Double -> C-String conversion

[Der Andere]

During every iteration in a loop, I need to convert a double value into a
char*. Simple type-casting did not work, so I thought of using
stringstreams. However, the stream would have to be *emptied* after each
iteration (and I could not conceive of a practical way doing that).

Cheers,
Matthias
--
Für emails Anweisung in der Adresse befolgen

[Rolf Magnus]

Re: Double -> C-String conversion

Der Andere wrote:
[color=blue]
> During every iteration in a loop, I need to convert a double value
> into a char*. Simple type-casting did not work, so I thought of using
> stringstreams.[/color]

Sounds as if you want a string representation of your double. A char* is
not a string. It's a pointer to char, nothing less, nothing more.
[color=blue]
> However, the stream would have to be *emptied* after each iteration
> (and I could not conceive of a practical way doing that).[/color]

Do the following:

thestream.str(" ");

[Karthik]

Re: Double -> C-String conversion

Der Andere wrote:
[color=blue]
> During every iteration in a loop, I need to convert a double value into a
> char*. Simple type-casting did not work, so I thought of using
> stringstreams. However, the stream would have to be *emptied* after each
> iteration (and I could not conceive of a practical way doing that).[/color]

How about using sprintf ?

sprintf(p, "%lf", doubleval);
// Remember to allocate memory to p before you do this.

HTH[color=blue]
>
> Cheers,
> Matthias
> --
> Für emails Anweisung in der Adresse befolgen
>
>[/color]


--
Karthik

------

Human Beings please 'removeme' for my email.

[Jeff Schwab]

Re: Double -> C-String conversion

Rolf Magnus wrote:[color=blue]
> Der Andere wrote:
>
>[color=green]
>>During every iteration in a loop, I need to convert a double value
>>into a char*. Simple type-casting did not work, so I thought of using
>>stringstreams .[/color]
>
>
> Sounds as if you want a string representation of your double. A char* is
> not a string. It's a pointer to char, nothing less, nothing more.
>
>[color=green]
>>However, the stream would have to be *emptied* after each iteration
>>(and I could not conceive of a practical way doing that).[/color]
>
>
> Do the following:
>
> thestream.str(" ");[/color]

Alternatively:

thestream.str( ).clear( );

[Der Andere]

Re: Double -> C-String conversion

> > During every iteration in a loop, I need to convert a double value[color=blue][color=green]
> > into a char*. Simple type-casting did not work, so I thought of using
> > stringstreams.[/color]
>
> Sounds as if you want a string representation of your double. A char* is
> not a string. It's a pointer to char, nothing less, nothing more.[/color]

C-string, yes. I thought I could indicate a c-string with char *: Would
char[] be better?
[color=blue][color=green]
> > However, the stream would have to be *emptied* after each iteration
> > (and I could not conceive of a practical way doing that).[/color]
>
> Do the following:
>
> thestream.str(" ");[/color]

It works now, cheers :-)

Matthias

[Rolf Magnus]

Re: Double -> C-String conversion

Jeff Schwab wrote:
[color=blue]
> Rolf Magnus wrote:[color=green]
>> Der Andere wrote:
>>
>>[color=darkred]
>>>During every iteration in a loop, I need to convert a double value
>>>into a char*. Simple type-casting did not work, so I thought of using
>>>stringstream s.[/color]
>>
>>
>> Sounds as if you want a string representation of your double. A char*
>> is not a string. It's a pointer to char, nothing less, nothing more.
>>
>>[color=darkred]
>>>However, the stream would have to be *emptied* after each iteration
>>>(and I could not conceive of a practical way doing that).[/color]
>>
>>
>> Do the following:
>>
>> thestream.str(" ");[/color]
>
> Alternatively:
>
> thestream.str( ).clear( );[/color]

According to Stroustrup, this shouldn't empty the stream, because str()
returns a copy of the string.

[Rolf Magnus]

Re: Double -> C-String conversion

Der Andere wrote:
[color=blue][color=green][color=darkred]
>> > During every iteration in a loop, I need to convert a double value
>> > into a char*. Simple type-casting did not work, so I thought of
>> > using stringstreams.[/color]
>>
>> Sounds as if you want a string representation of your double. A char*
>> is not a string. It's a pointer to char, nothing less, nothing more.[/color]
>
> C-string, yes. I thought I could indicate a c-string with char *:
> Would char[] be better?[/color]

No. Actually, char[] is only allowed if used as a paremter. I just
wanted to clarify that char* isn't C's string type. C doesn't have a
string type. Therefore, there is no built-in way to convert something
into a string, and that explains why casting to char* doesn't do what
you want.

