reading from a text file

[googler]

I'm trying to read from an input text file and print it out. I can do
this by reading each character, but I want to implement it in a more
efficient way. So I thought my program should read one line at a time
and print it out. How can I do this? I wrote the code below but it's
not correct since the fscanf reads one word (terminating in whitespace
or newline) at a time, instead of reading the whole line.

#include <stdio.h>
void main(void)
{
char str[120];

FILE *fp = fopen("test.txt ", "r");

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

So I get an output like "Thisisatest.It isnotworking.", when the input
file contains:
This is a test.
It is not working.

Any suggestion is appreciated. Thanks.

[Skarmander]

googler wrote:

I'm trying to read from an input text file and print it out. I can do
this by reading each character, but I want to implement it in a more
efficient way. So I thought my program should read one line at a time
and print it out. How can I do this? I wrote the code below but it's
not correct since the fscanf reads one word (terminating in whitespace
or newline) at a time, instead of reading the whole line.
<snip> Any suggestion is appreciated. Thanks.

fgets.

S.

[googler]

Skarmander wrote:

googler wrote:

I'm trying to read from an input text file and print it out. I can do
this by reading each character, but I want to implement it in a more
efficient way. So I thought my program should read one line at a time
and print it out. How can I do this? I wrote the code below but it's
not correct since the fscanf reads one word (terminating in whitespace
or newline) at a time, instead of reading the whole line.

<snip>

Any suggestion is appreciated. Thanks.

fgets.

S.

Sorry, I should have used fgets.. my bad.

One more question. My code looks like:
while (!feof(fp))
{
fgets(str, 120, fp);
printf("%s", str);
}

This prints the last line twice. I don't understand why. When it prints
the last line for the first time, it should have known that the end of
the file has been reached, so the next condition check for the while
loop should have failed. Why is it still entering the while loop and
printing the last line again?

Thanks.

[granvillemw]

Don't use feof. You can always use fgets to check if it gets to the end
of the file.

for example,
while( fgets(str, 120,fp) )
{
printfr("%s", str);
}

cheers

[AllaTurca]

Reading a line and a character are not much different in terms of
efficiency because a caching is already done by file reader function.
Meaning it reads some blocks (much more than you need) of file at one
time and stores it on the memory. I think you must use the appropriate
one according to your need. If you just want to print the file on the
screen the two would not make much difference but fgets would be a
little better. And in processing the sum of "a little"s are big.

[granvillemw]

Or actually, I should've done some explanation that, during your last
double-printing, fgets returned NULL whilst the "str" memory stayed the
same in terms of the content.

the workaround to your problem whilst keeping the feof would be as
follows:
while ( !feof(fp) )
....
if( fgets(...) )
printf....

[Richard Heathfield]

googler said:

I'm trying to read from an input text file and print it out. I can do
this by reading each character, but I want to implement it in a more
efficient way. So I thought my program should read one line at a time
and print it out. How can I do this? I wrote the code below but it's
not correct since the fscanf reads one word (terminating in whitespace
or newline) at a time, instead of reading the whole line.
Others have already answered your question, but nobody appears to have
pointed out yet that...

#include <stdio.h>
void main(void)

....in C, main returns int, not void. This is a common error, and those who
commit it often find it hard to believe that it's wrong. Nevertheless, no C
compiler is required to accept a main function that returns void unless it
specifically documents that acceptance - which few, if any, do.

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at above domain (but drop the www, obviously)

[Chris Torek]

In article <11************ **********@g14g 2000cwa.googleg roups.com>
googler <pi********@yah oo.com> wrote:

One more question. [The following code fragment, slightly edited for space]
while (!feof(fp)) {
fgets(str, 120, fp);
printf("%s", str);
}
... prints the last line twice. I don't understand why. When it prints
the last line for the first time, it should have known that the end of
the file has been reached,

What makes you believe that? Remember that fgets() is (loosely)
defined as:

int c;
while ((c = getc(fp)) != EOF && c != '\n') {
line[i++] = c;
if (c == '\n')
break;
}
line[i] = '\0';

(of course fgets() avoids overrunning your buffer, so it is a little
more complicated than that, but assume for the moment that no input
lines will be overlong).

Suppose the stdio "FILE *fp" is connected to a human being (at the
keyboard, or a terminal, or behind a network connection via telnet
or ssh or whatever), who has some way of signalling "end of file"
while still remaining connected (on many OSes this is done by
entering ^Z or ^D or @EOF or some similar character or string as
the only input on a line). The human types:

abc[enter]

so the first fgets() reads "abc\n". The fgets() call returns.
What should feof(fp) be?

