can't read file?

[nick]

the following code is used to read a file called "input.txt"
# include <stdio.h>
int main(int argc,char *argv[]){
FILE *fp;
int ch;
fp=fopen("input .txt","r");
ch=fscanf(fp,"% d",&ch);
printf("%d\n",c h);

fclose(fp);
return 0;
}

the contains in "input.txt"
3
3 14
2 8
-1 0
when i run the program the output is:
1

what's going on?
why the output is not 3?

Nick

[Keith Thompson]

nick <i1********@yah oo.com> writes:

the following code is used to read a file called "input.txt"
# include <stdio.h>
int main(int argc,char *argv[]){
FILE *fp;
int ch;
fp=fopen("input .txt","r");
ch=fscanf(fp,"% d",&ch);
printf("%d\n",c h);

fclose(fp);
return 0;
}

the contains in "input.txt"
3
3 14
2 8
-1 0
when i run the program the output is:
1

what's going on?
why the output is not 3?

fscanf() returns the number of items scanned, which in this case will
be 1 if it succeeds. You're passing &ch as an argument to fcanf()
*and* you're assigning the result to ch. Don't do that.

(I'm not certain whether it invokes undefined behavior, but it's
certainly not what you want to do .)

--
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.

[usr.root]

ch=fscanf(fp,"% d",&ch);
this line is the answer! because fscanf() return a int number 1,your ch
got it.
the true way was like this
fscanf(fp,"%d", &ch);
write like this is all ok,fscanf set value to ch,and you will found
the result is 3

best wishes

usr_root

[nick]

thanks!

[Keith Thompson]

"us******@gmail .com" <us******@gmail .com> writes:

ch=fscanf(fp,"% d",&ch);
this line is the answer! because fscanf() return a int number 1,your ch
got it.
the true way was like this
fscanf(fp,"%d", &ch);
write like this is all ok,fscanf set value to ch,and you will found
the result is 3

I think we've told you this before. Please pay attention this time.

You need to provide some context when you post a followup. Not
everyine has easy access to the parent article. It may not have
arrived yet, it may have expired, or somebody's newsreader might not
have a command to dispaly it.

If you want to post a followup via groups.google.c om, don't use
the broken "Reply" link at the bottom of the article. Click on
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"Reply" at the bottom of the article headers.

And ask Google to fix their broken interface.

--
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.

[Mike Wahler]

"nick" <i1********@yah oo.com> wrote in message
news:dg******** ***@justice.its c.cuhk.edu.hk.. .

the following code is used to read a file called "input.txt"
# include <stdio.h>
int main(int argc,char *argv[]){
FILE *fp;
int ch;
fp=fopen("input .txt","r");
ch=fscanf(fp,"% d",&ch);
printf("%d\n",c h);

fclose(fp);
return 0;
}

the contains in "input.txt"
3
3 14
2 8
-1 0
when i run the program the output is:
1

what's going on?
why the output is not 3?

Look in your textbook. What does 'fscanf()' return?

-Mike

[usr.root]

Keith Thompson 写�：

"us******@gmail .com" <us******@gmail .com> writes:

ch=fscanf(fp,"% d",&ch);
this line is the answer! because fscanf() return a int number 1,your ch
got it.
the true way was like this
fscanf(fp,"%d", &ch);
write like this is all ok,fscanf set value to ch,and you will found
the result is 3

I think we've told you this before. Please pay attention this time.

You need to provide some context when you post a followup. Not
everyine has easy access to the parent article. It may not have
arrived yet, it may have expired, or somebody's newsreader might not
have a command to dispaly it.

If you want to post a followup via groups.google.c om, don't use
the broken "Reply" link at the bottom of the article. Click on
"show options" at the top of the article, then click on the
"Reply" at the bottom of the article headers.

And ask Google to fix their broken interface.

--
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.

Thank you!i get it!

[Jack Klein]

On Fri, 23 Sep 2005 01:15:33 GMT, Keith Thompson <ks***@mib.or g> wrote
in comp.lang.c:

nick <i1********@yah oo.com> writes:

the following code is used to read a file called "input.txt"
# include <stdio.h>
int main(int argc,char *argv[]){
FILE *fp;
int ch;
fp=fopen("input .txt","r");
ch=fscanf(fp,"% d",&ch);
printf("%d\n",c h);

fclose(fp);
return 0;
}

the contains in "input.txt"
3
3 14
2 8
-1 0
when i run the program the output is:
1

what's going on?
why the output is not 3?

fscanf() returns the number of items scanned, which in this case will
be 1 if it succeeds. You're passing &ch as an argument to fcanf()
*and* you're assigning the result to ch. Don't do that.

