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(gawk.info)Array Intro

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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 one name cannot be
used in both ways (as an array and as a variable) in the same `awk'
program.

   Arrays in `awk' superficially resemble arrays in other programming
languages, but there are fundamental differences.  In `awk', it isn't
necessary to specify the size of an array before starting to use it.
Additionally, any number or string in `awk', not just consecutive
integers, may be used as an array index.

   In most other languages, arrays must be "declared" before use,
including a specification of how many elements or components they
contain.  In such languages, the declaration causes a contiguous block
of memory to be allocated for that many elements.  Usually, an index in
the array 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 only for as many elements as given in the declaration.  (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 the following
example, conceptually, if the element values are 8, `"foo"', `""', and
30:

     +---------+---------+--------+---------+
     |    8    |  "foo"  |   ""   |    30   |    Value
     +---------+---------+--------+---------+
          0         1         2         3        Index

Only the values are stored; the indices are implicit from the order of
the values.  8 is the value at index zero, because 8 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 3     Value 30
     Element 1     Value "foo"
     Element 0     Value 8
     Element 2     Value ""

The pairs are shown 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 a tenth element is added to the array
whose value is `"number ten"'.  The result is:

     Element 10    Value "number ten"
     Element 3     Value 30
     Element 1     Value "foo"
     Element 0     Value 8
     Element 2     Value ""

Now the array is "sparse", which just means some indices are missing.
It has elements 0-3 and 10, but doesn't have elements 4, 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, the following is an array that 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 Note: Using Numbers to
Subscript Arrays.  Here, the number `1' isn't
double-quoted, since `awk' automatically converts it to a string.

   The value of `IGNORECASE' has no effect upon array subscripting.
The identical string value used to store an array element must be used
to retrieve it.  When `awk' creates an array (e.g., with the `split'
built-in function), that array's indices are consecutive integers
starting at one.  (Note: String Manipulation Functions.
)

   `awk''s arrays are efficient--the time to access an element is
independent of the number of elements in the array.