[Jack Klein]

Re: Double -> C-String conversion

On Sun, 25 Apr 2004 16:37:02 -0700, Karthik
<removeme_kayka ydreamz@yahoo.c om> wrote in comp.lang.c++:
[color=blue]
> Der Andere wrote:
>[color=green]
> > During every iteration in a loop, I need to convert a double value into a
> > char*. Simple type-casting did not work, so I thought of using
> > stringstreams. However, the stream would have to be *emptied* after each
> > iteration (and I could not conceive of a practical way doing that).[/color]
>
> How about using sprintf ?[/color]

That's a possibility but...
[color=blue]
> sprintf(p, "%lf", doubleval);[/color]

In all versions of the ISO C standard prior to 1999, including the
1995 version that the C++ standard inherits from, "%lf" is an
ill-formed conversion specifier for the *printf() functions and
produces undefined behavior.

There is no need for separate conversion specifiers for float and
double in *printf, as floats are always promoted to double as is the
case for all optional parameters to all variadic functions in C and
C++.

The 1999 update to the C standard made the 'l' length modifier a no-op
with the all floating point conversion specifiers, but this is not
part of the current C++ standard. The "%lf" *printf conversion
specifier causes undefined behavior in C++.
[color=blue]
> // Remember to allocate memory to p before you do this.
>
> HTH[color=green]
> >
> > Cheers,
> > Matthias[/color][/color]

--
Jack Klein
Home: http://JK-Technology.Com
FAQs for
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alt.comp.lang.l earn.c-c++

http://www.contrib.andrew.cmu.edu/~ajo/docs/FAQ-acllc.html

[John Harrison]

Re: Double -&gt; C-String conversion


"Jeff Schwab" <jeffplus@comca st.net> wrote in message
news:3oadncIDba TV0xHdRVn-uQ@comcast.com. ..[color=blue]
> Rolf Magnus wrote:[color=green]
> > Der Andere wrote:
> >
> >[color=darkred]
> >>During every iteration in a loop, I need to convert a double value
> >>into a char*. Simple type-casting did not work, so I thought of using
> >>stringstreams .[/color]
> >
> >
> > Sounds as if you want a string representation of your double. A char* is
> > not a string. It's a pointer to char, nothing less, nothing more.
> >
> >[color=darkred]
> >>However, the stream would have to be *emptied* after each iteration
> >>(and I could not conceive of a practical way doing that).[/color]
> >
> >
> > Do the following:
> >
> > thestream.str(" ");[/color]
>
> Alternatively:
>
> thestream.str( ).clear( );[/color]

That doesn't work. It clears the error flags of the stream. That's what
clear does for any stream, it has nothing to do with emptying a string
stream.

john

[John Harrison]

Re: Double -&gt; C-String conversion


"Jack Klein" <jackklein@spam cop.net> wrote in message
news:sg1p80p2m8 hkber2rph0gqvoe 7aqhvaqpg@4ax.c om...[color=blue]
> On Sun, 25 Apr 2004 16:37:02 -0700, Karthik
> <removeme_kayka ydreamz@yahoo.c om> wrote in comp.lang.c++:
>[color=green]
> > Der Andere wrote:
> >[color=darkred]
> > > During every iteration in a loop, I need to convert a double value[/color][/color][/color]
into a[color=blue][color=green][color=darkred]
> > > char*. Simple type-casting did not work, so I thought of using
> > > stringstreams. However, the stream would have to be *emptied* after[/color][/color][/color]
each[color=blue][color=green][color=darkred]
> > > iteration (and I could not conceive of a practical way doing that).[/color]
> >
> > How about using sprintf ?[/color]
>
> That's a possibility but...
>[color=green]
> > sprintf(p, "%lf", doubleval);[/color]
>
> In all versions of the ISO C standard prior to 1999, including the
> 1995 version that the C++ standard inherits from, "%lf" is an
> ill-formed conversion specifier for the *printf() functions and
> produces undefined behavior.
>[/color]

How did this misunderstandin g become so widespread? So widespread in fact
that the C standard committee felt obliged to change the standard.

john

[Brian Rodenborn]

Re: Double -&gt; C-String conversion


Rolf Magnus <ramagnus@t-online.de> wrote in message
news:c6hld1$e3j $05$2@news.t-online.com...
[color=blue]
> No. Actually, char[] is only allowed if used as a paremter. I just
> wanted to clarify that char* isn't C's string type. C doesn't have a
> string type. Therefore, there is no built-in way to convert something
> into a string, and that explains why casting to char* doesn't do what
> you want.[/color]


Nor does C++ have a string type. It has a library template instantiation
called. C has a string concept, it is not an intrinsic type just as C++'s is
not. That C string, and you will find the term defined in the C standard, is
an array or array-like region of char terminated by a null character.

To convert a double to such a string, the sprintf() function can be used. I
would recommend using C++ strstreams instead.


Brian Rodenborn

[Ivan Vecerina]

Re: Double -&gt; C-String conversion

"John Harrison" <john_andronicu s@hotmail.com> wrote in message
news:c6i5q4$c6l s2$1@ID-196037.news.uni-berlin.de...[color=blue][color=green][color=darkred]
> > > sprintf(p, "%lf", doubleval);[/color]
> >
> > In all versions of the ISO C standard prior to 1999, including the
> > 1995 version that the C++ standard inherits from, "%lf" is an
> > ill-formed conversion specifier for the *printf() functions and
> > produces undefined behavior.[/color]
>
> How did this misunderstandin g become so widespread? So widespread in fact
> that the C standard committee felt obliged to change the standard.[/color]

Because when you want to read a double value, you use:
sscanf(p, "%lf", &doubleval); // for float: ... "%f", &floatval)

The implicit conversion of variadic function parameters from
float to double does not apply to pointers, as used in scan functions.

