8.1.6 Using Predefined Array Scanning Orders with gawk

This subsection describes a feature that is specific to gawk.

By default, when a for loop traverses an array, the order is undefined, meaning that the awk implementation determines the order in which the array is traversed. This order is usually based on the internal implementation of arrays and will vary from one version of awk to the next.

Often, though, you may wish to do something simple, such as “traverse the array by comparing the indices in ascending order,” or “traverse the array by comparing the values in descending order.” gawk provides two mechanisms that give you this control:

  • Set PROCINFO["sorted_in"] to one of a set of predefined values. We describe this now.
  • Set PROCINFO["sorted_in"] to the name of a user-defined function to use for comparison of array elements. This advanced feature is described later in Controlling Array Traversal and Array Sorting.

The following special values for PROCINFO["sorted_in"] are available:

"@unsorted"

Array elements are processed in arbitrary order, which is the default awk behavior.

"@ind_str_asc"

Order by indices in ascending order compared as strings; this is the most basic sort. (Internally, array indices are always strings, so with ‘a[2*5] = 1’ the index is "10" rather than numeric 10.)

"@ind_num_asc"

Order by indices in ascending order but force them to be treated as numbers in the process. Any index with a non-numeric value will end up positioned as if it were zero.

"@val_type_asc"

Order by element values in ascending order (rather than by indices). Ordering is by the type assigned to the element (see section Variable Typing and Comparison Expressions). All numeric values come before all string values, which in turn come before all subarrays. (Subarrays have not been described yet; see section Arrays of Arrays.)

If you choose to use this feature in traversing FUNCTAB (see section Built-in Variables That Convey Information), then the order is built-in functions first (see section Built-in Functions), then user-defined functions (see section User-Defined Functions) next, and finally functions loaded from an extension (see section Writing Extensions for gawk).

"@val_str_asc"

Order by element values in ascending order (rather than by indices). Scalar values are compared as strings. If the string values are identical, the index string values are compared instead. When comparing non-scalar values, "@val_type_asc" sort ordering is used, so subarrays, if present, come out last.

"@val_num_asc"

Order by element values in ascending order (rather than by indices). Scalar values are compared as numbers. Non-scalar values are compared using "@val_type_asc" sort ordering, so subarrays, if present, come out last. When numeric values are equal, the string values are used to provide an ordering: this guarantees consistent results across different versions of the C qsort() function,42 which gawk uses internally to perform the sorting. If the string values are also identical, the index string values are compared instead.

"@ind_str_desc"

Like "@ind_str_asc", but the string indices are ordered from high to low.

"@ind_num_desc"

Like "@ind_num_asc", but the numeric indices are ordered from high to low.

"@val_type_desc"

Like "@val_type_asc", but the element values, based on type, are ordered from high to low. Subarrays, if present, come out first.

"@val_str_desc"

Like "@val_str_asc", but the element values, treated as strings, are ordered from high to low. If the string values are identical, the index string values are compared instead. When comparing non-scalar values, "@val_type_desc" sort ordering is used, so subarrays, if present, come out first.

"@val_num_desc"

Like "@val_num_asc", but the element values, treated as numbers, are ordered from high to low. If the numeric values are equal, the string values are compared instead. If they are also identical, the index string values are compared instead. Non-scalar values are compared using "@val_type_desc" sort ordering, so subarrays, if present, come out first.

The array traversal order is determined before the for loop starts to run. Changing PROCINFO["sorted_in"] in the loop body does not affect the loop. For example:

$ gawk '
> BEGIN {
>    a[4] = 4
>    a[3] = 3
>    for (i in a)
>        print i, a[i]
> }'
-| 4 4
-| 3 3
$ gawk '
> BEGIN {
>    PROCINFO["sorted_in"] = "@ind_str_asc"
>    a[4] = 4
>    a[3] = 3
>    for (i in a)
>        print i, a[i]
> }'
-| 3 3
-| 4 4

When sorting an array by element values, if a value happens to be a subarray then it is considered to be greater than any string or numeric value, regardless of what the subarray itself contains, and all subarrays are treated as being equal to each other. Their order relative to each other is determined by their index strings.

Here are some additional things to bear in mind about sorted array traversal:

  • The value of PROCINFO["sorted_in"] is global. That is, it affects all array traversal for loops. If you need to change it within your own code, you should see if it’s defined and save and restore the value:
    …
    if ("sorted_in" in PROCINFO) {
        save_sorted = PROCINFO["sorted_in"]
        PROCINFO["sorted_in"] = "@val_str_desc" # or whatever
    }
    …
    if (save_sorted)
        PROCINFO["sorted_in"] = save_sorted
  • As already mentioned, the default array traversal order is represented by "@unsorted". You can also get the default behavior by assigning the null string to PROCINFO["sorted_in"] or by just deleting the "sorted_in" element from the PROCINFO array with the delete statement. (The delete statement hasn’t been described yet; see section The delete Statement.)

In addition, gawk provides built-in functions for sorting arrays; see Sorting Array Values and Indices with gawk.

Footnotes

(42)

When two elements compare as equal, the C qsort() function does not guarantee that they will maintain their original relative order after sorting. Using the string value to provide a unique ordering when the numeric values are equal ensures that gawk behaves consistently across different environments.