String reversing problem

[Albert]

Why doesn't:

#include <stdio.h>

void reverse(char[], int);

main()
{
char s[5];

s[0] = 'h';
s[1] = 'e';
s[2] = 'l';
s[3] = 'l';
s[4] = 'o';
reverse(s, 5);

for (int i=0; i<=4; i++)
putchar(s[i]);
return 0;
}

void reverse(char s[], int num_elements)
{
int i, j;

for (i=0,j=num_elem ents-1; (i<=num_element s-1) && (j>=0); i++,j--)
s[i] = s[j];
}

output:

olleh

?

[tmp123]

To Mr. Keith Thompson and Mr. Tim Rentsch:

Thanks for your replies, always open to learn something new.
The first comment mades me doubt: if strlen(s)==0, s==e, thus s!=e-- is
false and exit loop.
The second comments, about sequence points, well, I must recognize I do
not know what do you refer as "sequence points". An explanation or a
reference will be welcome.

To Mr. Christian Bau:

Your English seems not to be the most valid to be used in net, because
could be easily confused with agressive, specially by non-English
people. Moreover, it could be taken as a confusion between what is a
medium to interchange knowledgment, and what is a real programming
team.

Kind regards.

[Tim Rentsch]

"tmp123" <tm****@menta.n et> writes:

To Mr. Keith Thompson and Mr. Tim Rentsch:

Thanks for your replies, always open to learn something new.
The first comment mades me doubt: if strlen(s)==0, s==e, thus s!=e-- is
false and exit loop.

The problem is that the variable 'e' is decremented even if it is
equal to the address held in 's'. That means the decrement can
attempt to set 'e' to an address "before" the first element of
the array object holding the string, which is disallowed. It's
allowed to point to one element past the end of an array object,
but not allowed to point to one element before the beginning.

[Christian Bau]

In article <11************ **********@g49g 2000cwa.googleg roups.com>,
"tmp123" <tm****@menta.n et> wrote:

To Mr. Keith Thompson and Mr. Tim Rentsch:

Thanks for your replies, always open to learn something new.
The first comment mades me doubt: if strlen(s)==0, s==e, thus s!=e-- is
false and exit loop.
The second comments, about sequence points, well, I must recognize I do
not know what do you refer as "sequence points". An explanation or a
reference will be welcome.

To Mr. Christian Bau:

Your English seems not to be the most valid to be used in net, because
could be easily confused with agressive, specially by non-English
people. Moreover, it could be taken as a confusion between what is a
medium to interchange knowledgment, and what is a real programming
team.

Nothing wrong with my english. Lots wrong with your code. I don't care
too much about the undefined behavior, because you managed to write
completely incomprehensibl e code for a very simple task.

void reverse_string (char* s)
{
int i = 0;
int j = strlen (s) - 1;

while (i < j)
{
char tmp = s [i];
s [i] = s [j];
s [j] = tmp;
++i;
--j;
}
}

works and is easy to understand.

[tmp123]

Thanks for the explanation.

I do not know any compiler nor OS with problems for this pointer (it is
only assigned, not used, and probably the final address will be valid).
However, if you say that standard allows to pass the end of the array
but not point before, I can accept the reasoning.

Kind regards.

[Mark McIntyre]

On 30 Dec 2005 14:10:46 -0800, in comp.lang.c , "tmp123"
<tm****@menta.n et> wrote:

I do
not know what do you refer as "sequence points". An explanation or a
reference will be welcome.
©ISO/IEC ISO/IEC 9899:1999 (E)
Annex C
(informative)
Sequence points
1 The following are the sequence points described in 5.1.2.3:
— The call to a function, after the arguments have been evaluated
(6.5.2.2).
— The end of the first operand of the following operators: logical AND
&& (6.5.13);
logical OR || (6.5.14); conditional ? (6.5.15); comma , (6.5.17).
— The end of a full declarator: declarators (6.7.5);
— The end of a full expression: an initializer (6.7.8); the expression
in an expression
statement (6.8.3); the controlling expression of a selection statement
(if or switch)
(6.8.4); the controlling expression of a while or do statement
(6.8.5); each of the
expressions of a for statement (6.8.5.3); the expression in a return
statement
(6.8.6.4).
— Immediately before a library function returns (7.1.4).
— After the actions associated with each formatted input/output
function conversion
specifier (7.19.6, 7.24.2).
— Immediately before and immediately after each call to a comparison
function, and
also between any call to a comparison function and any movement of the
objects
passed as arguments to that call (7.20.5).

