Path: utzoo!dciem!nrcaer!scs!spl1!laidbak!att!pacbell!ames!pasteur!ucbvax!decwrl!hplabs!hpda!hpcuhb!hpcilzb!chris
From: chris@hpcilzb.HP.COM (Chris Toomey)
Newsgroups: comp.software-eng
Subject: Structured Analysis ques.
Message-ID: <4150003@hpcilzb.HP.COM>
Date: 1 Jun 88 23:26:02 GMT
Article-I.D.: hpcilzb.4150003
Organization: HP Design Tech Center - Santa Clara, CA
Lines: 43


  I'm doing structured analysis for the first time and am a little confused
about a few of its concepts.

  I'm using an automated tool (HP Teamwork) to develop a set of leveled
data flow diagrams and the accompanying data dictionary; I want to use
these in place of a formal written requirements spec.  I'm confused over
the identification of the net inputs/outputs of the system as specified
in the top-level DFD (the context diagram).  I'll use a simple example
to illustrate my point.

  Let's say I'm developing a very simple spelling program (similar to UNIX's
spell) which will repeatedly prompt the user for words, which are to be input
one at a time following the prompt. The program will check the spelling of the
words and place them accordingly into the files "correct" and "incorrect." This
will continue until the user enters ctrl-d at the prompt.

  Now, for the context diagram:



                       -------                   ---------
 ----------   words   /        \ --------------> correct
 | user   | -------->|  spell   |                ---------
 ----------           \        / --------------> incorrect
                       --------                  ---------


  As this DFD illustrates, the net input to the program is the words the user 
enters, and the net output is the two files correct and incorrect. Is this the
right assessment of the system I've described? If so, then it seems to me that
the system cannot be fully specified by this approach because it fails to 
consider the module(s) which are responsible for prompting the user and reading
words. In other words, how can DFDs and data dictionary entries be used in 
place of a written requirements spec. if they don't encompass the entire 
system being developed? Is my context diagram incorrect? Should the net input
be "keystrokes"?

  Also, if the system was extended to display a menu to the user, would the
menu be considered net output to the user or merely "control information" (and
thus not shown on the DFD)?

  Thanks for any insights.