set char to empty

Re: set char to empty

Magix wrote:
[color=blue]
> Hi,
>
> How do I set the char [ ] to empty ?
>
> let say I have,
> char test[20];[/color]

char test[20] = "";
if you want to initialize it to an empty string.
[color=blue]
> can I do like this :
> *test = NULL;[/color]

*test (or test[0]) is a char.
NULL is a pointer.
Thou shalt not assign pointer values to chars.

*test = 0;
test[0] = 0;

or any of several synonymous statements will do.

Re: set char to empty

On Mon, 2 Aug 2004 12:50:39 +0800, "Magix" <magix@asia.com > wrote:
[color=blue]
>Hi,
>
>How do I set the char [ ] to empty ?
>
>let say I have,
>char test[20];[/color]

You cannot set an array to empty. test is an array of 20 char. Each
of those 20 has a value (even if you don't know what the value is).

You can set the array to an empty (strlen = 0) string by setting
test[0] to '\0'. There are several ways to do this:

As part of the definition
char test[20] = "";
char test[20] = {'\0'};

Via an assignment
test[0] = '\0';

Via a function
strcpy(test,"") ;
memset(test,'\0 ',sizeof test);
[color=blue]
>
>can I do like this :
>*test = NULL;[/color]

No. You cannot place an array on the left of the assignment operator.


<<Remove the del for email>>

Re: set char to empty

Magix wrote on 02/08/04 :
[color=blue]
> How do I set the char [ ] to empty ?
>
> let say I have,
> char test[20];
>
> can I do like this :
> *test = NULL;[/color]

Where did you get that? Certainely not from a good C-book. 'test' being
an array, it's a constant pointer. It can't be modified.

What do you mean by 'empty'? An object has always a value. It is never
'empty'.

That said, a peculiar value could have the semantics of 'empty'. Its a
matter of convention. If you are talking about a string, it's
considered 'empty' or "" when you have a 0 at index 0.

char test[20];

test[0] = 0;

or
*test = 0;

or

strcpy (test, "");

because "" is hard coded

char anonymous[] = {0};

--
Emmanuel
The C-FAQ: http://www.eskimo.com/~scs/C-faq/faq.html

"C is a sharp tool"

Re: set char to empty


"Martin Ambuhl" <mambuhl@earthl ink.net> wrote in message
news:2n60itFsaj 2rU1@uni-berlin.de...[color=blue]
> Magix wrote:
>[color=green]
> > Hi,
> >
> > How do I set the char [ ] to empty ?
> >
> > let say I have,
> > char test[20];[/color]
>
> char test[20] = "";
> if you want to initialize it to an empty string.
>[color=green]
> > can I do like this :
> > *test = NULL;[/color]
>
> *test (or test[0]) is a char.
> NULL is a pointer.
> Thou shalt not assign pointer values to chars.
>
> *test = 0;
> test[0] = 0;
>
> or any of several synonymous statements will do.
>[/color]

I used test as string buffer. After strcat to test buffer in this process, i
will need to clear the buffer, and strcat another info to test buffer in
another process.

Re: set char to empty

Martin Ambuhl <mambuhl@earthl ink.net> wrote:
[color=blue]
> Magix wrote:
>[color=green]
> > can I do like this :
> > *test = NULL;[/color]
>
> *test (or test[0]) is a char.
> NULL is a pointer.
> Thou shalt not assign pointer values to chars.
>
> *test = 0;
> test[0] = 0;
>
> or any of several synonymous statements will do.[/color]

In particular, if you want to make it obvious that you're assigning a
char, not an int,

*test='\0';
or
test[0]='\0';

is completely equivalent, but looks more char-ish.

Richard

Re: set char to empty

Emmanuel Delahaye <emdel@yourbran oos.fr> scribbled the following:[color=blue]
> Magix wrote on 02/08/04 :[color=green]
>> How do I set the char [ ] to empty ?
>>
>> let say I have,
>> char test[20];
>>
>> can I do like this :
>> *test = NULL;[/color][/color]
[color=blue]
> Where did you get that? Certainely not from a good C-book. 'test' being
> an array, it's a constant pointer. It can't be modified.[/color]

He's modifying *test, not test. And arrays are not pointers.
[color=blue]
> What do you mean by 'empty'? An object has always a value. It is never
> 'empty'.[/color]
[color=blue]
> That said, a peculiar value could have the semantics of 'empty'. Its a
> matter of convention. If you are talking about a string, it's
> considered 'empty' or "" when you have a 0 at index 0.[/color]

This is true.

