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Question of ofstream / fstream methods to modify a specific line in atext file.

Hello,

I am using the ofstream class to create a text file with keys and
values like:

Key1=Value10
Key2=Value15
Key3=Value20

In case I need to set a new value for Key2, say value50 - I am able to
read and get the value, not sure how to replace that specific value
after '=' for a specific line - Please advice which class / method can
help me achieve this?

thanks
/R

Here is my code snippet:

-----
using namespace std;

#include <iostream>

bool SetVal4Key(std::string Key, std::string Value) {

ofstream cfgfile;
bool status = FALSE;
string sLine;
string buf;
UINT32 pos = 0;
string Delim = "=";
cfgfile.open ("/etc/config.txt", ios::noreplace | ios::app);
if (!cfgfile) {
cout << Failed to open config file - unable to continue" << endl;
return status;
}

while (!cfgfile.eof()) {

std::getline(cfgfile, buf);

// Dump the content for debugging purpose

len = buf.size();
pos = buf.find(Key, 0);

if (!pos) {
pos = buf.find(Delim, 0);

//Modify the string
buf.erase
buf= Key;
buf.append = Value;

// Write to the specific line in the file where the key is already
present
status = TRUE;
cfgfile.close();
status = TRUE;
}
cout << "Failed to locate the key" << endl;
}
return status;

}

----
Oct 17 '08 #1
6 4587
On Oct 17, 9:44*am, Ramesh <rrame...@gmail.comwrote:
Hello,

I am using the ofstream class to create a text file with keys and
values like:

Key1=Value10
Key2=Value15
Key3=Value20

In case I need to set a new value for Key2, say value50 - I am able to
read and get the value, not sure how to replace that specific value
after '=' for a specific line - Please advice which class / method can
help me achieve this?

thanks
/R

Here is my code snippet:

-----
using namespace std;

#include <iostream>

bool SetVal4Key(std::string Key, std::string Value) {

ofstream * * * *cfgfile;
bool * * * * * *status = FALSE;
string * * * * *sLine;
string * * * * *buf;
UINT32 * * * * *pos = 0;
string * * * * *Delim = "=";

cfgfile.open ("/etc/config.txt", ios::noreplace | ios::app);
if (!cfgfile) {
* * * * cout << Failed to open config file - unable to continue" << endl;
* * * * return status;

}

while (!cfgfile.eof()) {

* * * * std::getline(cfgfile, buf);

* * * * // Dump the content for debugging purpose

* * * * len = buf.size();
* * * * pos = buf.find(Key, 0);

* * * * if (!pos) {
* * * * * * * * pos = buf.find(Delim, 0);

* * * * * * * * //Modify the string
* * * * * * * * buf.erase
* * * * * * * * buf= Key;
* * * * * * * * buf.append = Value;

* * * * * * * * // Write to the specific line in the filewhere the key is already
present
* * * * * * * * status = TRUE;
* * * * * * * * cfgfile.close();
* * * * * * * * status = TRUE;
* * * * }
* * * * cout << "Failed to locate the key" << endl;}

return status;

}
ios::noreplace is none-standard.

There's no way to modify the file inplace with standard C++.
You can load the file into vector<string>
modifies the strings. then overwrite the original file.

if the file to too large to do so, find out the platform APIs to
modify inplace.

--
Best Regards
Barry
Oct 17 '08 #2
On Oct 16, 6:57*pm, Barry <dhb2...@gmail.comwrote:
On Oct 17, 9:44*am, Ramesh <rrame...@gmail.comwrote:
Hello,
I am using the ofstream class to create a text file with keys and
values like:
Key1=Value10
Key2=Value15
Key3=Value20
In case I need to set a new value for Key2, say value50 - I am able to
read and get the value, not sure how to replace that specific value
after '=' for a specific line - Please advice which class / method can
help me achieve this?
thanks
/R
Here is my code snippet:
-----
using namespace std;
#include <iostream>
bool SetVal4Key(std::string Key, std::string Value) {
ofstream * * * *cfgfile;
bool * * * * * *status = FALSE;
string * * * * *sLine;
string * * * * *buf;
UINT32 * * * * *pos = 0;
string * * * * *Delim = "=";
cfgfile.open ("/etc/config.txt", ios::noreplace | ios::app);
if (!cfgfile) {
* * * * cout << Failed to open config file - unable to continue" << endl;
* * * * return status;
}
while (!cfgfile.eof()) {
* * * * std::getline(cfgfile, buf);
* * * * // Dump the content for debugging purpose
* * * * len = buf.size();
* * * * pos = buf.find(Key, 0);
* * * * if (!pos) {
* * * * * * * * pos = buf.find(Delim, 0);
* * * * * * * * //Modify the string
* * * * * * * * buf.erase
* * * * * * * * buf= Key;
* * * * * * * * buf.append = Value;
* * * * * * * * // Write to the specific line in the file where the key is already
present
* * * * * * * * status = TRUE;
* * * * * * * * cfgfile.close();
* * * * * * * * status = TRUE;
* * * * }
* * * * cout << "Failed to locate the key" << endl;}
return status;
}

ios::noreplace is none-standard.

There's no way to modify the file inplace with standard C++.
You can load the file into vector<string>
modifies the strings. then overwrite the original file.

if the file to too large to do so, find out the platform APIs to
modify inplace.

