reading data from a file into a 2d array

[yourmycaffiene]

Okay, this if my first post so go easy on me plus I've only been using
C for a couple of weeks. I'm working on a program currently that
requires me to read data from a .dat file into a 2d array and then
print out the contents of the 2d array to the screen. I wil also need
to append data to the .dat file but mostly right now I'm worrying about
getting the info into the 2d array. My .dat file looks like this

1 20000
2 30000
3 40000

etc. The first number standing for an ID and the second standing for a
weight.

#include <stdio.h>
main()
{
FILE *pRead;
int array[20][3];
pRead = fopen("test.dat ", "r");

if (pRead==NULL)
printf("\nFile cannot be opened\n");

else
printf("\nConte nts of test.dat\n\n");
fscanf(pRead,"% s,%s", array);

while ( !feof(pRead) )
{
printf("%s%s\n" , array);
fscanf(pRead, "%s,%s", array);
}

}

I know I need to use a for loop or something to that idea so that I can
read in the column and then the row. I'm really drawing a blank here on
how to get that working. Could someone point me in the right direction?
Cause right now my output is coming out like this :

11
20002000
22
30003000

etc.
Also, if I use %d instead of %s here, I get segmentation faults. Can
anyone explain why this is happening? Thanks.

[darkchild50]

do you mind telling me why you used a multi-dimintinal array? to me it
seems also you are
using an integer array and filling it with strings?

[Keith Thompson]

da*********@gma il.com writes:

do you mind telling me why you used a multi-dimintinal array? to me it
seems also you are
using an integer array and filling it with strings?

Please read <http://cfaj.freeshell. org/google/>. Thanks.

--
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.

[Richard G. Riley]

On 2006-02-23, yourmycaffiene <dd****@gmail.c om> wrote:

Okay, this if my first post so go easy on me plus I've only been using
C for a couple of weeks. I'm working on a program currently that
requires me to read data from a .dat file into a 2d array and then
print out the contents of the 2d array to the screen. I wil also need
to append data to the .dat file but mostly right now I'm worrying about
getting the info into the 2d array. My .dat file looks like this

1 20000
2 30000
3 40000

etc. The first number standing for an ID and the second standing for a
weight.

#include <stdio.h>
main()
{
FILE *pRead;
int array[20][3];
pRead = fopen("test.dat ", "r");

if (pRead==NULL)
printf("\nFile cannot be opened\n");

else
printf("\nConte nts of test.dat\n\n");
fscanf(pRead,"% s,%s", array);

while ( !feof(pRead) )
{
printf("%s%s\n" , array);
fscanf(pRead, "%s,%s", array);
}

}

I know I need to use a for loop or something to that idea so that I can
read in the column and then the row. I'm really drawing a blank here on
how to get that working. Could someone point me in the right direction?
Cause right now my output is coming out like this :

11
20002000
22
30003000

etc.
Also, if I use %d instead of %s here, I get segmentation faults. Can
anyone explain why this is happening? Thanks.

Have a look at this code and work from there:

#include <stdio.h>
main()
{

unsigned long a,b;

FILE * pRead = fopen("test.dat ", "r");

if (pRead==NULL)
printf("File cannot be opened\n");

else
printf("Content s of test.dat : \n");

while ( !feof(pRead) ){
fscanf(pRead,"% u %u\n", &a,&b);
printf("%u %u\n",a,b);
}

}
Some pointers to help you on the way : fcanf can do the conversions
for you : use it if you use fscanf. You were reading in strings and
then storing them in "int" storage - this is naughty and very
unclean. Try to get a handle on the types and keep them
concistent. While C can and does allow you to stick data in the wrong
"type memory" it is a very bad idea.

The storage in the array we leave as an excercise :)

--
Remove evomer to reply

[Vladimir S. Oka]

yourmycaffiene wrote:

Okay, this if my first post so go easy on me plus I've only been using
C for a couple of weeks. I'm working on a program currently that
requires me to read data from a .dat file into a 2d array and then
print out the contents of the 2d array to the screen. I wil also need
to append data to the .dat file but mostly right now I'm worrying about
getting the info into the 2d array. My .dat file looks like this

1 20000
2 30000
3 40000

etc. The first number standing for an ID and the second standing for a
weight.