(Sample code snipped)

--
/-- Joona Palaste (palaste@cc.hel sinki.fi) ------------- Finland --------\
\-- http://www.helsinki.fi/~palaste --------------------- rules! --------/
"We sorcerers don't like to eat our words, so to say."
- Sparrowhawk

Re: set char to empty

Joona I Palaste wrote on 02/08/04 :[color=blue]
> Emmanuel Delahaye <emdel@yourbran oos.fr> scribbled the following:[color=green]
>> Magix wrote on 02/08/04 :[color=darkred]
>>> How do I set the char [ ] to empty ?
>>>
>>> let say I have,
>>> char test[20];
>>>
>>> can I do like this :
>>> *test = NULL;[/color][/color]
>[color=green]
>> Where did you get that? Certainely not from a good C-book. 'test' being
>> an array, it's a constant pointer. It can't be modified.[/color]
>
> He's modifying *test, not test. And arrays are not pointers.[/color]

Dammned. I missed the '*'...

--
Emmanuel
The C-FAQ: http://www.eskimo.com/~scs/C-faq/faq.html

"C is a sharp tool"

Re: set char to empty


"Emmanuel Delahaye" <emdel@YOURBRAn oos.fr> wrote in message
news:mn.12137d4 83a544452.15512 @YOURBRAnoos.fr ...[color=blue]
> Magix wrote on 02/08/04 :
>[color=green]
> > How do I set the char [ ] to empty ?
> >
> > let say I have,
> > char test[20];
> >
> > can I do like this :
> > *test = NULL;[/color]
>
> Where did you get that? Certainely not from a good C-book. 'test' being
> an array, it's a constant pointer. It can't be modified.
>
> What do you mean by 'empty'? An object has always a value. It is never
> 'empty'.
>[/color]
[color=blue]
> That said, a peculiar value could have the semantics of 'empty'. Its a
> matter of convention. If you are talking about a string, it's
> considered 'empty' or "" when you have a 0 at index 0.
>
> char test[20];
>
> test[0] = 0;
>
> or
> *test = 0;
>
> or
>
> strcpy (test, "");
>
> because "" is hard coded
>
> char anonymous[] = {0};
>
> --
> Emmanuel
> The C-FAQ: http://www.eskimo.com/~scs/C-faq/faq.html
>
> "C is a sharp tool"
>[/color]


Basical, I used it as string buffer. I want to clear the buffer for use of
another process.

let say:
char test[20];

strcat(test,"AB C");
...some routine
strcat(test, "DEF");
.... some routine
.... output test
// clear test buffer for another process
*test=0;
strcat(test, "GHI")
... some routine
strcat(test, "XYZ")
... output test

Re: set char to empty

Emmanuel Delahaye wrote on 02/08/04 :[color=blue]
> Magix wrote on 02/08/04 :
>[color=green]
>> How do I set the char [ ] to empty ?
>>
>> let say I have,
>> char test[20];
>>
>> can I do like this :
>> *test = NULL;[/color]
>
> Where did you get that? Certainely not from a good C-book. 'test' being an
> array, it's a constant pointer. It can't be modified.[/color]

FOrget that. I missed the '*'. That said, you should use 0 or '\0'
instead of NULL that has only a meaning in a pointer context.

--
Emmanuel
The C-FAQ: http://www.eskimo.com/~scs/C-faq/faq.html

"C is a sharp tool"

Re: set char to empty

Magix wrote on 02/08/04 :
[color=blue]
> Basical, I used it as string buffer. I want to clear the buffer for use of
> another process.
>
> let say:
> char test[20];
>
> strcat(test,"AB C");[/color]

Stop! You can't use strcat() on an uninitialized string.

add

*test = 0;

before the first strcat() call, or use strcpy() the first time...
[color=blue]
> ...some routine
> strcat(test, "DEF");
> ... some routine
> ... output test
> // clear test buffer for another process
> *test=0;[/color]

Ok.
[color=blue]
> strcat(test, "GHI")
> ... some routine
> strcat(test, "XYZ")
> ... output test[/color]

Ok. Just be sure that you don't overflow the string. strncat() can
help.

