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12.2.2 Sorting Array Values and Indices with gawk

In most awk implementations, sorting an array requires writing a sort() function. While this can be educational for exploring different sorting algorithms, usually that's not the point of the program. gawk provides the built-in asort() and asorti() functions (see String Functions) for sorting arrays. For example:

     populate the array data
     n = asort(data)
     for (i = 1; i <= n; i++)
         do something with data[i]

After the call to asort(), the array data is indexed from 1 to some number n, the total number of elements in data. (This count is asort()'s return value.) data[1] <= data[2] <= data[3], and so on. The comparison is based on the type of the elements (see Typing and Comparison). All numeric values come before all string values, which in turn come before all subarrays.

An important side effect of calling asort() is that the array's original indices are irrevocably lost. As this isn't always desirable, asort() accepts a second argument:

     populate the array source
     n = asort(source, dest)
     for (i = 1; i <= n; i++)
         do something with dest[i]

In this case, gawk copies the source array into the dest array and then sorts dest, destroying its indices. However, the source array is not affected.

asort() accepts a third string argument to control comparison of array elements. As with PROCINFO["sorted_in"], this argument may be one of the predefined names that gawk provides (see Controlling Scanning), or the name of a user-defined function (see Controlling Array Traversal).

NOTE: In all cases, the sorted element values consist of the original array's element values. The ability to control comparison merely affects the way in which they are sorted.

Often, what's needed is to sort on the values of the indices instead of the values of the elements. To do that, use the asorti() function. The interface is identical to that of asort(), except that the index values are used for sorting, and become the values of the result array:

     { source[$0] = some_func($0) }
     
     END {
         n = asorti(source, dest)
         for (i = 1; i <= n; i++) {
             Work with sorted indices directly:
             do something with dest[i]
             ...
             Access original array via sorted indices:
             do something with source[dest[i]]
         }
     }

Similar to asort(), in all cases, the sorted element values consist of the original array's indices. The ability to control comparison merely affects the way in which they are sorted.

Sorting the array by replacing the indices provides maximal flexibility. To traverse the elements in decreasing order, use a loop that goes from n down to 1, either over the elements or over the indices.1

Copying array indices and elements isn't expensive in terms of memory. Internally, gawk maintains reference counts to data. For example, when asort() copies the first array to the second one, there is only one copy of the original array elements' data, even though both arrays use the values.

Because IGNORECASE affects string comparisons, the value of IGNORECASE also affects sorting for both asort() and asorti(). Note also that the locale's sorting order does not come into play; comparisons are based on character values only.2 Caveat Emptor.


Footnotes

[1] You may also use one of the predefined sorting names that sorts in decreasing order.

[2] This is true because locale-based comparison occurs only when in POSIX compatibility mode, and since asort() and asorti() are gawk extensions, they are not available in that case.