File: gawk.info,  Node: Scanning an Array,  Next: Controlling Scanning,  Prev: Array Example,  Up: Array Basics

8.1.5 Scanning All Elements of an Array
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In programs that use arrays, it is often necessary to use a loop that
executes once for each element of an array.  In other languages, where
arrays are contiguous and indices are limited to nonnegative integers,
this is easy: all the valid indices can be found by counting from the
lowest index up to the highest.  This technique won't do the job in
'awk', because any number or string can be an array index.  So 'awk' has
a special kind of 'for' statement for scanning an array:

     for (VAR in ARRAY)
         BODY

This loop executes BODY once for each index in ARRAY that the program
has previously used, with the variable VAR set to that index.

   The following program uses this form of the 'for' statement.  The
first rule scans the input records and notes which words appear (at
least once) in the input, by storing a one into the array 'used' with
the word as the index.  The second rule scans the elements of 'used' to
find all the distinct words that appear in the input.  It prints each
word that is more than 10 characters long and also prints the number of
such words.  *Note String Functions:: for more information on the
built-in function 'length()'.

     # Record a 1 for each word that is used at least once
     {
         for (i = 1; i <= NF; i++)
             used[$i] = 1
     }

     # Find number of distinct words more than 10 characters long
     END {
         for (x in used) {
             if (length(x) > 10) {
                 ++num_long_words
                 print x
             }
         }
         print num_long_words, "words longer than 10 characters"
     }

*Note Word Sorting:: for a more detailed example of this type.

   The order in which elements of the array are accessed by this
statement is determined by the internal arrangement of the array
elements within 'awk' and in standard 'awk' cannot be controlled or
changed.  This can lead to problems if new elements are added to ARRAY
by statements in the loop body; it is not predictable whether the 'for'
loop will reach them.  Similarly, changing VAR inside the loop may
produce strange results.  It is best to avoid such things.

   As a point of information, 'gawk' sets up the list of elements to be
iterated over before the loop starts, and does not change it.  But not
all 'awk' versions do so.  Consider this program, named 'loopcheck.awk':

     BEGIN {
         a["here"] = "here"
         a["is"] = "is"
         a["a"] = "a"
         a["loop"] = "loop"
         for (i in a) {
             j++
             a[j] = j
             print i
         }
     }

   Here is what happens when run with 'gawk' (and 'mawk'):

     $ gawk -f loopcheck.awk
     -| here
     -| loop
     -| a
     -| is

   Contrast this to BWK 'awk':

     $ nawk -f loopcheck.awk
     -| loop
     -| here
     -| is
     -| a
     -| 1