scanf (yes/no) - doesn't work + deprecation errors scanf, fopen etc.

[Martin Jørgensen]

Hi,

Consider:
------------
char stringinput[64]

..bla. bla. bla.

do
{
printf("Write to result.txt (y/n)? ");
scanf("%s", stringinput);
}
while (writechar != 'y' || != 'n');
The compiler complaints. It says: error C2059: syntax error : '!='

Another problem is that MS VS 2005 keeps complaining about deprecated
commands such as:

1. warning C4996: 'fscanf' was declared deprecated..... ..: see
declaration of 'fscanf' - Message: 'This function or variable may be
unsafe. Consider using fscanf_s instead. To disable deprecation, use
_CRT_SECURE_NO_ DEPRECATE. See online help for details.'

2. warning C4996: 'scanf' was declared deprecated: see declaration of
'scanf'... Message: 'This function or variable may be unsafe. Consider
using scanf_s instead. To disable deprecation, use ...

3. warning C4996: 'fopen' was declared deprecated.... : see declaration
of 'fopen' Message: 'This function or variable may be unsafe. Consider
using fopen_s instead. To disable deprecation, use
_CRT_SECURE_NO_ DEPRECATE. See online help for details.'
How do I fix these problems? Sorry, but I'm not very experienced with C
programming.
Med venlig hilsen / Best regards
Martin Jørgensen

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[Martin Jørgensen]

Barry Schwarz wrote:
-snip-

This should be :
while (writechar != 'y' || writechar != 'n');

While it is now syntactically correct, the expression will always
evaluate to true. Use &&, not ||.

Damn... Thanks...
Med venlig hilsen / Best regards
Martin Jørgensen

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[Martin Joergensen]

"Michael Mair" <Mi**********@i nvalid.invalid> skrev i en meddelelse
news:45******** ****@individual .net...
-snip-

Another thing: There is no equivalent to the run-time determination
of field width and precision for printf(). If you do not want to
have magic numbers you always have something like that:

#define STRINGIZE(S) #s
#define XSTR(S) STRINGIZE(S)

#define BUFSIZE 20

...
char buffer[BUFSIZE+1];
....
ret = sscanf("%" XSTR(BUFSIZE) "s", buffer);
....

Not really what I'd call flexible. And I am a friend of sscanf().

How about using getchar() like this for a yes/no test:

printf("Write to file result.txt (y/n)? ");
while ( tolower(writech ar = getchar() ) != 'y' && tolower(writech ar) != 'n')
{
if (writechar != '\n')
printf("Write to file result.txt (y/n)? ");
};

if (tolower(writec har) == 'j')
bla. bla. (write to file)
if no, then quit etc...
Best regards / Med venlig hilsen
Martin Jørgensen

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[Emmanuel Delahaye]

Broeisi a écrit :

This code will do what you want... __fpurge(stdin) ; // clear the stdin to strip off the previous newline char.

Bad avice. This function is not standard.

You should use fgets() with an array of 3 (at least) char...

--
A+

Emmanuel Delahaye

[stathis gotsis]

"Martin Joergensen" <un*********@sp am.jay.net> wrote in message
news:td******** ****@news.tdc.d k...

How about using getchar() like this for a yes/no test:
It would be better if you posted a fully compilable program. I assume you
included <ctype.h> and declared writechar as int. There are still some
problems with your implementation.
printf("Write to file result.txt (y/n)? ");
while ( tolower(writech ar = getchar() ) != 'y' && tolower(writech ar) != 'n') {
if (writechar != '\n')
printf("Write to file result.txt (y/n)? ");
};

You are not taking into account that a call to getchar() may return EOF
except for a character. This may happen if end of file is encountered or in
case of an error.

What happens if the user of your program types more than one character and
then presses <Enter>?
1) If the first character is one of those you are looking for
('y','n','Y','N '), the while loop exits and the rest of the typed characters
still reside in stdin. If you read the stdin later on, you will first read
those characters.
2) If the first character is not one of the expected ones, the message
printed in the body of your loop, will be printed repeatedly until all
characters in the stdin are consumed.
I do not think this is the behaviour you desire, so you should focus on
fixing these problems.

[Martin Jørgensen]

stathis gotsis wrote:

"Martin Joergensen" <un*********@sp am.jay.net> wrote in message
news:td******** ****@news.tdc.d k... -snip-
You are not taking into account that a call to getchar() may return EOF
except for a character. This may happen if end of file is encountered or in
case of an error.
EOF when I read from keyboard?
What happens if the user of your program types more than one character and
then presses <Enter>?
Yeah, I know it's a problem I'm trying to fix...
1) If the first character is one of those you are looking for
('y','n','Y','N '), the while loop exits and the rest of the typed characters
still reside in stdin. If you read the stdin later on, you will first read
those characters.
2) If the first character is not one of the expected ones, the message
printed in the body of your loop, will be printed repeatedly until all
characters in the stdin are consumed.
I do not think this is the behaviour you desire, so you should focus on
fixing these problems.

