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Arrays
Arrays in awk
An array is a table of values, called elements. The
elements of an array are distinguished by their indices. Indices
may be either numbers or strings. awk maintains a single set
of names that may be used for naming variables, arrays and functions
(see section User-defined Functions).
Thus, you cannot have a variable and an array with the same name in the
same awk program.
Introduction to Arrays
The awk language provides one-dimensional arrays for storing groups
of related strings or numbers.
Every awk array must have a name. Array names have the same
syntax as variable names; any valid variable name would also be a valid
array name. But you cannot use one name in both ways (as an array and
as a variable) in one awk program.
Arrays in awk superficially resemble arrays in other programming
languages; but there are fundamental differences. In awk, you
don't need to specify the size of an array before you start to use it.
Additionally, any number or string in awk may be used as an
array index, not just consecutive integers.
In most other languages, you have to declare an array and specify how many elements or components it contains. In such languages, the declaration causes a contiguous block of memory to be allocated for that many elements. An index in the array usually must be a positive integer; for example, the index zero specifies the first element in the array, which is actually stored at the beginning of the block of memory. Index one specifies the second element, which is stored in memory right after the first element, and so on. It is impossible to add more elements to the array, because it has room for only as many elements as you declared. (Some languages allow arbitrary starting and ending indices, e.g., `15 .. 27', but the size of the array is still fixed when the array is declared.)
A contiguous array of four elements might look like this,
conceptually, if the element values are eight, "foo",
"" and 30:
Only the values are stored; the indices are implicit from the order of the values. Eight is the value at index zero, because eight appears in the position with zero elements before it.
Arrays in awk are different: they are associative. This means
that each array is a collection of pairs: an index, and its corresponding
array element value:
Element 4 Value 30 Element 2 Value "foo" Element 1 Value 8 Element 3 Value ""
We have shown the pairs in jumbled order because their order is irrelevant.
One advantage of associative arrays is that new pairs can be added
at any time. For example, suppose we add to the above array a tenth element
whose value is "number ten". The result is this:
Element 10 Value "number ten" Element 4 Value 30 Element 2 Value "foo" Element 1 Value 8 Element 3 Value ""
Now the array is sparse, which just means some indices are missing: it has elements 1--4 and 10, but doesn't have elements 5, 6, 7, 8, or 9.
Another consequence of associative arrays is that the indices don't have to be positive integers. Any number, or even a string, can be an index. For example, here is an array which translates words from English into French:
Element "dog" Value "chien" Element "cat" Value "chat" Element "one" Value "un" Element 1 Value "un"
Here we decided to translate the number one in both spelled-out and numeric form--thus illustrating that a single array can have both numbers and strings as indices. (In fact, array subscripts are always strings; this is discussed in more detail in section Using Numbers to Subscript Arrays.)
When awk creates an array for you, e.g., with the split
built-in function,
that array's indices are consecutive integers starting at one.
(See section Built-in Functions for String Manipulation.)
Referring to an Array Element
The principal way of using an array is to refer to one of its elements. An array reference is an expression which looks like this:
array[index]
Here, array is the name of an array. The expression index is the index of the element of the array that you want.
The value of the array reference is the current value of that array
element. For example, foo[4.3] is an expression for the element
of array foo at index `4.3'.
If you refer to an array element that has no recorded value, the value
of the reference is "", the null string. This includes elements
to which you have not assigned any value, and elements that have been
deleted (see section The delete Statement). Such a reference
automatically creates that array element, with the null string as its value.
(In some cases, this is unfortunate, because it might waste memory inside
awk.)
You can find out if an element exists in an array at a certain index with the expression:
index in array
This expression tests whether or not the particular index exists,
without the side effect of creating that element if it is not present.
The expression has the value one (true) if array[index]
exists, and zero (false) if it does not exist.
For example, to test whether the array frequencies contains the
index `2', you could write this statement:
if (2 in frequencies)
print "Subscript 2 is present."
Note that this is not a test of whether or not the array
frequencies contains an element whose value is two.
(There is no way to do that except to scan all the elements.) Also, this
does not create frequencies[2], while the following
(incorrect) alternative would do so:
if (frequencies[2] != "")
print "Subscript 2 is present."
Assigning Array Elements
Array elements are lvalues: they can be assigned values just like
awk variables:
array[subscript] = value
Here array is the name of your array. The expression subscript is the index of the element of the array that you want to assign a value. The expression value is the value you are assigning to that element of the array.
Basic Array Example
The following program takes a list of lines, each beginning with a line number, and prints them out in order of line number. The line numbers are not in order, however, when they are first read: they are scrambled. This program sorts the lines by making an array using the line numbers as subscripts. It then prints out the lines in sorted order of their numbers. It is a very simple program, and gets confused if it encounters repeated numbers, gaps, or lines that don't begin with a number.
{
if ($1 > max)
max = $1
arr[$1] = $0
}
END {
for (x = 1; x <= max; x++)
print arr[x]
}
The first rule keeps track of the largest line number seen so far;
it also stores each line into the array arr, at an index that
is the line's number.
