File: gawk.info,  Node: Programs Exercises,  Prev: Programs Summary,  Up: Sample Programs

11.5 Exercises
==============

  1. Rewrite 'cut.awk' (*note Cut Program::) using 'split()' with '""'
     as the separator.

  2. In *note Egrep Program::, we mentioned that 'egrep -i' could be
     simulated in versions of 'awk' without 'IGNORECASE' by using
     'tolower()' on the line and the pattern.  In a footnote there, we
     also mentioned that this solution has a bug: the translated line is
     output, and not the original one.  Fix this problem.

  3. The POSIX version of 'id' takes options that control which
     information is printed.  Modify the 'awk' version (*note Id
     Program::) to accept the same arguments and perform in the same
     way.

  4. The 'split.awk' program (*note Split Program::) assumes that
     letters are contiguous in the character set, which isn't true for
     EBCDIC systems.  Fix this problem.  (Hint: Consider a different way
     to work through the alphabet, without relying on 'ord()' and
     'chr()'.)

  5. In 'uniq.awk' (*note Uniq Program::, the logic for choosing which
     lines to print represents a "state machine", which is "a device
     which can be in one of a set number of stable conditions depending
     on its previous condition and on the present values of its
     inputs."(1)  Brian Kernighan suggests that "an alternative approach
     to state machines is to just read the input into an array, then use
     indexing.  It's almost always easier code, and for most inputs
     where you would use this, just as fast."  Rewrite the logic to
     follow this suggestion.

  6. Why can't the 'wc.awk' program (*note Wc Program::) just use the
     value of 'FNR' in 'endfile()'?  Hint: Examine the code in *note
     Filetrans Function::.

  7. Manipulation of individual characters in the 'translate' program
     (*note Translate Program::) is painful using standard 'awk'
     functions.  Given that 'gawk' can split strings into individual
     characters using '""' as the separator, how might you use this
     feature to simplify the program?

  8. The 'extract.awk' program (*note Extract Program::) was written
     before 'gawk' had the 'gensub()' function.  Use it to simplify the
     code.

  9. Compare the performance of the 'awksed.awk' program (*note Simple
     Sed::) with the more straightforward:

          BEGIN {
              pat = ARGV[1]
              repl = ARGV[2]
              ARGV[1] = ARGV[2] = ""
          }

          { gsub(pat, repl); print }

  10. What are the advantages and disadvantages of 'awksed.awk' versus
     the real 'sed' utility?

  11. In *note Igawk Program::, we mentioned that not trying to save the
     line read with 'getline' in the 'pathto()' function when testing
     for the file's accessibility for use with the main program
     simplifies things considerably.  What problem does this engender
     though?

  12. As an additional example of the idea that it is not always
     necessary to add new features to a program, consider the idea of
     having two files in a directory in the search path:

     'default.awk'
          This file contains a set of default library functions, such as
          'getopt()' and 'assert()'.

     'site.awk'
          This file contains library functions that are specific to a
          site or installation; i.e., locally developed functions.
          Having a separate file allows 'default.awk' to change with new
          'gawk' releases, without requiring the system administrator to
          update it each time by adding the local functions.

     One user suggested that 'gawk' be modified to automatically read
     these files upon startup.  Instead, it would be very simple to
     modify 'igawk' to do this.  Since 'igawk' can process nested
     '@include' directives, 'default.awk' could simply contain
     '@include' statements for the desired library functions.  Make this
     change.

  13. Modify 'anagram.awk' (*note Anagram Program::), to avoid the use
     of the external 'sort' utility.

   ---------- Footnotes ----------

   (1) This definition is from
.