NEWBIE: how to paste strings to make a command for system()

[Atropo]

Hi all.

Having several strings how do i combine them to construct a command;
lets say to run the date command.

string str = "14/10/08 19:06:09";
strDD = str.substr(0,2) ;
strMM = str.substr(3,2) ;
strAA = str.substr(6,2) ;
strhh = str.substr(9,2) ;
strmm = str.substr(12,2 );
strss = str.substr(15,2 );

strtarget<<"dat e -u "<<strMM<<strDD <<strhh<<strmm< <strAA<<"."<<st rss<<
" 2>&1 /dev/null;"<<endl;

system(strtarge t);

Not sure if the problem it's syntax in the strtarget line (59). when
I compile i get this

rtchk.cpp: In function `int main (int, char **)':
rtchk.cpp:59: no match for `string & << const char[9]'

Thanks in advance. really appreciate any advise.

[Juha Nieminen]

James Kanze wrote:

On Oct 16, 3:42 pm, Juha Nieminen <nos...@thanks. invalidwrote:

>Atropo wrote:

>>strtarget<<"d ate -u "<<strMM<<strDD <<strhh<<strmm< <strAA<<"."<<st rss<<
" 2>&1 /dev/null;"<<endl;

>> system(strtarge t);

>You really want to look at the strftime() standard function
rather than doing that ugly hack with the system() function.

The system function is being used to set the date.

At least here the -u parameter specifies that the date should be in
UTC. It says nothing about setting the date.

strftime() doesn't do what he wants, and any use of it here
would doubtlessly be less efficient, since it involves
conversions to int and back.

How can a call to strftime() be less efficient than spawning a
command-line interpreter and running the 'date' command, which itself
most probably calls strftime()?

[James Kanze]

On Oct 20, 2:20*pm, [e-mail] x.net (Yannick Tremblay) wrote:

In article <E5%Jk.67$1K... @read4.inet.fi> ,
Juha Nieminen *<nos...@thanks .invalidwrote:

Efficiency is unlikely to be important. *strftime would be of
no use because the date is in human readable format as
suggested above, not as a tm struct. *He could possibly use
strptime to get a tm then strftime to get a string back but
simple string manipulation would do the job at least as well
and probably faster.

Just a nit, but there is no strptime in standard C++. It's a
Unix extension.

You are not comparing strftime with system(), you are
comparing strftime+strpti me with string manipulation. (but
efficiency is very unlikely to be critical)

Compared to the time necessary to spawn the child process, very,
very unlikely. Depending on what his real specification is,
there can be an advantage in passing through the numeric values
and struct tm. It's more flexible, even if it is more work.
But you still have to parse the input (boost::regex seems the
best solution for that), then if you want numeric values, use
istringstream to convert the parsed substrings. (If strptime is
available, it might be slightly easier to use, but IMHO,
learning to use boost::regex for this sort of thing is worth the
effort.) Then you reformat the data you have to get what you
want; if it's all strings, you can use either ostringstream or
string concatenation; if you have numeric values, it has to be
ostringstream.

--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]

[James Kanze]

On Oct 17, 1:58*pm, Juha Nieminen <nos...@thanks. invalidwrote:

James Kanze wrote:

On Oct 16, 3:42 pm, Juha Nieminen <nos...@thanks. invalidwrote:

Atropo wrote:
strtarget<<"da te -u "<<strMM<<strDD <<strhh<<strmm< <strAA<<"."<<st rss<<
" 2>&1 /dev/null;"<<endl;

>* * * * system(strtarge t);

You really want to look at the strftime() standard function
rather than doing that ugly hack with the system() function.

The system function is being used to set the date.

At least here the -u parameter specifies that the date should
be in UTC. It says nothing about setting the date.

The command he generated is the command to set the system time
under Unix. And using system is probably the most portable way
of doing it; it will work under all Unix, where as all of the
other ways will only work under a specific Unix.

strftime() doesn't do what he wants, and any use of it here
would doubtlessly be less efficient, since it involves
conversions to int and back.

How can a call to strftime() be less efficient than spawning a
command-line interpreter and running the 'date' command, which
itself most probably calls strftime()?

Because he still needs to spawn a command line and execute an
external function to set the system time. All using strftime()
means is that he absolutely has to convert the numeric values in
his string to numbers (ints), and that he also has to program
around buffer length issues.

--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, [phone]