File: gawk.info,  Node: Array Intro,  Next: Reference to Elements,  Up: Array Basics

8.1.1 Introduction to Arrays
----------------------------

     Doing linear scans over an associative array is like trying to club
     someone to death with a loaded Uzi.
                            -- _Larry Wall_

   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, 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 nonnegative 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 *note Figure 8.1:
figure-array-elements, conceptually, if the element values are eight,
'"foo"', '""', and 30.


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

Figure 8.1: A contiguous array

Only the values are stored; the indices are implicit from the order of
the values.  Here, 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:

        Index   Value
------------------------
        '3'     '30'
        '1'     '"foo"'
        '0'     '8'
        '2'     '""'

The pairs are shown in jumbled order because their order is
irrelevant.(1)

   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:

        Index   Value
-------------------------------
        '10'    '"number
                ten"'
        '3'     '30'
        '1'     '"foo"'
        '0'     '8'
        '2'     '""'

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 nonnegative integers.  Any number, or even a string, can be
an index.  For example, the following is an array that translates words
from English to French:

        Index   Value
------------------------
        '"dog"' '"chien"'
        '"cat"' '"chat"'
        '"one"' '"un"'
        '1'     '"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.  There are some subtleties to how numbers work when used as
array subscripts; this is discussed in more detail in *note Numeric
Array Subscripts::.)  Here, the number '1' isn't double-quoted, because
'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 Functions::.)

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

   ---------- Footnotes ----------

   (1) The ordering will vary among 'awk' implementations, which
typically use hash tables to store array elements and values.