#include <stdio.h>
#include <ctype.h>

#define buffer_length 2

main()
{
char writechar[buffer_length];

writechar[0]='a'; // initialize it to something other than y/n

while ( tolower(writech ar[0] ) != 'y' && tolower(writech ar[0]) != 'n')
{
if (writechar[0] != '\n')
printf("Write to the file result.txt (y/n)? ");
fgets(writechar , buffer_length, stdin);
}

if (tolower(writec har[0]) == 'y')
printf("You typed \"y\"\n");
else if (tolower(writec har[0]) == 'n')
printf("You typed \"n\"\n");
}
Now, even though I made "#define buffer_length 2" I would expect that if
the user typed something like "asdfa", then the question wouldn't be
printed 5 times... But it is printed 5 times... Without having to
download any libraries or whatever, how to fix this final problem? I
assume the definition of writechar is ok (or should be it an array of
length 3)?
Med venlig hilsen / Best regards
Martin Jørgensen

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[Keith Thompson]

Martin Jørgensen <un*********@sp am.jay.net> writes:

stathis gotsis wrote:

"Martin Joergensen" <un*********@sp am.jay.net> wrote in message
news:td******** ****@news.tdc.d k... -snip-

You are not taking into account that a call to getchar() may return EOF
except for a character. This may happen if end of file is encountered or in
case of an error.

EOF when I read from keyboard?

Yes, of course, typically triggered by control-D or control-Z.

[snip]
#include <stdio.h>
#include <ctype.h>

#define buffer_length 2

main()
{
char writechar[buffer_length];

writechar[0]='a'; // initialize it to something other than y/n

while ( tolower(writech ar[0] ) != 'y' && tolower(writech ar[0]) != 'n')
{
if (writechar[0] != '\n')
printf("Write to the file result.txt (y/n)? ");
fgets(writechar , buffer_length, stdin);
}

if (tolower(writec har[0]) == 'y')
printf("You typed \"y\"\n");
else if (tolower(writec har[0]) == 'n')
printf("You typed \"n\"\n");
}
Now, even though I made "#define buffer_length 2" I would expect that
if the user typed something like "asdfa", then the question wouldn't
be printed 5 times... But it is printed 5 times... Without having to
download any libraries or whatever, how to fix this final problem? I
assume the definition of writechar is ok (or should be it an array of
length 3)?

If fgets() tries to read a line longer than the specified length
(buffer_length) , the remainder of the line is left on the input
stream, to be read by the next call to fgets(). Repeated calls to
fgets() will read the entire line, a chunk at a time.

You probably don't want to examine every character on the input line,
just the first one. Or maybe you want to skip whitespace.

You can use fgets() with larger size argument to read the entire input
line into a string, then examine just the first character of the
string. If fgets() didn't read the entire line, the last character of
the string won't be a '\n' character; you can then discard the rest of
the line.

Or you can read a single character, check whether it's 'y' or 'n', and
then discard the rest of the line (up to the first '\n'). Decide what
you want to do on an empty input line (i.e., if the first character
you read is '\n'), and how you want to handle an EOF condition.

--
Keith Thompson (The_Other_Keit h) 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.

[stathis gotsis]

"Martin Jørgensen" <un*********@sp am.jay.net> wrote in message
news:c2******** ****@news.tdc.d k...

stathis gotsis wrote:

"Martin Joergensen" <un*********@sp am.jay.net> wrote in message
news:td******** ****@news.tdc.d k... -snip-

You are not taking into account that a call to getchar() may return EOF
except for a character. This may happen if end of file is encountered or in case of an error.

EOF when I read from keyboard?

Yes, it is possible when you type something like CTRL-D or CTRL-Z(It is
CTRL-Z typed at the start of the input line on a windows dos console). Also,
getchar() may return EOF in case of an error, so you should consider
comparing the return value of getchar() to EOF too.
#include <stdio.h>
#include <ctype.h>

#define buffer_length 2

main()
{
char writechar[buffer_length];

writechar[0]='a'; // initialize it to something other than y/n

while ( tolower(writech ar[0] ) != 'y' && tolower(writech ar[0]) != 'n')
{
if (writechar[0] != '\n')
printf("Write to the file result.txt (y/n)? ");
fgets(writechar , buffer_length, stdin);
fgets() will read up to (buffer_length-1 = 1) characters, so the problem
remains unsolved. Even if you increase buffer_lenth, the user might input
long lines so that fgets() does not read all of the input. I think you
should stick with your previous implementation where you used getchar(). You
will just have to read the first character each time, then eat up rest of
the input until you reach the newline. In all cases, handle EOF.
}

if (tolower(writec har[0]) == 'y')
printf("You typed \"y\"\n");
else if (tolower(writec har[0]) == 'n')
printf("You typed \"n\"\n");
}

[Michael Mair]

Martin Jørgensen schrieb:

stathis gotsis wrote:

"Martin Joergensen" <un*********@sp am.jay.net> wrote in message
news:td******** ****@news.tdc.d k...