(I'm not certain whether it invokes undefined behavior, but it's
certainly not what you want to do .)

Can't have undefined behavior, 'ch' in modified twice, but there are
at least two sequence points between the assignments. The assignment
to 'ch' via the pointer passed to fscanf() must take place and
complete at a sequence point before the statement returning the value,
which itself is terminated by another sequence point.

And the assignment of the return value to 'ch' cannot take place until
after that return statement in fscanf().

No different than:

#include <stdio.h>
int int_3(int *ip)
{
*ip = 3; /* sequence point #1 */
return 1; /* sequence point #2 */
}
int main(void)
{
int i;
i = int_3(&i);
printf("%d\n", i);
return 0;
}

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.l earn.c-c++
http://www.contrib.andrew.cmu.edu/~a...FAQ-acllc.html

[Keith Thompson]

Jack Klein <ja*******@spam cop.net> writes:

On Fri, 23 Sep 2005 01:15:33 GMT, Keith Thompson <ks***@mib.or g> wrote
in comp.lang.c:

nick <i1********@yah oo.com> writes:

> the following code is used to read a file called "input.txt"
> # include <stdio.h>
> int main(int argc,char *argv[]){
> FILE *fp;
> int ch;
> fp=fopen("input .txt","r");
> ch=fscanf(fp,"% d",&ch);
> printf("%d\n",c h);
>
> fclose(fp);
> return 0;
> }

[...] fscanf() returns the number of items scanned, which in this case will
be 1 if it succeeds. You're passing &ch as an argument to fcanf()
*and* you're assigning the result to ch. Don't do that.

(I'm not certain whether it invokes undefined behavior, but it's
certainly not what you want to do .)

Can't have undefined behavior, 'ch' in modified twice, but there are
at least two sequence points between the assignments. The assignment
to 'ch' via the pointer passed to fscanf() must take place and
complete at a sequence point before the statement returning the value,
which itself is terminated by another sequence point.

And the assignment of the return value to 'ch' cannot take place until
after that return statement in fscanf().

Ok -- but doesn't that assume that fscanf() is implemented in C?

--
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.

[Jack Klein]

On Fri, 23 Sep 2005 07:45:05 GMT, Keith Thompson <ks***@mib.or g> wrote
in comp.lang.c:

Jack Klein <ja*******@spam cop.net> writes:

On Fri, 23 Sep 2005 01:15:33 GMT, Keith Thompson <ks***@mib.or g> wrote
in comp.lang.c:

nick <i1********@yah oo.com> writes:
> the following code is used to read a file called "input.txt"
> # include <stdio.h>
> int main(int argc,char *argv[]){
> FILE *fp;
> int ch;
> fp=fopen("input .txt","r");
> ch=fscanf(fp,"% d",&ch);
> printf("%d\n",c h);
>
> fclose(fp);
> return 0;
> } [...] fscanf() returns the number of items scanned, which in this case will
be 1 if it succeeds. You're passing &ch as an argument to fcanf()
*and* you're assigning the result to ch. Don't do that.

(I'm not certain whether it invokes undefined behavior, but it's
certainly not what you want to do .)

Can't have undefined behavior, 'ch' in modified twice, but there are
at least two sequence points between the assignments. The assignment
to 'ch' via the pointer passed to fscanf() must take place and
complete at a sequence point before the statement returning the value,
which itself is terminated by another sequence point.

And the assignment of the return value to 'ch' cannot take place until
after that return statement in fscanf().

Ok -- but doesn't that assume that fscanf() is implemented in C?

No, the C library does not need to be written in C, but it is defined
and prototyped in terms of C functions. No matter how it is actually
implemented, it must behave "as if" it was written in C.

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
comp.lang.c http://www.eskimo.com/~scs/C-faq/top.html
comp.lang.c++ http://www.parashift.com/c++-faq-lite/
alt.comp.lang.l earn.c-c++
http://www.contrib.andrew.cmu.edu/~a...FAQ-acllc.html