Given that the implicit conversion exists in printf (the parameters
received are always double, never float), it kind of makes sense
that %f and %lf be made equivalent, or even that %lf be the primary
choice. (But as Jack pointed out, %lf is formally UB in C++98 and C90).


Regards,
Ivan
--
http://ivan.vecerina.com/contact/?subject=NG_POST <- e-mail contact form

[Jeff Schwab]

Re: Double -&gt; C-String conversion

Rolf Magnus wrote:[color=blue]
> Jeff Schwab wrote:
>
>[color=green]
>>Rolf Magnus wrote:
>>[color=darkred]
>>>Der Andere wrote:
>>>
>>>
>>>
>>>>During every iteration in a loop, I need to convert a double value
>>>>into a char*. Simple type-casting did not work, so I thought of using
>>>>stringstrea ms.
>>>
>>>
>>>Sounds as if you want a string representation of your double. A char*
>>>is not a string. It's a pointer to char, nothing less, nothing more.
>>>
>>>
>>>
>>>>However, the stream would have to be *emptied* after each iteration
>>>>(and I could not conceive of a practical way doing that).
>>>
>>>
>>>Do the following:
>>>
>>>thestream.st r("");[/color]
>>
>>Alternatively :
>>
>> thestream.str( ).clear( );[/color]
>
>
> According to Stroustrup, this shouldn't empty the stream, because str()
> returns a copy of the string.[/color]

Whoops! You're absolutely right.

[Jeff Schwab]

Re: Double -&gt; C-String conversion

John Harrison wrote:[color=blue]
> "Jeff Schwab" <jeffplus@comca st.net> wrote in message
> news:3oadncIDba TV0xHdRVn-uQ@comcast.com. ..
>[color=green]
>>Rolf Magnus wrote:
>>[color=darkred]
>>>Der Andere wrote:
>>>
>>>
>>>
>>>>During every iteration in a loop, I need to convert a double value
>>>>into a char*. Simple type-casting did not work, so I thought of using
>>>>stringstrea ms.
>>>
>>>
>>>Sounds as if you want a string representation of your double. A char* is
>>>not a string. It's a pointer to char, nothing less, nothing more.
>>>
>>>
>>>
>>>>However, the stream would have to be *emptied* after each iteration
>>>>(and I could not conceive of a practical way doing that).
>>>
>>>
>>>Do the following:
>>>
>>>thestream.st r("");[/color]
>>
>>Alternatively :
>>
>> thestream.str( ).clear( );[/color]
>
>
> That doesn't work. It clears the error flags of the stream. That's what
> clear does for any stream, it has nothing to do with emptying a string
> stream.[/color]

No, John, you've mistaken the clear() method of the stream for the
clear() method of the underlying stream. The str() method here returns
a string, not a stream.

This is a great example of why unnecessary abbreviations are such a bad
idea when choosing identifiers.

[Jeff Schwab]

Re: Double -&gt; C-String conversion

John Harrison wrote:[color=blue]
> "Jeff Schwab" <jeffplus@comca st.net> wrote in message
> news:3oadncIDba TV0xHdRVn-uQ@comcast.com. ..
>[color=green]
>>Rolf Magnus wrote:
>>[color=darkred]
>>>Der Andere wrote:
>>>
>>>
>>>
>>>>During every iteration in a loop, I need to convert a double value
>>>>into a char*. Simple type-casting did not work, so I thought of using
>>>>stringstrea ms.
>>>
>>>
>>>Sounds as if you want a string representation of your double. A char* is
>>>not a string. It's a pointer to char, nothing less, nothing more.
>>>
>>>
>>>
>>>>However, the stream would have to be *emptied* after each iteration
>>>>(and I could not conceive of a practical way doing that).
>>>
>>>
>>>Do the following:
>>>
>>>thestream.st r("");[/color]
>>
>>Alternatively :
>>
>> thestream.str( ).clear( );[/color]
>
>
> That doesn't work. It clears the error flags of the stream. That's what
> clear does for any stream, it has nothing to do with emptying a string
> stream.
>
> john
>
>[/color]

No, John, you're wrong. You've mistaken the clear() method of the
stream for the clear() method of the underlying stream. The str()
method here returns a string, not a stream. This does clear the string
representation of the stream's contents, not the stream's error flags.
(Unfortunately, as Rolf pointed out, it clears only a temporary copy of
the string, so the stream is unaffected. I wonder, why isnt the
temporary string const?)

This is a great example of why unnecessary abbreviations are such a bad
idea when choosing identifiers.