-snip-

You are not taking into account that a call to getchar() may return EOF
except for a character. This may happen if end of file is encountered
or in
case of an error.

EOF when I read from keyboard?

As the others have not mentioned it: You can redirect a file
to stdin on many systems. I like to do that for C courses;
redirect stdin/stdout/stderr and let a bash script or batch
file check whether everything has been done correctly...

Interestingly, if I feed the elsethread (*) posted get_double
programme with an invalid file (containing only ".\n"), the
programme is caught in an endless loop as I forgot to break
the loop on file end and file error conditions... ;-)
This was not a problem for interactive input as the EOF
condition was given once and afterwards the loop could
continue, but in this case, we always get EOF. So much for
sufficient error handling.

<snip>
Cheers
Michael

(*) <45************ @individual.net >
--
E-Mail: Mine is an /at/ gmx /dot/ de address.

[Martin Joergensen]

"stathis gotsis" <st***********@ hotmail.com> skrev i en meddelelse
news:dt******** **@ulysses.noc. ntua.gr...
-snip-

EOF when I read from keyboard?

Yes, it is possible when you type something like CTRL-D or CTRL-Z(It is
CTRL-Z typed at the start of the input line on a windows dos console).
Also,
getchar() may return EOF in case of an error, so you should consider
comparing the return value of getchar() to EOF too.

Ok, clear...

#include <stdio.h>
#include <ctype.h>

#define buffer_length 2

main()
{
char writechar[buffer_length];

writechar[0]='a'; // initialize it to something other than y/n

while ( tolower(writech ar[0] ) != 'y' && tolower(writech ar[0]) != 'n')
{
if (writechar[0] != '\n')
printf("Write to the file result.txt (y/n)? ");
fgets(writechar , buffer_length, stdin);

fgets() will read up to (buffer_length-1 = 1) characters, so the problem
remains unsolved. Even if you increase buffer_lenth, the user might input
long lines so that fgets() does not read all of the input. I think you
should stick with your previous implementation where you used getchar().
You
will just have to read the first character each time, then eat up rest of
the input until you reach the newline. In all cases, handle EOF.

Hmm. Okay, but I'm not really sure about how to discard the rest of the
line....... This program that was posted earlier had an error with the
__fpurge(stdin) - how do I modify it to work with standard C without
downloading any libraries or anything?:
#include <stdio.h>

int main(void)
{
char stringInput;

do
{
printf("\nWrite to result.txt (y/n)? ");
stringInput = getchar();
__fpurge(stdin) ; // clear the stdin to strip off the previous newline char.
}
while (stringInput != 'y' && stringInput != 'n');

printf("Thank you very much\n");

return 0;
}

Best regards / Med venlig hilsen
Martin Jørgensen

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[Michael Mair]

Martin Joergensen schrieb:

"stathis gotsis" <st***********@ hotmail.com> skrev i en meddelelse <snip!>

fgets() will read up to (buffer_length-1 = 1) characters, so the problem
remains unsolved. Even if you increase buffer_lenth, the user might input
long lines so that fgets() does not read all of the input. I think you
should stick with your previous implementation where you used getchar().
You
will just have to read the first character each time, then eat up rest of
the input until you reach the newline. In all cases, handle EOF.

Hmm. Okay, but I'm not really sure about how to discard the rest of the
line....... This program that was posted earlier had an error with the
__fpurge(stdin) - how do I modify it to work with standard C without
downloading any libraries or anything?:

#include <stdio.h>

int main(void)
{
char stringInput;

int tmp;
do
{
printf("\nWrite to result.txt (y/n)? ");
stringInput = getchar();
Hmmm, go back to the comp.lang.c FAQ and read it carefully.

getchar() returns int, where the characters are cast to
unsigned char and EOF is a negative int value.
Every character literal in C is of type int.
If you make stringInput type char you can miss EOF returns or
returns >= SCHAR_MAX.
__fpurge(stdin) ; // clear the stdin to strip off the previous newline char.
while (EOF != (tmp = getchar()))
if ('\n' == tmp)
break;
or
do {
tmp = getchar();
} while(EOF != tmp && '\n' != tmp);
}
while (stringInput != 'y' && stringInput != 'n');

printf("Thank you very much\n");

return 0;
}

Cheers
Michael
--
E-Mail: Mine is an /at/ gmx /dot/ de address.