GAWK: Effective AWK Programming

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122 GAWK: Effective AWK Programminginput has been read, to print out all the lines. When this program is run with the followinginput:5 I am the Five man2 Who are you? The new number two!4 . . . And four on the floor1 Who is number one?3 I three you.Its output is:1 Who is number one?2 Who are you? The new number two!3 I three you.4 . . . And four on the floor5 I am the Five manIf a line number is repeated, the last line with a given number overrides the others. Gapsin the line numbers can be handled with an easy improvement to the program’s END rule,as follows:END {for (x = 1; x
Chapter 7: Arrays in awk 123}# Find number of distinct words more than 10 characters longEND {for (x in used) {if (length(x) > 10) {++num_long_wordsprint x}}print num_long_words, "words longer than 10 characters"}See Section 13.3.5 [Generating Word-Usage Counts], page 242, for a more detailed exampleof this type.The order in which elements of the array are accessed by this statement is determinedby the internal arrangement of the array elements within awk and cannot be controlled orchanged. This can lead to problems if new elements are added to array by statements in theloop body; it is not predictable whether the for loop will reach them. Similarly, changingvar inside the loop may produce strange results. It is best to avoid such things.7.6 The delete StatementTo 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 longeravailable. It is as if the element had never been referred to or had been given a value. Thefollowing 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 isdeleted, a subsequent for statement to scan the array does not report that element and thein 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 nullvalue (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. If ‘--lint’ is provided on thecommand line (see Section 11.2 [Command-Line Options], page 177), gawk issues a warningmessage 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 thesubscript in the delete statement, as follows:
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GAWK: Effective AWK Programming

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