    88      B
>     andrea  90.5    A
>     jasper  85      B
>     dunce   64.5    D
>     ellis   93.5    A

>     Class Average:  83.4167
>     At or Above Average:    4
>     Below Average:  2
>     A:      2
>     B:      2
>     C:      1
>     D:      1

### 8.4.2 Testing for Membership in an Array

The keyword `in` is also an operator that can be used in a conditional expression to test that a subscript is a member of an array. The expression:

>     item in array

returns 1 if `array[item]` exists and 0 if it does not. For example, the following conditional statement is true if the string "BASIC" is a subscript of the array `acro`.

>     if ( "BASIC" in acro )
>     	print "Found BASIC"

This is true if "BASIC" is a subscript used to access an element of `acro`. This syntax cannot tell you whether "BASIC" is the value of an element of `acro`. This expression is the same as writing a loop to check that such a subscript exists, although the above expression is much easier to write, and much more efficient to execute.

### 8.4.3 A Glossary Lookup Script

This program reads a series of glossary entries from a file named glossary and puts them into an array. The user is prompted to enter a glossary term and if it is found, the definition of the term is printed.

Here's the `lookup` program:

>     awk '# lookup -- reads local glossary file and prompts user for query

>     #0
>     BEGIN { FS = "\t"; OFS = "\t"
>     	# prompt user
>     	printf("Enter a glossary term: ")
>     }

>     #1 read local file named glossary
>     FILENAME == "glossary" {
>     	# load each glossary entry into an array
>     	entry[$1] = $2
>     	next
>     }

>     #2 scan for command to exit program
>     $0 ~ /^(quit|[qQ]|exit|[Xx])$/ { exit }

>     #3 process any non-empty line
>     $0 != "" {
>     	if ( $0 in entry ) {
>     		# it is there, print definition
>     		print entry[$0]
>     	} else
>     		print $0 " not found"
>     }

>     #4 prompt user again for another term
>     {
>     	printf("Enter another glossary term (q to quit): ")
>     }' glossary -

The pattern-matching rules are numbered to make this discussion easier. As we look at the individual rules, we'll discuss them in the order in which they are encountered in the flow of the script. Rule #0 is the `BEGIN` rule, which is performed only once before any input is read. It sets `FS` and `OFS` to a tab and then prompts the user to enter a glossary item. The response will come from standard input, but that is read after the glossary file.

Rule #1 tests to see if the current filename (the value of `FILENAME`) is "glossary" and is therefore only applied while reading input from this file. This rule loads the glossary entries into an array:

>     entry[term] = definition

where $1 is the term and $2 is the definition. The `next` statement at the end of rule #1 is used to skip other rules in the script and causes a new line of input to be read. So, until all the entries in the glossary file are read, no other rule is evaluated.

Once input from glossary is exhausted, awk reads from standard input because "-" is specified on the command line. Standard input is where the user's response comes from. Rule #3 tests that the input line ($0) is not empty. This rule should match whatever the user types. The action uses `in` to see if the input line is an index in the array. If it is, it simply prints out the corresponding value. Otherwise, we tell the user that no valid entry was found.

After rule #3, rule #4 will be evaluated. This rule simply prompts the user for another entry. Note that regardless of whether a valid entry was processed in rule #3, rule #4 is executed. The prompt also tells the user how to quit the program. After this rule, awk looks for the next line of input.

If the user chooses to quit by entering "q" as the next line of input, rule #2 will be matched. The pattern looks for a complete line consisting of alternative words or single letters that the user might enter to quit. The "^" and "$" are important, signifying that the input line contains no other characters but these; otherwise a "q" appearing in a glossary entry would be matched. Note that the placement of this rule in the sequence of rules is significant. It must appear before rules #3 and #4 because these rules will match anything, including the words "quit" and "exit."

Let's look at how the program works. For this example, we will make a copy of the acronyms file and use it as the glossary file.

>     $ cp acronyms glossary
>     $ lookup
>     Enter a glossary term: GIGO
>     Garbage in, garbage out
>     Enter another glossary term (q to quit): BASIC
>     Beginner's All-Purpose Symbolic Instruction Code
>     Enter another glossary term (q to quit): q

As you can see, the program is set up to prompt the user for additional items until the user enters "q".

Note that this program can be easily revised to read a glossary anywhere on the file system, including the user's home directory. The shell script that invokes awk could handle command-line options that allow the user to specify the glossary filename. You could also read a shared glossary file and then read a local one by writing separate rules to process the entries.

### 8.4.4 Using split() to Create Arrays

The built-in function `split()` can parse any string into elements of an array. This function can be useful to extract "subfields" from a field. The syntax of the `split()` function is:

> `n = split`(`string`, `array`, `separator`)

string is the input string to be parsed into elements of the named array. The array's indices start at 1 and go to `n`, the number of elements in the array. The elements will be split based on the specified separator character. If a separator is not specified, then the field separator (`FS`) is used. The separator can be a full regular expression, not just a single character. Array splitting behaves identically to field splitting; see the section "Referencing and Separating Fields" in Chapter 7.