--
Emmanuel
The C-FAQ: http://www.eskimo.com/~scs/C-faq/faq.html

"C is a sharp tool"

Re: set char to empty

Magix wrote:
[color=blue]
> "Martin Ambuhl" <mambuhl@earthl ink.net> wrote in message
> news:2n60itFsaj 2rU1@uni-berlin.de...
>[color=green]
>>Magix wrote:
>>
>>[color=darkred]
>>>Hi,
>>>
>>>How do I set the char [ ] to empty ?
>>>
>>>let say I have,
>>>char test[20];[/color]
>>
>> char test[20] = "";
>>if you want to initialize it to an empty string.
>>
>>[color=darkred]
>>>can I do like this :
>>>*test = NULL;[/color]
>>
>> *test (or test[0]) is a char.
>> NULL is a pointer.
>> Thou shalt not assign pointer values to chars.
>>
>> *test = 0;
>> test[0] = 0;
>>
>>or any of several synonymous statements will do.
>>[/color]
>
>
> I used test as string buffer. After strcat to test buffer in this process, i
> will need to clear the buffer,[/color]

No, you don't. You just need to mark it with an immediate end-of-string
'\0'.
[color=blue]
> and strcat another info to test buffer in
> another process.[/color]

For which my above suggestions will work just fine.

Re: set char to empty


"Emmanuel Delahaye" <emdel@YOURBRAn oos.fr> wrote in message
news:mn.12447d4 880efb036.15512 @YOURBRAnoos.fr ...[color=blue]
> Magix wrote on 02/08/04 :
>[color=green]
> > Basical, I used it as string buffer. I want to clear the buffer for use[/color][/color]
of[color=blue][color=green]
> > another process.
> >
> > let say:
> > char test[20];
> >
> > strcat(test,"AB C");[/color]
>
> Stop! You can't use strcat() on an uninitialized string.
>
> add
>
> *test = 0;
>
> before the first strcat() call, or use strcpy() the first time...
>[color=green]
> > ...some routine
> > strcat(test, "DEF");
> > ... some routine
> > ... output test
> > // clear test buffer for another process
> > *test=0;[/color]
>
> Ok.
>[color=green]
> > strcat(test, "GHI")
> > ... some routine
> > strcat(test, "XYZ")
> > ... output test[/color]
>
> Ok. Just be sure that you don't overflow the string. strncat() can
> help.
>
> --
> Emmanuel
> The C-FAQ: http://www.eskimo.com/~scs/C-faq/faq.html
>
> "C is a sharp tool"
>[/color]

I initialise:
char test[20]="";

The first print out, I got ABCDEF
The second print out, I got GHIXYZABCDEF, which I expect to get GHIXYZ
Why ?

Re: set char to empty

Magix wrote on 02/08/04 :
[color=blue]
> I initialise:
> char test[20]="";[/color]

Good practice. Ok.
[color=blue][color=green][color=darkred]
>>> strcat(test,"AB C");[/color][/color][/color]
[color=blue][color=green][color=darkred]
>>> ...some routine
>>> strcat(test, "DEF");
>>> ... some routine
>>> ... output test
>>> // clear test buffer for another process
>>> *test=0;[/color]
>>[color=darkred]
>>> strcat(test, "GHI")
>>> ... some routine
>>> strcat(test, "XYZ")
>>> ... output test[/color][/color][/color]
[color=blue]
> The first print out, I got ABCDEF[/color]

Ok.
[color=blue]
> The second print out, I got GHIXYZABCDEF, which I expect to get GHIXYZ
> Why ?[/color]

Because something wrong probably occured in 'some routine'.

--
Emmanuel
The C-FAQ: http://www.eskimo.com/~scs/C-faq/faq.html

"C is a sharp tool"

Re: set char to empty

In article <mn.136d7d48e2f 54434.15512@YOU RBRAnoos.fr>,
Emmanuel Delahaye <emdel@YOURBRAn oos.fr> wrote:
[color=blue][color=green][color=darkred]
>>>> // clear test buffer for another process
>>>> *test=0;
>>>
>>>> strcat(test, "GHI")
>>>> ... some routine
>>>> strcat(test, "XYZ")
>>>> ... output test[/color][/color][/color]
[color=blue][color=green]
>> The second print out, I got GHIXYZABCDEF, which I expect to get GHIXYZ
>> Why ?[/color]
>
>Because something wrong probably occured in 'some routine'.[/color]

Although in theory undefined behaviour could do anything, it's hard to
see how in practice a string could end "DEF" immediately after
strcat(test, "XYZ").

What exactly is the output test? Just a printf?

-- Richard