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From: catfood@NCoast.ORG (Mark W. Schumann)
Newsgroups: comp.lang.c
Subject: Re: Trouble with curses
Message-ID: <1990Oct29.144020.17442@NCoast.ORG>
Date: 29 Oct 90 14:40:20 GMT
References: <1990Oct27.173826.29771@NCoast.ORG>
Distribution: usa
Organization: North Coast Public Access *NIX, Cleveland, OH
Lines: 71

In article <1990Oct27.173826.29771@NCoast.ORG> ramsey@NCoast.ORG
(Connie using Cedric Ramsey's account) writes:

>Hi, this is Connie. I'm accessing this net with a friends account. 
   ... and welcome.  Too bad about the Browns the other day, huh?

>I have a question about the curses package. Heres the skeletal code:
> [Much deleted for brevity]

>do_escape() {
>  switch(tolower(getch())) {
>  case 'e': 
>    do_exit(); 
>    break;
>  [ ... irrelevant stuff deleted ... ]
>  }

>Basically what is happening is the the user presses ESCAPE key follow by 
>'E' key to exit. But I have to press 'E' twice after pressing ESCAPE 
>to get an exit. I can't figure out why. Does anybody know ?

Betcha your tolower is implemented as a macro, something like this:

#define tolower(c) (isupper (c) ? c + 'a' - 'A' : c)

(Portability purists will quibble with this, but what the hey it works
in DOS.  My point is this:)  Notice that there is no way for this macro
to help but evaluate the argument 'c' twice.  The code generated in your
case would be:

  do_escape() {
    switch (isupper (getch()) ? getch() + 'a' - 'A' : getch()) {
       case 'e':
          ....dah dah dah...
    }

So your object code evaluates isupper(getch()) [gets a keystroke here],
then depending on the result it evaluates getch() + 'a' - 'A' OR
plain getch() [gets *a new* keystroke here in either case].

Your problem is the usual macro yuckies--expanding code inline causes
this type of re-evaluation.  Try this instead:

   do_escape() {
   int c;  
      c = getch();
      switch (tolower (c)) {
         .........
         }
      }

The variable 'c' of course gets evaluated only once as 'getch().'  THEN
you process its value without risking the side effects.

Alternative:
    do_escape() {
       switch (getch()) {
           case 'e':
           case 'E':
              do_exit();          /* K & R says be careful, but it works */
              break;
           ......

Hope this helps and that I wasn't over-explaining.

-- 
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Mark W. Schumann  3111 Mapledale Avenue, Cleveland 44109 USA
Domain: catfood@ncoast.org
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