Replace word in text file

[William Payne]

Hello, I need to write a program that opens a text file and scans the entire
file for a specific line and when that line is found, a particular word on
that line is to be replaced. The new word is given as an argument to the
program. I wrote a small test program that doesn't work because strcmp()
fails to find a matching line. Here's the code:

#include <stdio.h>
#include <string.h>

int main(int argc, char** argv)
{
FILE* in_file = NULL;
FILE* out_file = NULL;
char* temporary_file_ name = NULL;
char line[128];

if(argc < 2)
{
fprintf(stderr, "You must supply a word.\n");

return 1;
}

in_file = fopen("test.txt ", "r");

if(in_file == NULL)
{
fprintf(stderr, "Error opening test.txt\n");

return 1;
}

temporary_file_ name = tmpnam(NULL);

out_file = fopen(temporary _file_name, "w");

while(fgets(lin e, sizeof(line), in_file))
{
printf("Line read: %s\n", line);

if(strcmp(line, "set imap_user=root" ) == 0)
{
printf("Replaci ng.\n");
strcpy(line, "set imap_user=");
strcat(line, argv[1]);
}

fputs(line, out_file);
}

fclose(in_file) ;
fflush(out_file );
fclose(out_file );

remove("test.tx t");
rename(temporar y_file_name, "test.txt") ;

return 0;
}

And here's the original file test.txt:
set spoolfile=imap://sysinst.ida.liu .se/INBOX
set folder=imap://sysinst.ida.liu .se/INBOX
set imap_user=root
set ssl_starttls=no
set sendmail="/usr/sbin/sendmail -oem -oi"

Why doesn't it work?

/ William Payne

[Richard Heathfield]

William Payne wrote:

Hello, I need to write a program that opens a text file and scans the
entire file for a specific line and when that line is found, a particular
word on that line is to be replaced. The new word is given as an argument
to the program. I wrote a small test program that doesn't work because
strcmp() fails to find a matching line.
That's because fgets doesn't strip the newline (assuming it finds one in the
available space).
while(fgets(lin e, sizeof(line), in_file))
{
printf("Line read: %s\n", line);

if(strcmp(line, "set imap_user=root" ) == 0)

I suggest you replace this line with:

if(strcmp(line, "set imap_user=root\ n") == 0)

--
Richard Heathfield : bi****@eton.pow ernet.co.uk
"Usenet is a strange place." - Dennis M Ritchie, 29 July 1999.
C FAQ: http://www.eskimo.com/~scs/C-faq/top.html
K&R answers, C books, etc: http://users.powernet.co.uk/eton

[Ed Morton]

On 11/5/2003 2:58 PM, William Payne wrote:

Hello, I need to write a program that opens a text file and scans the entire
file for a specific line and when that line is found, a particular word on
that line is to be replaced. The new word is given as an argument to the
program. I wrote a small test program that doesn't work because strcmp()
fails to find a matching line. Here's the code:
<snip> while(fgets(lin e, sizeof(line), in_file))
{
printf("Line read: %s\n", line);

if(strcmp(line, "set imap_user=root" ) == 0)

fgets() copies the newline char to the end of the string, so "line" ends in a
newline whereas "set imap_user=root" doesn't.

Ed.

[Mike Wahler]

"William Payne" <mi************ **@student.liu. se> wrote in message
news:bo******** **@news.island. liu.se...

Hello, I need to write a program that opens a text file and scans the entire file for a specific line and when that line is found, a particular word on
that line is to be replaced. The new word is given as an argument to the
program. I wrote a small test program that doesn't work because strcmp()
fails to find a matching line. Here's the code:

#include <stdio.h>
#include <string.h>

int main(int argc, char** argv)
{
FILE* in_file = NULL;
FILE* out_file = NULL;
char* temporary_file_ name = NULL;
char line[128];

if(argc < 2)
{
fprintf(stderr, "You must supply a word.\n");

return 1;
}

in_file = fopen("test.txt ", "r");

if(in_file == NULL)
{
fprintf(stderr, "Error opening test.txt\n");

return 1;
}

temporary_file_ name = tmpnam(NULL);

out_file = fopen(temporary _file_name, "w");

while(fgets(lin e, sizeof(line), in_file))
{
printf("Line read: %s\n", line);

if(strcmp(line, "set imap_user=root" ) == 0)
Change the above line to:

if(strcmp(line, "set imap_user=root\ n") == 0)
{
printf("Replaci ng.\n");
strcpy(line, "set imap_user=");
strcat(line, argv[1]);
strcat(line, "\n");

/* you should also provide protection against 'strcat()'
overflowing the array 'line' */
}

fputs(line, out_file);
}

fclose(in_file) ;
fflush(out_file );
fclose(out_file );

remove("test.tx t");
rename(temporar y_file_name, "test.txt") ;

return 0;
}

And here's the original file test.txt:
set spoolfile=imap://sysinst.ida.liu .se/INBOX
set folder=imap://sysinst.ida.liu .se/INBOX
set imap_user=root
set ssl_starttls=no
set sendmail="/usr/sbin/sendmail -oem -oi"

Why doesn't it work?