The second rule runs after all the input has been read, to print out all the lines.
When this program is run with the following input:
5 I am the Five man 2 Who are you? The new number two! 4 . . . And four on the floor 1 Who is number one? 3 I three you.
its output is this:
1 Who is number one? 2 Who are you? The new number two! 3 I three you. 4 . . . And four on the floor 5 I am the Five man
If a line number is repeated, the last line with a given number overrides the others.
Gaps in the line numbers can be handled with an easy improvement to the
program's END rule:
END {
for (x = 1; x <= max; x++)
if (x in arr)
print arr[x]
}
Scanning All Elements of an Array
In programs that use arrays, you often need a loop that executes
once for each element of an array. In other languages, where arrays are
contiguous and indices are limited to positive integers, this is
easy: you can
find all the valid indices by counting from the lowest index
up to the highest. This
technique won't do the job in awk, since any number or string
can be an array index. So awk has a special kind of for
statement for scanning an array:
for (var in array) body
This loop executes body once for each index in array that your program has previously used, with the variable var set to that index.
Here is a program that uses this form of the for statement. The
first rule scans the input records and notes which words appear (at
least once) in the input, by storing a one into the array used with
the word as index. The second rule scans the elements of used to
find all the distinct words that appear in the input. It prints each
word that is more than 10 characters long, and also prints the number of
such words. See section Built-in Functions for String Manipulation, for more information
on the built-in function length.
# Record a 1 for each word that is used at least once.
{
for (i = 1; i <= NF; i++)
used[$i] = 1
}
# Find number of distinct words more than 10 characters long.
END {
for (x in used)
if (length(x) > 10) {
++num_long_words
print x
}
print num_long_words, "words longer than 10 characters"
}
See section Generating Word Usage Counts, for a more detailed example of this type.
The order in which elements of the array are accessed by this statement
is determined by the internal arrangement of the array elements within
awk and cannot be controlled or changed. This can lead to
problems if new elements are added to array by statements in
the loop body; you cannot predict whether or not the for loop will
reach them. Similarly, changing var inside the loop may produce
strange results. It is best to avoid such things.
The delete Statement
You can remove an individual element of an array using the delete
statement:
delete array[index]
Once you have deleted an array element, you can no longer obtain any value the element once had. It is as if you had never referred to it and had never given it any value.
Here is an example of deleting elements in an array:
for (i in frequencies) delete frequencies[i]
This example removes all the elements from the array frequencies.
If you delete an element, a subsequent for statement to scan the array
will not report that element, and the in operator to check for
the presence of that element will return zero (i.e. false):
delete foo[4]
if (4 in foo)
print "This will never be printed"
It is important to note that deleting an element is not the
same as assigning it a null value (the empty string, "").
foo[4] = "" if (4 in foo) print "This is printed, even though foo[4] is empty"
It is not an error to delete an element that does not exist.
You can delete all the elements of an array with a single statement,
by leaving off the subscript in the delete statement.
delete array
This ability is a gawk extension; it is not available in
compatibility mode (see section Command Line Options).
Using this version of the delete statement is about three times
more efficient than the equivalent loop that deletes each element one
at a time.
The following statement provides a portable, but non-obvious way to clear out an array.
# thanks to Michael Brennan for pointing this out
split("", array)
The split function
(see section Built-in Functions for String Manipulation)
clears out the target array first. This call asks it to split
apart the null string. Since there is no data to split out, the
function simply clears the array and then returns.
Using Numbers to Subscript Arrays
An important aspect of arrays to remember is that array subscripts are always strings. If you use a numeric value as a subscript, it will be converted to a string value before it is used for subscripting (see section Conversion of Strings and Numbers).
This means that the value of the built-in variable CONVFMT can potentially
affect how your program accesses elements of an array. For example:
xyz = 12.153
data[xyz] = 1
CONVFMT = "%2.2f"
if (xyz in data)
printf "%s is in data\n", xyz
else
printf "%s is not in data\n", xyz
This prints `12.15 is not in data'. The first statement gives
xyz a numeric value. Assigning to
data[xyz] subscripts data with the string value "12.153"
(using the default conversion value of CONVFMT, "%.6g"),
and assigns one to data["12.153"]. The program then changes
the value of CONVFMT. The test `(xyz in data)' generates a new
string value from xyz, this time "12.15", since the value of
CONVFMT only allows two significant digits. This test fails,
since "12.15" is a different string from "12.153".
According to the rules for conversions
(see section Conversion of Strings and Numbers), integer
values are always converted to strings as integers, no matter what the
value of CONVFMT may happen to be. So the usual case of:
for (i = 1; i <= maxsub; i++)
do something with array[i]
will work, no matter what the value of CONVFMT.
Like many things in awk, the majority of the time things work
as you would expect them to work. But it is useful to have a precise
knowledge of the actual rules, since sometimes they can have a subtle
effect on your programs.
