sscanf function

[touchstone]

Code Select Expand



include<stdio.h>
#include<stdlib.h>


int main()


{

FILE *infile = fopen("d:\\sample.txt","r");

if(infile ==NULL) printf("can not open ");


char buff[90];

int n ;

while(!feof(infile))

{

fgets(buff,70,infile);

sscanf(buff,"%d",&n);

printf("%d\n",n);

}

}



sample.txt
--------------
16 18 20 25 18  23 22 55

11 22 23 25 28 29 27 31

4 2 78 67 90 23 25 56

12 0  16 17 87 79 0 89


ouput of the program
-----------------------------
16
16
11
11
4
4
12
12
12


why the data has been repeated  two times ?

why not the output like below?

16
11
4
12

any explanation?

[Yoni]

Haven't read entire source code, but I spotted the most likely source of the problem.

Code Select Expand


sscanf(buff,"%d",&n);


When you use "%d" as a format string for *scanf, it reads an integer and that's it. The next time you try to read, it'll encounter the whitespace and fail.
You can instruct *scanf to skip whitespaces by adding a space in the format string, as such:

Code Select Expand


sscanf(buff, "%d ", &n);


(I changed the string from "%d" to "%d ", by adding a space after the d.)

[MoNksBaNe_Agahnim]

Code Select Expand


while(!feof(infile))


maybe make it f.eof() ... not sure but that is what you have to do with fstream

[Kp]

Code Select Expand

while(!feof(infile))maybe make it f.eof() ... not sure but that is what you have to do with fstream

No.  If you look at his code, nowhere does there exist a variable f which could even have an eof method.

If you're only scanning one number, I'd suggest using strtol instead.  Just a matter of preference.

[touchstone]

hi  i did  this (added space after %d)
as per your suggestion....

Code Select Expand



sscanf(buff, "%d ", &n);



still... it it did not give me  result.

why it is not giving. i am  learning to use fgets() and sscanf()

do you think i have done any mistake in the code?

...but according to logic it should print the number only once. is not it?i think my logic is correct.

datas are poured into buff and then sscanf is extracting out the integer from  the buff string.

i am trying a lot to find out  the cause....but could not....whats wrong?

[iago]

Your code looks fine, except:
Max Length should be a constant (your buffer length and the amount you get can be the same)
If the file can't be opened, it will still try reading it
Besides those, it looks great.. I don't have a C compiler here, but I will try it in Linux

<edit>
include <stdio.h> should be #include <stdio.h> (I assume that's a copy mistake)
<edit2>
hah, I found the problem just before I ran it.  The problem is that the file has blank lines, which are read in and they aren't parsed correctly.  
</edit2>
</edit>

[touchstone]

>Max Length should be a constant

what do you mean? i have alloted maximum 90 character in the buff[]....its enough.

>If the file can't be opened

 well,   file is opening... i did not do NULL checking.
...but no doubt its opening

>include <stdio.h> should be #include <stdio.h>
yes....its a copy mistake.

> found the problem just before I ran it.  The problem is that the file has blank lines, which are read in and they aren't parsed correctly.  

....i am not agree with you.

let me explain why....

fget() will read  70 elements ...... as soon as it gets a newline it will stop and it will insert newline into buff and also a  '\0' character will be automatically added into buff.

file pointer is now in the next row(because of newline)


now sscanf() will extract the first  integer stored in the buff.


this  should be the case for one go of the while loop.

same thing will be repeating  until  fget() returns end of file.

i am just  experimenting with this code to  understand the behaviour of fgets() and sscanf()

but i did not find any answer  why  i am getting wrong output.

thanks

[iago]

>Max Length should be a constant

what do you mean? i have alloted maximum 90 character in the buff[]....its enough.

I mean, it is better coding practice to do this:
#define MAXLENGTH 90
...
char line[MAXLENGTH];
...
fgets(line, MAXLENGTH, file);
...

>If the file can't be opened

 well,   file is opening... i did not do NULL checking.
...but no doubt its opening

But if the file doesn't open for some reason, it will still attempt to process and die.

> found the problem just before I ran it.  The problem is that the file has blank lines, which are read in and they aren't parsed correctly.  

....i am not agree with you.

let me explain why....

fget() will read  70 elements ...... as soon as it gets a newline it will stop and it will insert newline into buff and also a  '\0' character will be automatically added into buff.

file pointer is now in the next row(because of newline)

You're exactly right.  But your file looks like this:

Code Select Expand

16 18 20 25 18  23 22 55\n
\n
11 22 23 25 28 29 27 31\n
\n
4 2 78 67 90 23 25 56\n
\n
12 0  16 17 87 79 0 89EOF

After it reads the first line, it reads the second, which is simply "\n".  The sscanf() runs and does nothing, because there is nothing there to parse.  Then the string is displayed, but because sscanf() did nothing, the string hasn't changed.

Try checking the return value of sscanf() (I forget what it should be, but you can look it up) to see if anything happened before displaying.

[Skywing]

sscanf returns the number of arguments scanned in, so you should compare it to 1 in this situation.

[iago]

sscanf returns the number of arguments scanned in, so you should compare it to 1 in this situation.

Ah, that's what I thought, but I wasn't 100% sure so I didn't want to say.  Thanks!

[touchstone]

hi, i was away for some days....

however, i am convinced by your  explanation....i hava understood the cause of mine previous output.

and  i should go for checking of sscanf() conversion before printing.

i wrote

Code Select Expand



.........
..........
int p = sscanf(buff,"%d",&n);
if(p==1)
{

printf("%d\n",n);
............



....and this checking is giving
16
11
4
12

its ok now. ....thanks

[iago]

Another way to do it would be this:

Code Select Expand

if(sscanf(buff,"%d",&n) == 1)
{
 ....
}