--
Best Regards
Barry
Yeah just learnt about replace after the compiler didnt like it - am
using ios::out | ios::app in its place.
I am still digging to see if fseek / fsetpos related functions in
cstdio can be handy, but no clear idea yet :)

But thanks a bunch for your quick response.
Oct 17 '08 #3
Ramesh wrote:
On Oct 16, 6:57 pm, Barry <dhb2...@gmail.comwrote:
>On Oct 17, 9:44 am, Ramesh <rrame...@gmail.comwrote:
>>Hello,
I am using the ofstream class to create a text file with keys and
values like:
Key1=Value10
Key2=Value15
Key3=Value20
In case I need to set a new value for Key2, say value50 - I am able to
read and get the value, not sure how to replace that specific value
after '=' for a specific line - Please advice which class / method can
help me achieve this?
thanks
/R
Here is my code snippet:
-----
using namespace std;
#include <iostream>
bool SetVal4Key(std::string Key, std::string Value) {
ofstream cfgfile;
bool status = FALSE;
string sLine;
string buf;
UINT32 pos = 0;
string Delim = "=";
cfgfile.open ("/etc/config.txt", ios::noreplace | ios::app);
if (!cfgfile) {
cout << Failed to open config file - unable to continue" << endl;
return status;
}
while (!cfgfile.eof()) {
std::getline(cfgfile, buf);
// Dump the content for debugging purpose
len = buf.size();
pos = buf.find(Key, 0);
if (!pos) {
pos = buf.find(Delim, 0);
//Modify the string
buf.erase
buf= Key;
buf.append = Value;
// Write to the specific line in the file where the key is already
present
status = TRUE;
cfgfile.close();
status = TRUE;
}
cout << "Failed to locate the key" << endl;}
return status;
}
ios::noreplace is none-standard.

There's no way to modify the file inplace with standard C++.
You can load the file into vector<string>
modifies the strings. then overwrite the original file.

if the file to too large to do so, find out the platform APIs to
modify inplace.

--
Best Regards
Barry

Yeah just learnt about replace after the compiler didnt like it - am
using ios::out | ios::app in its place.
I am still digging to see if fseek / fsetpos related functions in
cstdio can be handy, but no clear idea yet :)
The problem is that if you tweak the file inplace, and your replacement
text is bigger, you will clobber text:

e.g.
Key=Value15
Key=Value20

If you replace "Value15" with "Value100", you will get

Key=Value15Key=Value100

It's much safer to read it all, modify it, and write it back out.

Oct 17 '08 #4
On Oct 17, 4:09*am, Ramesh <rrame...@gmail.comwrote:
>
Yeah just learnt about replace after the compiler didnt like it - am
using ios::out | ios::app in its place.
I am still digging to see if fseek / fsetpos related functions in
cstdio can be handy, but no clear idea yet :)
I'm afraid that fseek & fsetpos won't be much help as you're dealing
with ASCII files. I'd go with Barry's advice to read the whole file
and replace the specific line which needs to be modified before
writing the contents back to another file.

Cheers
Chris

Oct 17 '08 #5
On Oct 17, 1:28*am, news.chris.th...@gmail.com wrote:
On Oct 17, 4:09*am, Ramesh <rrame...@gmail.comwrote:
Yeah just learnt about replace after the compiler didnt like it - am
using ios::out | ios::app in its place.
I am still digging to see if fseek / fsetpos related functions in
cstdio can be handy, but no clear idea yet :)

I'm afraid that fseek & fsetpos won't be much help as you're dealing
with ASCII files. I'd go with Barry's advice to read the whole file
and replace the specific line which needs to be modified before
writing the contents back to another file.

Cheers
Chris
Thanks, yes I gave up - really not worth spending time :)
right now doing a removal of the old file and modified contents go
into a new file.

Regards
Ramesh
Oct 17 '08 #6
On 2008-10-17 18:45:18 -0400, Ramesh <rr******@gmail.comsaid:
On Oct 17, 1:28*am, news.chris.th...@gmail.com wrote:
>On Oct 17, 4:09*am, Ramesh <rrame...@gmail.comwrote:
>>Yeah just learnt about replace after the compiler didnt like it - am
using ios::out | ios::app in its place.
I am still digging to see if fseek / fsetpos related functions in
cstdio can be handy, but no clear idea yet :)

I'm afraid that fseek & fsetpos won't be much help as you're dealing
with ASCII files. I'd go with Barry's advice to read the whole file
and replace the specific line which needs to be modified before
writing the contents back to another file.

Cheers
Chris

Thanks, yes I gave up - really not worth spending time :)
right now doing a removal of the old file and modified contents go
into a new file.
Right. The key notion here is that file streams are streams: a sequence
of bytes, one after another. The usual approach to modifying a file is,
indeed, copying up to the point where you need to change, writing out
the changed text, and copying the remainder. Then, after the new file
has been written and closed, deleting the old one and renaming the new
one.

--
Pete
Roundhouse Consulting, Ltd. (www.versatilecoding.com) Author of "The
Standard C++ Library Extensions: a Tutorial and Reference
(www.petebecker.com/tr1book)

Oct 18 '08 #7

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