(Of Christian's comment)Your English seems not to be the most valid to be used in net,

Grow a thicker skin.

Personally I also thought the code was an abhomination and had I
discovered it in a project I was running I'd have told the programmer
to remove it forthwith.

Mark McIntyre
--

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[Mark McIntyre]

On 30 Dec 2005 15:04:08 -0800, in comp.lang.c , "tmp123"
<tm****@menta.n et> wrote:

Thanks for the explanation.

I do not know any compiler nor OS with problems for this pointer

Thats not relevant. If it breaks the rules of the C standard, then its
not guaranteed to work, and you should not do it. One day your code
will be run on an OS which does care, and you will be fired / lose
money / lose face or whatever because of your code error.

Mark McIntyre
--

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[Tim Rentsch]

"tmp123" <tm****@menta.n et> writes:

Thanks for the explanation.
You're welcome, glad it was of help.

I do not know any compiler nor OS with problems for this pointer (it is
only assigned, not used, and probably the final address will be valid).
In most practical cases it won't be a problem. However, the Standard
is quite unambiguous that it is potentially a problem, and there are
some implementations (I'm pretty sure) where it fails.

However, if you say that standard allows to pass the end of the array
but not point before, I can accept the reasoning.
One way to learn about these things is to get a copy of the
Standard, and read it yourself. There's an updated version
you can get just by downloading:

http://www.open-std.org/jtc1/sc22/wg...docs/n1124.pdf

If you get a copy then you can read up on sequence points or
whatever else (including array indexing and pointer arithmetic)
and many of the comments on comp.lang.c will make a lot more
sense.

Kind regards.

Likewise. And Happy New Year.

[Richard Heathfield]

tmp123 said:

Thanks for the explanation.

I do not know any compiler nor OS with problems for this pointer

The backroom boys are working on one right now, for all you know. And it
might just be tomorrow's sensational new toy for other, unrelated reasons.
And your boss might just say, "let's migrate all our code to this new
thing", as bosses often do. And at that point, the guys who stuck to the
rules will have working code, and the guys who didn't, won't.

So it pays to do things properly.

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at above domain (but drop the www, obviously)

[tmp123]

Hi,

See inlines:

Christian Bau wrote:

tmp123 wrote:

To Mr. Christian Bau:

Your English seems not to be the most valid to be used in net, because
could be easily confused with agressive, specially by non-English
people. Moreover, it could be taken as a confusion between what is a
medium to interchange knowledgment, and what is a real programming
team.
Nothing wrong with my english. Lots wrong with your code. I don't care
too much about the undefined behavior, because you managed to write
completely incomprehensibl e code for a very simple task.

Code that has been clearly state as "just for fun", that is, as an
academic experiment. At least, me, I get new knoledgment about pointer
before arrays and sequence points. Thanks to persons who have provided
it.

And taken into account that this piece of code has been presented as
"the good one" in a real programming team, some comments about:

void reverse_string (char* s)
{
int i = 0;
int j = strlen (s) - 1;
1) It is not the same initialize a variable as give a variable the
first value it will take.
2) Not always a stack to add variables is available, or to add more
variables to it. Sometimes only modify code is allowed (i.e: patching
firmware in real time systems without stop them).
3) Relation between names "i" and "j" and their meaning/usage is
totally lost.

while (i < j)
4) Never heard about "for" statement?. It is used to enclose in an easy
to read statement all control of the loop iterators (initialization s,
exit condition and state update).
{
char tmp = s [i];
5) Declare char here, far of the semantically parent of it (char *s) it
is only a way to hide things. And lots of compilers will ignore it (no
new frame).
s [i] = s [j];
s [j] = tmp;
6) It seems this code must always be compiled with latest version of
advanced compilers. The responsability to convert array index
calculations to pointer operations, even integers to pointers, is
transferred to compiler.
++i;
--j;
7) Lost lines here?
}
}

works and is easy to understand.

