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8.2 The delete Statement

To remove an individual element of an array, use the delete statement:

     delete array[index-expression]

Once an array element has been deleted, any value the element once had is no longer available. It is as if the element had never been referred to or been given a value. The following is an example of deleting elements in an array:

     for (i in frequencies)
       delete frequencies[i]

This example removes all the elements from the array frequencies. Once an element is deleted, a subsequent for statement to scan the array does not report that element and the in operator to check for the presence of that element returns zero (i.e., false):

     delete foo[4]
     if (4 in foo)
         print "This will never be printed"

It is important to note that deleting an element is not the same as assigning it a null value (the empty string, ""). For example:

     foo[4] = ""
     if (4 in foo)
       print "This is printed, even though foo[4] is empty"

It is not an error to delete an element that does not exist. However, if --lint is provided on the command line (see Options), gawk issues a warning message when an element that is not in the array is deleted.

All the elements of an array may be deleted with a single statement by leaving off the subscript in the delete statement, as follows:

     delete array

Using this version of the delete statement is about three times more efficient than the equivalent loop that deletes each element one at a time.

NOTE: For many years, using delete without a subscript was a gawk extension. As of September, 2012, it was accepted for inclusion into the POSIX standard. See the Austin Group website. This form of the delete statement is also supported by Brian Kernighan's awk and mawk, as well as by a number of other implementations (see Other Versions).

The following statement provides a portable but nonobvious way to clear out an array:1

     split("", array)

The split() function (see String Functions) clears out the target array first. This call asks it to split apart the null string. Because there is no data to split out, the function simply clears the array and then returns.

CAUTION: Deleting an array does not change its type; you cannot delete an array and then use the array's name as a scalar (i.e., a regular variable). For example, the following does not work:
     a[1] = 3
     delete a
     a = 3

Footnotes

[1] Thanks to Michael Brennan for pointing this out.