trying to find the error

[mdh]

K & R 5-5 asks for a strncat function ( concat n characters of t);

void mystrncat(char *s, char *t, int n)

{

while ( *s++); /* find end of s */ /* <<<<< 1 */

/* stops at '\0' */ /* <<<<<2 */

while ( *t && n-- 0)

*s++ = *t++; /* /*<<<<< 3 */

while ( n -- 0);
*s++ = '\0';

}

Now, with 1 I **Thought** that *s++ fails when *s == '\0', so that s
points to '\0'.
So, when 3 occurs, I thought the first char of t ( *t) is assigned to
the "Old" position of s, which should be '\0', but it is not.
What am I missing.
Thanks in advance.

[Eric Sosman]

Re: trying to find the error

mdh wrote:

K & R 5-5 asks for a strncat function ( concat n characters of t);
>
void mystrncat(char *s, char *t, int n)
>
{
>
while ( *s++); /* find end of s */ /* <<<<< 1 */
>
/* stops at '\0' */ /* <<<<<2 */
>
while ( *t && n-- 0)
>
*s++ = *t++; /* /*<<<<< 3 */
>
while ( n -- 0);
*s++ = '\0';
>
}
>
Now, with 1 I **Thought** that *s++ fails when *s == '\0', so that s
points to '\0'.
So, when 3 occurs, I thought the first char of t ( *t) is assigned to
the "Old" position of s, which should be '\0', but it is not.
What am I missing.

The loop stops when `s' points at '\0' *before* being
incremented, but the increment happens regardless of the
outcome of the test. After the loop, `s' points not at
the '\0', but at the character immediately after it.

--
Eric Sosman
esosman@ieee-dot-org.invalid

[mdh]

Re: trying to find the error

On May 1, 8:22 pm, Eric Sosman <esos...@ieee-dot-org.invalidwrot e:
, but the increment happens regardless of the

outcome of the test.

Ok...I understand. Thank you.

[mdh]

Re: trying to find the error

     The loop stops when `s' points at '\0' *before* being
incremented, but the increment happens regardless of the
outcome of the test.

May I just ask this. I recently had an extensive explanation about
this from one of the members of the group. Is this then a fair
statement about what happens.
When the loop fails, it is still proceeds to the next sequence point.
In other words, it is not like a "break" statement in a loop.
thanks

[Ian Collins]

Re: trying to find the error

mdh wrote:

> The loop stops when `s' points at '\0' *before* being
>incremented, but the increment happens regardless of the
>outcome of the test.

>
May I just ask this. I recently had an extensive explanation about
this from one of the members of the group. Is this then a fair
statement about what happens.
When the loop fails, it is still proceeds to the next sequence point.
In other words, it is not like a "break" statement in a loop.

The loop does not fail, the test yields false. The expression "s++" is
always evaluated before the result of the expression is tested.

If you want s to point to the end of the string, use

while( *s ) { s++; }

--
Ian Collins.

[mdh]

Re: trying to find the error

On May 1, 9:20 pm, Ian Collins <ian-n...@hotmail.co mwrote:

>
The loop does not fail, the test yields false.  The expression "s++" is
always evaluated before the result of the expression is tested.
>

thank you.

[jt]

Re: trying to find the error

On May 2, 9:39 am, mdh <m...@comcast.n etwrote:

On May 1, 9:20 pm, Ian Collins <ian-n...@hotmail.co mwrote:
>
>
>

The loop does not fail, the test yields false. The expression "s++" is
always evaluated before the result of the expression is tested.

>
thank you.

and moreover the unary operators are right to left associative.sin ce
here its post incrementation, s is incremented in the next statement.
probably you can overcome this by using

while(*s)s++;

[Ian Collins]

Re: trying to find the error

jt wrote:

On May 2, 9:39 am, mdh <m...@comcast.n etwrote:

>On May 1, 9:20 pm, Ian Collins <ian-n...@hotmail.co mwrote:
>>
>>
>>

>>The loop does not fail, the test yields false. The expression "s++" is
>>always evaluated before the result of the expression is tested.

>thank you.

>
and moreover the unary operators are right to left associative.sin ce
here its post incrementation, s is incremented in the next statement.

s++ is the statement, there isn't a next one.

probably you can overcome this by using
>
while(*s)s++;

Which is exactly what I wrote....

--
Ian Collins.

[rio]

Re: trying to find the error


"mdh" <mdeh@comcast.n etha scritto nel messaggio
news:75bdeebe-38ca-4219-9dab-c335a122aa41@b5 g2000pri.google groups.com...

>K & R 5-5 asks for a strncat function ( concat n characters of t);
>
void mystrncat(char *s, char *t, int n)
>
{
>
while ( *s++); /* find end of s */ /* <<<<< 1 */

if s==0 there is an error
if *s=='\0' there is an error

is it better? "if(s) {while(*s) ++s;}
else return 0;
"

/* stops at '\0' */ /* <<<<<2 */

while ( *t && n-- 0)

*s++ = *t++; /* /*<<<<< 3 */

why do you want the below and not a simple
"if(n-->0) *s=0;
else return error;"

while ( n -- 0);
*s++ = '\0';
>
}
>
Now, with 1 I **Thought** that *s++ fails when *s == '\0', so that s
points to '\0'.

this suggest to me that "while ( *s++); " is not the right loop

So, when 3 occurs, I thought the first char of t ( *t) is assigned to
the "Old" position of s, which should be '\0', but it is not.
What am I missing.
Thanks in advance.