NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c-part1/5059

Internet Message Format | 1996-08-05 | 4.0 KB

Path: mail2news.demon.co.uk!genesis.demon.co.uk
From: Lawrence Kirby <[e-mail]>
Newsgroups: comp.lang.c
Subject: Re: newbie's problems w/ linux
Date: Mon, 12 Feb 96 20:06:33 GMT
Organization: none
Message-ID: <[e-mail]>
References: <4fnrq8$[e-mail]>
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In article <4fnrq8$[e-mail]> [e-mail] "
" writes:

>i'm currently taking a beginners C class, and i was attempting to make a
>program which figures out GPA for my classes. I'm running windows 95, and
>when i compile it for DOS, it runs fine, but, when i compile it for
>linux, using gcc, i keep getting this same error, basically what happens
>is that after taking input through scanf(), it will skip the next
>scanf(), even though i'm using fflush(). here is a sample output of what
>happens.
>
>
>GPA version .9beta, written by [e-mail]
>
>
>How many classes do you have? 2
>What grade did you get in period 1? a
>What grade did you get in period 2? Do you have any AP classes?
>
>Your Total GPA for 2 period(s) is 0!
>Which is equivilent to a(n) F
>
>
>here is the source, if you can figure out what's wrong w/ it, please
>mail me at [e-mail], thanks in advance.
>
>#include <stdio.h>
>void convert(void);
>void endo(void);
>float converted, gpa = 0., ap, donegpa;
>int periods;
>char grade, endofit[2], yesno;
>void main()

In C main returns an int.

>{
>short int counter, counter2;

int is typically mapped to the most efficient type the archetecture
can handle therefore you should use it for auxiliary variables unless
you need something longer. Shorts have very little use other than in
structures and arrays.

>printf("\n\n\n\n\n\n\nGPA version .9beta, written by
> [e-mail]\n\n\n");
>printf("How many classes do you have? ");
>scanf("%i", &periods);

Look up the difference between the %i and %d scanf format specifiers. I'm
not saying %i is necessarily wrong here (although it may be if you
aren't using an ANSI compiler/library), just that %d more commonly
corresponds to what was wanted.

>fflush(stdin);

This results in undefined behaviour. fflush() is only defined on output
streams or update streams where the last operation was a write. You can
also pass NULL to fflush (many pre-ANSI libraries didn't support this
though.

>for ( counter = 1; counter <= periods; ++counter )
>{
>printf("What grade did you get in period %i? ", counter);

%d and %i means the same thing in printf formats. However %d has been
supported much longer by C compilers/libraries and is more commonly used.

Here is where you may need a fflush(). The C runtime system may have set
stdout as line buffered here in which case it won't write anything until
the program writes a newline to stdout. Alternatively you can force buffered
output to be written using:

fflush(stdout);

>scanf("%d", &grade);

Here you are passing a pointer to a char to a format specifier which
requires a pointer to an int. GCC should have warned you about this
if you used an adequate warning level (-Wall will probably do it).

>fflush(stdin);
>convert();
>gpa += converted;
>}
>printf("Do you have any AP classes? ");
>scanf("%d",& yesno);

Again you are trying to read an int value into a char and passing the
wring type of pointer which is illegal. It looks from below that you were
trying to read a charater in which case you could use:

scanf(" %c",& yesno);

Note the space is important since it skips over leading white-space in the
input data (which %c doesn't do automatically).

A generally better approach for getting input is to read in whole lines
using fgets() into a buffer and then scanning them with sscanf, atoi,
strtok etc. scanf by itself isn't too hot at dealing with line based
input.

--
-----------------------------------------
Lawrence Kirby | [e-mail]
Wilts, England | [e-mail]
-----------------------------------------