getc/fgets

[Richard Weeks]

Below is a fragment from a program that calculates statistics on
x,y data. I want the user to be able to predict one or more
predicted values of y from x, given the line of best fit. I have
a procedural problem.
predict:
printf("\npredict y? (y/n): ");
if((getc(stdin)=='n')) exit(EXIT_SUCCESS);
//if((fgets(response, 1, stdin)=="n")) exit(EXIT_SUCCESS);
else
{
printf("\nenter x: ");
scanf("%lf", &xdatum);
printf("\ny = %f\n", y_int + (slope * xdatum));
fflush(stdin);
goto predict;
}

It works fine using getc, but when I use fgets execution jumps
directly into the else{} block and prompts for x without waiting
for user response to the y/n prompt. What am I doing wrong?

Richard

[santosh]

santosh wrote:

Richard Weeks wrote:

>Below is a fragment from a program that calculates statistics on
x,y data. I want the user to be able to predict one or more
predicted values of y from x, given the line of best fit. I have
a procedural problem.
predict:
printf("\npredict y? (y/n): ");

Unless output is terminated with a newline, it might appear immediately on
the screen.

That should read;

.... might *not* appear immediately on the screen.

An alternative is to use fflush(stdout) just after the printf.

[Old Wolf]

On Aug 1, 9:46 am, Richard Weeks <[e-mail]:

//if((fgets(response, 1, stdin)=="n")) exit(EXIT_SUCCESS);

It works fine using getc, but when I use fgets execution jumps
directly into the else{} block and prompts for x without waiting
for user response to the y/n prompt. What am I doing wrong?

>From the C standard (ISO/IEC 9899:1999 7.19.7.2):

[#2] The fgets function reads at most one less than the
number of characters specified by n from the stream pointed
to by stream into the array pointed to by s.

My guess is that since you specified n=1, fgets
determines that it needs to read at most 0
characters. Since doing nothing is the same
as reading 0 characters, it does nothing.

Also, as santosh pointed out, your method of
checking the result is not correct.

[Keith Thompson]

Richard Weeks <rw****@nomail.netwrites:

Below is a fragment from a program that calculates statistics on x,y
data. I want the user to be able to predict one or more predicted
values of y from x, given the line of best fit. I have a procedural
problem.
predict:
printf("\npredict y? (y/n): ");

Why do you print a newline at the beginning of the prompt? If you
just want a blank line above the prompt, that's fine; if you think
it's required, it isn't.

You should add 'fflush(stdout);' here to ensure that your prompt
appears. It may not be necessary on most systems, but stdout could be
line-buffered.

if((getc(stdin)=='n')) exit(EXIT_SUCCESS);
//if((fgets(response, 1, stdin)=="n")) exit(EXIT_SUCCESS);

You can't use the "==" operator to compare strings, at least not
meaningfully.

Take a look at the definition of fgets from the standard:

The fgets function reads at most one less than the number of
characters specified by n from the stream pointed to by stream
into the array pointed to by s.

Your fgets call reads at most zero characters.

else
{
printf("\nenter x: ");

Again, 'fflush(stdout);'.

scanf("%lf", &xdatum);
printf("\ny = %f\n", y_int + (slope * xdatum));
fflush(stdin);

Just what do you think 'fflush(stdin);' is going to do?

It invokes undefined behavior.

goto predict;

Style point: this would be better written as a loop.

}

[snip]

The comp.lang.c FAQ is at <http://www.c-faq.com/>. Please read it.

--
Keith Thompson (The_Other_Keith) ks***@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <* <http://users.sdsc.edu/~kst>
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

[Justin Spahr-Summers]

On Jul 31, 4:46 pm, Richard Weeks <[e-mail]:

It works fine using getc, but when I use fgets execution jumps
directly into the else{} block and prompts for x without waiting
for user response to the y/n prompt. What am I doing wrong?

All previously mentioned points aside, I have to wonder: why do you
want to use fgets() when all you're doing at that point is reading one
character anyways?

[Army1987]

On Wed, 01 Aug 2007 03:29:53 +0530, santosh wrote:

Richard Weeks wrote:

[snip]

> if((getc(stdin)=='n')) exit(EXIT_SUCCESS);

getc can return EOF on end-of-file or error. EOF is not a valid character
value. You must test for EOF and *then* test against 'n' and 'N'.

What the hell? If getc(stdin) return EOF, the program will do
exactly the same thing as if it returned '\n', ' ', or anything
else other than 'n'.
A better thing to do would be islower(getchar()) != 'y', so the
default would be "no".
But (k = getc(stdin)) != EOF && k == 'n' will always do the exact
same thing as getc(stdin) == 'n', except that it needs an
otherwise useless temporary variable.
--
Army1987 (Replace "NOSPAM" with "email")
"Never attribute to malice that which can be adequately explained
by stupidity." -- R. J. Hanlon (?)