(gawk) Scanning an Array

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 Scanning All Elements of an Array
 =================================
 
    In programs that use arrays, you often need a loop that executes
 once for each element of an array.  In other languages, where arrays are
 contiguous and indices are limited to positive integers, this is easy:
 you can find all the valid indices by counting from the lowest index up
 to the highest.  This technique won't do the job in `awk', since 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 your program
 has previously used, with the variable VAR set to that index.
 
    Here is a program that 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 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.   Built-in Functions for String Manipulation 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"
      }
 
  Generating Word Usage Counts 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 cannot be controlled or changed.  This can
 lead to problems if new elements are added to ARRAY by statements in
 the loop body; you cannot predict whether or not the `for' loop will
 reach them.  Similarly, changing VAR inside the loop may produce
 strange results.  It is best to avoid such things.
 

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