'fgets()' stores the newline (if any) that it reads.

My above changes aren't foolproof, i.e. if the line read
has 'trailing spaces' before the newline, or if 'fgets()'
doesn't read a newline, then it won't work.

-Mike

[William Payne]

"Richard Heathfield" <do******@addre ss.co.uk.invali d> wrote in message
news:bo******** **@titan.btinte rnet.com...

William Payne wrote:

Hello, I need to write a program that opens a text file and scans the
entire file for a specific line and when that line is found, a particular word on that line is to be replaced. The new word is given as an argument to the program. I wrote a small test program that doesn't work because
strcmp() fails to find a matching line.
That's because fgets doesn't strip the newline (assuming it finds one in

the available space).

while(fgets(lin e, sizeof(line), in_file))
{
printf("Line read: %s\n", line);

if(strcmp(line, "set imap_user=root" ) == 0)

I suggest you replace this line with:

if(strcmp(line, "set imap_user=root\ n") == 0)

--
Richard Heathfield : bi****@eton.pow ernet.co.uk
"Usenet is a strange place." - Dennis M Ritchie, 29 July 1999.
C FAQ: http://www.eskimo.com/~scs/C-faq/top.html
K&R answers, C books, etc: http://users.powernet.co.uk/eton

Thanks for the quick reply, Mr Heathfield. It appears that you are right
about fgets() not stripping the newline because I got an empty line between
the printf()-statements in the while()-loop. So I wrote strcmp() as you
suggested, but it still fails to match the string. =( Any ideas?

/ William Payne

[William Payne]

"Mike Wahler" <mk******@mkwah ler.net> wrote in message
news:SI******** ********@newsre ad4.news.pas.ea rthlink.net...

"William Payne" <mi************ **@student.liu. se> wrote in message
news:bo******** **@news.island. liu.se...

Hello, I need to write a program that opens a text file and scans the

entire

file for a specific line and when that line is found, a particular word on that line is to be replaced. The new word is given as an argument to the
program. I wrote a small test program that doesn't work because strcmp()
fails to find a matching line. Here's the code:

#include <stdio.h>
#include <string.h>

int main(int argc, char** argv)
{
FILE* in_file = NULL;
FILE* out_file = NULL;
char* temporary_file_ name = NULL;
char line[128];

if(argc < 2)
{
fprintf(stderr, "You must supply a word.\n");

return 1;
}

in_file = fopen("test.txt ", "r");

if(in_file == NULL)
{
fprintf(stderr, "Error opening test.txt\n");

return 1;
}

temporary_file_ name = tmpnam(NULL);

out_file = fopen(temporary _file_name, "w");

while(fgets(lin e, sizeof(line), in_file))
{
printf("Line read: %s\n", line);

if(strcmp(line, "set imap_user=root" ) == 0)

Change the above line to:

if(strcmp(line, "set imap_user=root\ n") == 0)

{
printf("Replaci ng.\n");
strcpy(line, "set imap_user=");
strcat(line, argv[1]);

strcat(line, "\n");

/* you should also provide protection against 'strcat()'
overflowing the array 'line' */

}

fputs(line, out_file);
}

fclose(in_file) ;
fflush(out_file );
fclose(out_file );

remove("test.tx t");
rename(temporar y_file_name, "test.txt") ;

return 0;
}

And here's the original file test.txt:
set spoolfile=imap://sysinst.ida.liu .se/INBOX
set folder=imap://sysinst.ida.liu .se/INBOX
set imap_user=root
set ssl_starttls=no
set sendmail="/usr/sbin/sendmail -oem -oi"

Why doesn't it work?

'fgets()' stores the newline (if any) that it reads.

My above changes aren't foolproof, i.e. if the line read
has 'trailing spaces' before the newline, or if 'fgets()'
doesn't read a newline, then it won't work.