#include <stdio.h>
Vertical space is cheap...
main()
Better spell it out:

int main(void)
{
FILE *pRead;
int array[20][3];
Why 3 as a second dimension if you only have two columns. Avoiding
zero-based indexing in this way is a waste of memory. Imagine if the
first dimension was 200,000.
pRead = fopen("test.dat ", "r");

if (pRead==NULL)
printf("\nFile cannot be opened\n");
Vertical space here hinders reading...

You also probably want to abort execution at this point. The way it
stands, you'll still try to read from the file you failed to open.
else
printf("\nConte nts of test.dat\n\n");
Apart from lack of vertical spacing, you should really indent properly.
E.g.:

else
printf("Blah.\n ");
fscanf(pRead,"% s,%s", array);
Quite a few problems here. You're specifying `%s` which expects a
string from the file, but you tell `fscanf` to put it into a
two-dimensional array of `int`s. Apart from failing to do what you
want, along that path lies the monster of buffer overflows. You also
tell `fscanf` to expect two strings, but pass only one address of a
variable to store them in.

BTW, what was wrong with including the above line into `while` loop? As
it stands, you'd have to guarantee the file is not empty (impossible).
Also, if you can guarantee that, a `do` loop would have been clearer.
while ( !feof(pRead) )
{
printf("%s%s\n" , array);
Now you're trying to print contents of an array of `int`s as a string.
Also, you're telling `printf` to expect two strings, but only pass it
one pointer (of the wrong type, as well).
fscanf(pRead, "%s,%s", array);
Same comment as for the similar line above. Plus, You should really
have swapped the above two (or dropped this one, and put the first one
like this in the loop).
}
You should close the file as well:

fclose(pFile);

C99 will let you omit final `return 0;` but it's good practice to spell
things out.

return 0;
}
This is probably what you want:

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

#define DAT_MAX_LINES 20

int main(void)
{
int array[DAT_MAX_LINES][2];
int n = 0;
int i;

FILE * pRead = fopen("test.dat ", "r");

if (!pRead)
{
printf("File cannot be opened\n");
return EXIT_FAILURE;
}

printf("Content s of test.dat:\n");

while ( (n < DAT_MAX_LINES) && (!feof(pRead)) )
{
fscanf(pRead,"% d%d\n", &array[n][0], &array[n][1]);
++n;
}

if ( (!feof(pFile)) && (n == DAT_MAX_FILES) )
{
printf("Error: file to large for array.\n");
}

fclose(pFile);

for (i = 0; i < n; ++i)
{
printf("%d: %d, %d\n", i, array[i][0], array[i][1]);
}

return EXIT_SUCCESS;
}

<snip>
Also, if I use %d instead of %s here, I get segmentation faults. Can
anyone explain why this is happening? Thanks.

See comment above about matching the formats and the parameters (and
the number of the latter). Your compiler should have warned you about
these (mine did). Turn up the warining level as high as you can.

--
BR, Vladimir

[Ben Bacarisse]

On Thu, 23 Feb 2006 08:48:59 +0000, Richard G. Riley wrote:

Have a look at this code and work from there:

#include <stdio.h>
main()
{

unsigned long a,b;

FILE * pRead = fopen("test.dat ", "r");

if (pRead==NULL)
printf("File cannot be opened\n");

else
printf("Content s of test.dat : \n");

while ( !feof(pRead) ){
fscanf(pRead,"% u %u\n", &a,&b);
printf("%u %u\n",a,b);
}

}

This is a dodgy pattern to recommend. You get, I think, undefined
behaviour if you test a NULL FILE * with feof (I get a core dump rather
than any flying demons -- YMMV). You definitely get UB if you test for
end-of-file before reading anything (because there may be no data to read)
and your input is very sensitive to the format used (not technically a
problem, of course, but a beginenr will be baffled by the change in
behaviour caused by switching to "%u %u" from "%u %u\n").