The human *might* be *about* to press ^D or ^Z or type @EOF or
whatever it is that will signal EOF. Should feof(fp) wait until
he does so? What should it do if, instead, he types "def" and
presses ENTER?

You are effectively expecting the feof() predicate to predict the
future. There is no way for it to do that. It *could*, of course,
try to read input from the file -- in effect, waiting for the human
to signal EOF or enter "def\n". But it does not do that. Predicting
the future is too difficult. C is a simple language. It is much
easier to "predict" the past ... so that is what feof() does!
Instead of telling you "a future attempt to read is not going to
work because EOF is coming up", it tells you "a previous attempt
to read that failed, failed because EOF came up."

Suppose, now, that instead of a human, the stdio FILE *fp is
connected to a file on a floppy disk (or CD-ROM or DVD or whatever).
Suppose further that the floppy has been corrupted (someone used
a magnet to hold it up on the fridge, or scratched the CD-ROM, or
whatever). Your program/OS knows that the file should be 271483
bytes long, but partway in, the media turns out to be unreadable.
The fgetc() function -- or its getc() equivalent -- will return
EOF, indicating that it is unable to continue reading.

What should feof(fp) be? The file size is known (271483 bytes)
but you have at this point successfully read only 65536 bytes.
Should feof(fp) return nonzero (true)? You have not reached the
end of the file!

As before, feof() does not try to predict the future; instead, it
"predicts" the past. It tells you whether the getc() that returned
EOF did so because of end-of-file. In this case, it is *not* the
end of the file -- so feof(fp) is 0 (i.e., false). The other
predicate, ferror(fp), will be nonzero (i.e., true). It is
"predicting " the past, and telling you that the getc() failed due
to error. (Of course, the ability to distinguish between "normal
end of file" and "error reading data" is O/S and sometimes filesystem
or device specific, but it is fairly common.)

Because feof() only tells you about *previous* failures, and --
worse -- only tells you about EOF and not about errors, any loop
of the form:

while (!feof(fp))

is virtually *guaranteed* to be wrong. If you ever see this in
C code, be very suspicious.

As for why the last line prints twice, well, that one is a FAQ. :-)
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) [phone]
email: forget about it http://web.torek.net/torek/index.html
Reading email is like searching for food in the garbage, thanks to spammers.

[pete]

googler wrote:

I'm trying to read from an input text file and print it out. I can do
this by reading each character, but I want to implement it in a more
efficient way. So I thought my program should read one line at a time
and print it out. How can I do this? I wrote the code below but it's
not correct since the fscanf reads one word (terminating in whitespace
or newline) at a time, instead of reading the whole line.

#include <stdio.h>
void main(void)
{
char str[120];

FILE *fp = fopen("test.txt ", "r");

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

So I get an output like "Thisisatest.It isnotworking.", when the input
file contains:
This is a test.
It is not working.

Any suggestion is appreciated. Thanks.

/* BEGIN new.c */

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

#define SOURCE "test.txt"
#define LINE_LEN 119
#define str(s) # s
#define xstr(s) str(s)

int main(void)
{
int rc;
FILE *fd;
char line[LINE_LEN + 1];

fd = fopen(SOURCE, "r");
if (fd == NULL) {
fprintf(stderr,
"\nfopen() problem with \"%s\"\n", SOURCE);
exit(EXIT_FAILU RE);
}
do {
rc = fscanf(fd, "%" xstr(LINE_LEN) "[^\n]%*[^\n]", line);
if (!feof(fd)) {
getc(fd);
}
if (rc == 0) {
*line = '\0';
++rc;
}
printf("%s\n", line);
} while (rc == 1);
fclose(fd);
return 0;
}

/* END new.c */
--
pete

[pete]

pete wrote:

googler wrote:

I'm trying to read from an input text file and print it out.
I can do
this by reading each character, but I want to implement it in a more
efficient way. So I thought my program should read one line at a
time
and print it out. How can I do this? I wrote the code below but it's
not correct since the fscanf reads one word
(terminating in whitespace
or newline) at a time, instead of reading the whole line.

#include <stdio.h>
void main(void)
{
char str[120];

FILE *fp = fopen("test.txt ", "r");

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

So I get an output like "Thisisatest.It isnotworking.",
when the input
file contains:
This is a test.
It is not working.

Any suggestion is appreciated. Thanks.

/* BEGIN new.c */

new.c outputs the last line double.
I'm working on it.

--
pete