Using Uninitialized Variables as Subscripts
Suppose you want to print your input data in reverse order. A reasonable attempt at a program to do so (with some test data) might look like this:
$ echo 'line 1
> line 2
> line 3' | awk '{ l[lines] = $0; ++lines }
> END {
> for (i = lines-1; i >= 0; --i)
> print l[i]
> }'
-| line 3
-| line 2
Unfortunately, the very first line of input data did not come out in the output!
At first glance, this program should have worked. The variable lines
is uninitialized, and uninitialized variables have the numeric value zero.
So, the value of l[0] should have been printed.
The issue here is that subscripts for awk arrays are always
strings. And uninitialized variables, when used as strings, have the
value "", not zero. Thus, `line 1' ended up stored in
l[""].
The following version of the program works correctly:
{ l[lines++] = $0 }
END {
for (i = lines - 1; i >= 0; --i)
print l[i]
}
Here, the `++' forces l to be numeric, thus making
the "old value" numeric zero, which is then converted to "0"
as the array subscript.
As we have just seen, even though it is somewhat unusual, the null string
("") is a valid array subscript (d.c.). If `--lint' is provided
on the command line (see section Command Line Options),
gawk will warn about the use of the null string as a subscript.
Multi-dimensional Arrays
A multi-dimensional array is an array in which an element is identified
by a sequence of indices, instead of a single index. For example, a
two-dimensional array requires two indices. The usual way (in most
languages, including awk) to refer to an element of a
two-dimensional array named grid is with
grid[x,y].
Multi-dimensional arrays are supported in awk through
concatenation of indices into one string. What happens is that
awk converts the indices into strings
(see section Conversion of Strings and Numbers) and
concatenates them together, with a separator between them. This creates
a single string that describes the values of the separate indices. The
combined string is used as a single index into an ordinary,
one-dimensional array. The separator used is the value of the built-in
variable SUBSEP.
For example, suppose we evaluate the expression `foo[5,12] = "value"'
when the value of SUBSEP is "@". The numbers five and 12 are
converted to strings and
concatenated with an `@' between them, yielding "5@12"; thus,
the array element foo["5@12"] is set to "value".
Once the element's value is stored, awk has no record of whether
it was stored with a single index or a sequence of indices. The two
expressions `foo[5,12]' and `foo[5 SUBSEP 12]' are always
equivalent.
The default value of SUBSEP is the string "\034",
which contains a non-printing character that is unlikely to appear in an
awk program or in most input data.
The usefulness of choosing an unlikely character comes from the fact
that index values that contain a string matching SUBSEP lead to
combined strings that are ambiguous. Suppose that SUBSEP were
"@"; then `foo["a@b", "c"]' and `foo["a",
"b@c"]' would be indistinguishable because both would actually be
stored as `foo["a@b@c"]'.
You can test whether a particular index-sequence exists in a "multi-dimensional" array with the same operator `in' used for single dimensional arrays. Instead of a single index as the left-hand operand, write the whole sequence of indices, separated by commas, in parentheses:
(subscript1, subscript2, ...) in array
The following example treats its input as a two-dimensional array of fields; it rotates this array 90 degrees clockwise and prints the result. It assumes that all lines have the same number of elements.
awk '{
if (max_nf < NF)
max_nf = NF
max_nr = NR
for (x = 1; x <= NF; x++)
vector[x, NR] = $x
}
END {
for (x = 1; x <= max_nf; x++) {
for (y = max_nr; y >= 1; --y)
printf("%s ", vector[x, y])
printf("\n")
}
}'
When given the input:
1 2 3 4 5 6 2 3 4 5 6 1 3 4 5 6 1 2 4 5 6 1 2 3
it produces:
4 3 2 1 5 4 3 2 6 5 4 3 1 6 5 4 2 1 6 5 3 2 1 6
Scanning Multi-dimensional Arrays
There is no special for statement for scanning a
"multi-dimensional" array; there cannot be one, because in truth there
are no multi-dimensional arrays or elements; there is only a
multi-dimensional way of accessing an array.
However, if your program has an array that is always accessed as
multi-dimensional, you can get the effect of scanning it by combining
the scanning for statement
(see section Scanning All Elements of an Array) with the
split built-in function
(see section Built-in Functions for String Manipulation).
It works like this:
for (combined in array) {
split(combined, separate, SUBSEP)
...
}
This sets combined to
each concatenated, combined index in the array, and splits it
into the individual indices by breaking it apart where the value of
SUBSEP appears. The split-out indices become the elements of
the array separate.
Thus, suppose you have previously stored a value in array[1, "foo"];
then an element with index "1\034foo" exists in
array. (Recall that the default value of SUBSEP is
the character with code 034.) Sooner or later the for statement
will find that index and do an iteration with combined set to
"1\034foo". Then the split function is called as
follows:
split("1\034foo", separate, "\034")
The result of this is to set separate[1] to "1" and
separate[2] to "foo". Presto, the original sequence of
separate indices has been recovered.