I think the following is more beginner-friendly:

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

int main(void)
{
unsigned long a, b;
FILE *pRead = fopen("test.dat ", "r");

if (pRead == NULL)
printf("File cannot be opened\n");
else {
printf("Content s of test.dat : \n");

while (fscanf(pRead," %u %u", &a,&b) == 2)
printf("%u %u\n", a, b);
}
return 0;
}

--
Ben.

[Ben Bacarisse]

On Thu, 23 Feb 2006 16:36:39 +0000, Ben Bacarisse wrote:

I think the following is more beginner-friendly:

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

second include is, of course, not needed. I was panning to "exit" and
decided to "return" instead!

--
Ben.

[Richard G. Riley]

On 2006-02-23, Ben Bacarisse <be********@bsb .me.uk> wrote:

On Thu, 23 Feb 2006 08:48:59 +0000, Richard G. Riley wrote:

Have a look at this code and work from there:

#include <stdio.h>
main()
{

unsigned long a,b;

FILE * pRead = fopen("test.dat ", "r");

if (pRead==NULL)
printf("File cannot be opened\n");

else
printf("Content s of test.dat : \n");

while ( !feof(pRead) ){
fscanf(pRead,"% u %u\n", &a,&b);
printf("%u %u\n",a,b);
}

}
This is a dodgy pattern to recommend. You get, I think, undefined

Thanks. You're right : I missed the return or bracketing at null check -
but then I wasnt looking to write it for him just to indicate the type
problems he had.
behaviour if you test a NULL FILE * with feof (I get a core dump rather
than any flying demons -- YMMV). You definitely get UB if you test for
end-of-file before reading anything (because there may be no data to read)
and your input is very sensitive to the format used (not technically a
problem, of course, but a beginenr will be baffled by the change in
behaviour caused by switching to "%u %u" from "%u %u\n").

I think the following is more beginner-friendly:

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

int main(void)
{
unsigned long a, b;
FILE *pRead = fopen("test.dat ", "r");

if (pRead == NULL)
printf("File cannot be opened\n");
else {
printf("Content s of test.dat : \n");

while (fscanf(pRead," %u %u", &a,&b) == 2)
printf("%u %u\n", a, b);
}
return 0;
}

--
Remove evomer to reply

[Vladimir S. Oka]

Ben Bacarisse wrote:

<snip>

This is a dodgy pattern to recommend. You get, I think, undefined
behaviour if you test a NULL FILE * with feof (I get a core dump rather
True, as it probably internally dereferences NULL pointer.
than any flying demons -- YMMV). You definitely get UB if you test for
end-of-file before reading anything (because there may be no data to read)
This however I believe is not the case. The Standard says only this:

7.19.10.2p2
The feof function tests the end-of-file indicator for the stream
pointed to by stream.

7.19.10.2p3
The feof function returns nonzero if and only if the end-of-file
indicator is set for
stream.

I see no requirement for any data to exist in the file (think empty
files), or any data to be read beforehand (think empty files again).
and your input is very sensitive to the format used (not technically a
problem, of course, but a beginenr will be baffled by the change in
behaviour caused by switching to "%u %u" from "%u %u\n").

I think the following is more beginner-friendly:

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

int main(void)
{
unsigned long a, b;
FILE *pRead = fopen("test.dat ", "r");

if (pRead == NULL)
printf("File cannot be opened\n");
else {
printf("Content s of test.dat : \n");
Here, you make the same mistake as both OP and Richard. Even if the
`fopen` fails you'll try to read from the file.

while (fscanf(pRead," %u %u", &a,&b) == 2)
printf("%u %u\n", a, b);
}
return 0;
}

I believe my earlier post gave better advice.

--
BR, Vladimir

[stathis gotsis]

"Ben Bacarisse" <be********@bsb .me.uk> wrote in message
news:pa******** *************** *****@bsb.me.uk ...

I think the following is more beginner-friendly:

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

int main(void)
{
unsigned long a, b;
FILE *pRead = fopen("test.dat ", "r");

if (pRead == NULL)
printf("File cannot be opened\n");
else {
printf("Content s of test.dat : \n");

while (fscanf(pRead," %u %u", &a,&b) == 2)
printf("%u %u\n", a, b);
You should consider closing the file here: fclose(pRead); }
return 0;
}