For example, if you had a record in which the first field consisted of the person's full name, you could use the `split()` function to extract the person's first and last names. The following statement breaks up the first field into elements of the array `fullname`:

>     z = split($1, fullname, " ")

A space is specified as the delimiter. The person's first name can be referenced as:

>     fullname[1]

and the person's last name can be referenced as:

>     fullname[z]

because `z` contains the number of elements in the array. This works, regardless of whether the person's full name contains a middle name. If `z` is the value returned by `split()`, you can write a loop to read all the elements of this array.

>     z = split($1, array, " ")
>     for (i = 1; i <= z; ++i)
>     	print i, array[i]

The next section contains additional examples of using the `split()` function.

### 8.4.5 Making Conversions

This section looks at two examples that demonstrate similar methods of converting output from one format to another.

When working on the index program shown in Chapter 12, Full-Featured Applications, we needed a quick way to assign roman numerals to volume numbers. In other words, volume 4 needed to be identified as "IV" in the index. Since there was no immediate prospect of the number of volumes exceeding 10, we wrote a script that took as input a number between 1 and 10 and converted it to a roman numeral.

This shell script takes the first argument from the command line and echoes it as input to the awk program.

>     echo $1 |
>     awk '# romanum -- convert number 1-10 to roman numeral

>     # define numerals as list of roman numerals 1-10
>     BEGIN {
>     	# create array named numerals from list of roman numerals
>     	split("I,II,III,IV,V,VI,VII,VIII,IX,X", numerals, ",")
>     }

>     # look for number between 1 and 10
>     $1 > 0 && $1 <= 10 {
>     	# print specified element
>     	print numerals[$1]
>     	exit
>     }

>     { 	print "invalid number"
>       	exit
>     }'

This script defines a list of 10 roman numerals, then uses `split()` to load them into an array named `numerals`. This is done in the `BEGIN` action because it only needs to be done once.

The second rule checks that the first field of the input line contains a number between 1 and 10. If it does, this number is used as the index to the `numerals` array, retrieving the corresponding element. The `exit` statement terminates the program. The last rule is executed only if there is no valid entry.

Here's an example of how it works:

>     $ romanum 4
>     IV

Following along on the same idea, here's a script that converts dates in the form "mm-dd-yy" or "mm/dd/yy" to "month day, year."

>     awk '
>     # date-month -- convert mm/dd/yy or mm-dd-yy to month day, year

>     # build list of months and put in array.
>     BEGIN {
>     	# the 3-step assignment is done for printing in book
>     	listmonths = "January,February,March,April,May,June,"
>     	listmonths = listmonths "July,August,September,"
>     	listmonths = listmonths "October,November,December"
>     	split(listmonths, month, ",")
>     }

>     # check that there is input
>     $1 != "" {

>     # split on "/" the first input field into elements of array
>     	sizeOfArray = split($1, date, "/")

>     # check that only one field is returned
>     	if (sizeOfArray == 1)
>     		# try to split on "-"
>     		sizeOfArray = split($1, date, "-")

>     # must be invalid
>     	if (sizeOfArray == 1)
>     		exit

>     # add 0 to number of month to coerce numeric type
>     	date[1] += 0

>     # print month day, year
>     	print month[date[1]], (date[2] ", 19" date[3])
>     }'

This script reads from standard input. The `BEGIN` action creates an array named `month` whose elements are the names of the months of the year. The second rule verifies that we have a non-empty input line. The first statement in the associated action splits the first field of input looking for "/" as the delimiter. `sizeOfArray` contains the number of elements in the array. If awk was unable to parse the string, it creates the array with only one element. Thus, we can test the value of `sizeOfArray` to determine if we have several elements. If we do not, we assume that perhaps "-" was used as the delimiter. If that fails to produce an array with multiple elements, we assume the input is invalid, and exit. If we have successfully parsed the input, `date[1]` contains the number of the month. This value can be used as the index to the array `month`, nesting one array inside another. However, before using `date[1]`, we coerce the type of `date[1]` by adding 0 to it. While awk will correctly interpret "11" as a number, leading zeros may cause a number to be treated as a string. Thus, "06" might not be recognized properly without type coercion. The element referenced by `date[1]` is used as the subscript for `month`.

Here's a sample run:

>     $ echo "5/11/55" | date-month
>     May 11, 1955

### 8.4.6 Deleting Elements of an Array

Awk provides a statement for deleting an element of an array. The syntax is:

> `delete` `array`[`subscript`]

The brackets are required. This statement removes the element indexed by subscript from array. In particular, the `in` test for subscript will now return false. This is different than just assigning the empty string to that element; in that case `in` would still be true. See the `lotto` script in the next chapter for an example of using the `delete` statement.

8.3 Other Statements That Affect Flow Control |  | 8.5 An Acronym Processor