8) "Works" is not a measure of quality.

This is my last post in this subject. I don not like to be troll, nor
feed trolls.

Kind regards.

[slebetman]

tmp123 wrote:

Hi,

See inlines:

Christian Bau wrote:

tmp123 wrote:

To Mr. Christian Bau:

Your English seems not to be the most valid to be used in net, because
could be easily confused with agressive, specially by non-English
people. Moreover, it could be taken as a confusion between what is a
medium to interchange knowledgment, and what is a real programming
team.
Nothing wrong with my english. Lots wrong with your code. I don't care
too much about the undefined behavior, because you managed to write
completely incomprehensibl e code for a very simple task.

Code that has been clearly state as "just for fun", that is, as an
academic experiment. At least, me, I get new knoledgment about pointer
before arrays and sequence points. Thanks to persons who have provided
it.

"For fun" still doesn't make your code correct. You stated that it
works but the experts here have pointed out that even so it is still
not correct "C". So that doesn't invalidate Mark's comments - your code
still is horrible, learn and move on.

As for your comment about Mark's tone being aggressive, you have to
learn that this is Usenet, and taking on an aggressive tone is a
tradition of the net long before you showed up. Some groups liks
comp.lang.tcl may be less aggressive but comp.lang.c is aggressive (I
learned that the hard way). Maybe it's because so many people keep
asking the same stupid questions here over and over again (and keep
making the same stupid mistakes that others have made before).

void reverse_string (char* s)
{
int i = 0;
int j = strlen (s) - 1;

1) It is not the same initialize a variable as give a variable the
first value it will take.

What are you trying to say here? That code is correct.
2) Not always a stack to add variables is available, or to add more
variables to it. Sometimes only modify code is allowed (i.e: patching
firmware in real time systems without stop them).
I don't see how this is different from your code since you yourself
introduce the variable 'r'.
3) Relation between names "i" and "j" and their meaning/usage is
totally lost.
Lost? I understood it. The code is short and clear so it is
stylistically correct to use simple variable names (n, x, y, i, j
etc..). For that matter, the "relation between" 'r' and 's' in your
code and "their meaning/usage" also fall into the same category.

while (i < j)

4) Never heard about "for" statement?. It is used to enclose in an easy
to read statement all control of the loop iterators (initialization s,
exit condition and state update).

"Easy to read"? Certainly not your code. And in Mark's code "while" is
quite natural - use the right tool for the right job.

{
char tmp = s [i];

5) Declare char here, far of the semantically parent of it (char *s) it
is only a way to hide things. And lots of compilers will ignore it (no
new frame).

This is correct and valid in C99 (unlike your code which is incorrect
and invalid in any C standard). Just because there are no C99 compilers
around doesn't mean that this code is not "C".

s [i] = s [j];
s [j] = tmp;

6) It seems this code must always be compiled with latest version of
advanced compilers. The responsability to convert array index
calculations to pointer operations, even integers to pointers, is
transferred to compiler.

Nope, the above fragment of code will compile on any C compiler, I
suspect it is even valid in the original K&R "C" but I'm not sure.

works and is easy to understand.

8) "Works" is not a measure of quality.

True, as the experts here have pointed out about YOUR code. But "easy
to understand" IS a measure of quality.
This is my last post in this subject. I don not like to be troll, nor
feed trolls.

Mark have not been acting like a troll. He was merely scolding you for
writing poor code. You however are increasingly acting in a troll like
manner by insisting to argue even when you've been proven wrong.