-Mike

Thanks for your reply, Mr Wahler. I tried this loop as you and others
suggested:

while(fgets(lin e, sizeof(line), in_file))
{
printf("Line read: %s", line);

if(strcmp(line, "set imap_user=root\ n") == 0)
{
printf("Replaci ng.\n");
strcpy(line, "set imap_user=");
strcat(line, argv[1]);
strcat(line, "\n");
}

fputs(line, out_file);
}

But strcmp() never returns 0 so no replacement is made. So then I tried
replacing the call to strcmp() with the following:
if(strstr(line, "set imap_user=root" ) != NULL)

and now it founds a match one the correct line and correctly replaces the
word. Very good, but it annoys me that I don't know why strcmp() doesn't
work.

/ William Payne

[Ed Morton]

On 11/5/2003 3:14 PM, William Payne wrote:

"Richard Heathfield" <do******@addre ss.co.uk.invali d> wrote in message
news:bo******** **@titan.btinte rnet.com... <snip>

I suggest you replace this line with:

if(strcmp(line, "set imap_user=root\ n") == 0)

<snip> Thanks for the quick reply, Mr Heathfield. It appears that you are right
about fgets() not stripping the newline because I got an empty line between
the printf()-statements in the while()-loop. So I wrote strcmp() as you
suggested, but it still fails to match the string. =( Any ideas?
Not all OSs just use "\n" as the line terminator. Try adding "\n\r" instead of
just "\n". This approach still fails if "fgets()" doesn't read to the end of
the line...

Ed.
/ William Payne

[Alex]

Ed Morton <mo************ ****@lucent.com > wrote:

On 11/5/2003 3:14 PM, William Payne wrote:

"Richard Heathfield" <do******@addre ss.co.uk.invali d> wrote in message
news:bo******** **@titan.btinte rnet.com... <snip>

I suggest you replace this line with:

if(strcmp(line, "set imap_user=root\ n") == 0)

<snip>

Thanks for the quick reply, Mr Heathfield. It appears that you are right
about fgets() not stripping the newline because I got an empty line between
the printf()-statements in the while()-loop. So I wrote strcmp() as you
suggested, but it still fails to match the string. =( Any ideas?

Not all OSs just use "\n" as the line terminator. Try adding "\n\r" instead of
just "\n". This approach still fails if "fgets()" doesn't read to the end of
the line...

ITYM "\r\n". Also, I think that you're wrong.

IIRC, when a file is opened as a text stream, '\n' is the newline
character, regardless of the system particulars. When dealing with
binary streams, this does become and issue, but this is not the case
here.

At any rate, I think that it is far better to either truncate the
newline sequence, or use strncmp.

Alex

[Ed Morton]

On 11/5/2003 3:25 PM, William Payne wrote:
<snip>

But strcmp() never returns 0 so no replacement is made. So then I tried
replacing the call to strcmp() with the following:
if(strstr(line, "set imap_user=root" ) != NULL)

and now it founds a match one the correct line and correctly replaces the
word. Very good, but it annoys me that I don't know why strcmp() doesn't
work.
See my previous response. This should work for you:

while(fgets(lin e, sizeof(line), in_file))
{
printf("Line read: %s\n", line);

if(strncmp(line , "set imap_user=root" , sizeof "set imap_user=root" - 1 ) =
= 0)
{

i.e. just compare the number of chars that you want to match, ignoring any
new-line characters from the input file so your code isn't dependent on the
OS/tool that produced the input file. In your final code, you'll probably want
to define a string constant or macro for "set imap_user=root" rather than
hard-coding it twice, but you get the idea. You should probably also add a
"strlen" check to ensure that line isn't actually longer than you expected (e.g.
"set imap_user=rootc rop")

Ed.
/ William Payne

[Ed Morton]

On 11/5/2003 3:42 PM, Alex wrote:

Ed Morton <mo************ ****@lucent.com > wrote: <snip>

Not all OSs just use "\n" as the line terminator. Try adding "\n\r" instead of
just "\n". This approach still fails if "fgets()" doesn't read to the end of
the line...

ITYM "\r\n". Also, I think that you're wrong.

Could be, but just using "\n" doesn't work for him. I suppose there could just
be white-space at the end of the line. Either way...

<snip> At any rate, I think that it is far better to either truncate the
newline sequence, or use strncmp.
Truncating the newline sequence might be tough if it really isn't always "\n"
and you'd also want to truncate trailing white-space. I'd go with "strncmp()"
plus "strlen()".

Ed.
Alex