(gawk.info)Array Intro

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:

     +---------+---------+--------+---------+
     |    8    |  "foo"  |   ""   |    30   |    value
     +---------+---------+--------+---------+
          0         1         2         3        index

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 Note: Using Numbers to
Subscript Arrays.)

   The value of `IGNORECASE' has no effect upon array subscripting.
You must use the exact same string value to retrieve an array element
as you used to store it.

   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.
(Note: Built-in Functions for String Manipulation.)