[pointers and arrays]: The difference between an array name and a pointer

[iamchiaweilin]

Hello all:

What's the difference between p and q in the following statements?


char p[] = "Hello";
char *q = "Hello";

I know q stores the address of 'H'.
Question is: does p store the address of 'H' too?
I know p is the name of the array that contains "Hello". Is array name
a pointer?
In other words, is p exactly the same as &p[0]?

p and q are the same if you want to print them out by %s.
Is there a case where p and q (array name and pointer to the array)
can not be used interchangably? I knew there is, such as sizeof( ).

p.s. I remembered reading somewhere on the Net says that the statement
char *q = "Hello"
is not a good style of programming. coz you do not know whether q
points to a valid address or not.
Is it true?

 

[pemo]

Hello all:

What's the difference between p and q in the following statements?


char p[] = "Hello";
char *q = "Hello";

I know q stores the address of 'H'.
Question is: does p store the address of 'H' too?
I know p is the name of the array that contains "Hello". Is array name
a pointer?
In other words, is p exactly the same as &p[0]?

p and q are the same if you want to print them out by %s.
Is there a case where p and q (array name and pointer to the array)
can not be used interchangably? I knew there is, such as sizeof( ).

One difference is that *q = ? [or q[0] = ???] may not work - as "Hello" here
may *not* be writeable [readonly] - however *p = ? and p[0] = ? will work -
it's shorthand for char p[] = { 'H', 'e', ... };, i.e., simple array
initialisation. Anyway, yes, p stores the address of *its* 'H' as does q.

p.s. I remembered reading somewhere on the Net says that the statement
char *q = "Hello"
is not a good style of programming. coz you do not know whether q
points to a valid address or not.
Is it true?

No - if the code compiles, and there isn't some catastrophic runtime error,
then q points to the address where 'H' is stored.

 

[Lew Pitcher]

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1

Hello all:

What's the difference between p and q in the following statements?


char p[] = "Hello";
char *q = "Hello";

I know q stores the address of 'H'.
Question is: does p store the address of 'H' too?

No. p stores an array 6 of characters.

I know p is the name of the array that contains "Hello". Is array name
a pointer?

No, but it can often be treated /as if/ it were a pointer.

In other words, is p exactly the same as &p[0]?

It depends on the context.

sizeof(p) will return 6 (the length of the reserved space), and
sizeof (&p[0]) will return the size of a character pointer

but

*p is equal to 'H', and
*(&p[0]) is also equal to 'H'

p and q are the same if you want to print them out by %s.
Is there a case where p and q (array name and pointer to the array)
can not be used interchangably? I knew there is, such as sizeof( ).

So you already know that p is /not/ a pointer.

p.s. I remembered reading somewhere on the Net says that the statement
char *q = "Hello"
is not a good style of programming. coz you do not know whether q
points to a valid address or not.
Is it true?

Not necessarily. You left out a lot of context.

Often
char *q = "Hello";
is better style than
char p[] = "Hello";
when you want the string to not be modifiable (initially).

But, because neither variable is declared as non-modifiable,
p can be overwritten with a string of 0 to 5 characters + the EOS,
and
q can be overwritten with a pointer to something (either NULL or
a real object)

HTH
- --
Lew Pitcher

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.3 (MingW32) - WinPT 0.11.12

iD8DBQFFIUZ9agVFX4UWr64RAuMJAKDjlafFWoEsc2+YyZzrTjxYEKz4GACfULBM
+TThgTgF4qKJVzH9lGcNxoQ=
=Qd3q
-----END PGP SIGNATURE-----

 

[Scott Fluhrer]

Hello all:

What's the difference between p and q in the following statements?


char p[] = "Hello";

Here's a paraphrase of what this means: "Mr. Compiler Sir: Please create an
array of 6 char's, which I will refer to as 'p'. Also, make its initial
contents the chars 'H', 'e', 'l', 'l', 'o', 0. Oh, and I might just write
new values in the array, if I feel like it"

char *q = "Hello";

Here's a paraphrase of what this means: "Mr. Compiler Sir: Please create a
pointer to char, which I will refer to as 'q'. Also, make its initial
contents point to the first char of an unnamed array of 6 chars with the
initial contents 'H', 'e', 'l', 'l', 'o', 0. I promise not to write into
the unnamed array, but I might just point q somewhere else".

I know q stores the address of 'H'.
Question is: does p store the address of 'H' too?

No; p is an array, not a pointer.

I know p is the name of the array that contains "Hello". Is array name
a pointer?

No, it is not. In many contexts, an array name is automatically converted
into a pointer to the first element, but not all.

In other words, is p exactly the same as &p[0]?

No, not exactly. For example:
sizeof(p) == 6
sizeof(&p[0]) == sizeof (char *)

p and q are the same if you want to print them out by %s.
Is there a case where p and q (array name and pointer to the array)
can not be used interchangably? I knew there is, such as sizeof( ).

Here's two others:
p = "Bye!"; /* Error! */
q = "Bye!"; /* Works great! */
And:
p[0] = 'J'; /* Yummm! */
q[0] = 'J'; /* Undefined behavior (unless you've set q to point to a
writable char) */

p.s. I remembered reading somewhere on the Net says that the statement
char *q = "Hello"
is not a good style of programming. coz you do not know whether q
points to a valid address or not.
Is it true?

Nonsense. Look at the paraphrase of the definition of q I gave above --
you told the compiler to point q at a valid memory location (the first char
of the unnamed array).

 

[Richard Heathfield]

(e-mail address removed) said:

Hello all:

What's the difference between p and q in the following statements?


char p[] = "Hello";
char *q = "Hello";

p is an aggregate object, six bytes in size. sizeof p will confirm this. The
bytes contain 'H', 'e', 'l', 'l', 'o', and the null terminator character
'\0', in that order.

q is a pointer, its type being "pointer to char". Its value refers to the
place in memory where the string literal "Hello" (a six-character array
with static storage duration) is located.

I know q stores the address of 'H'.

That's a rather informal way of putting it. Specifically, it stores the
address of the particular 'H' that the compiler inserts somewhere in memory
and directly follows with 'e', 'l', 'l', 'o', and the null terminator
character '\0'.

Question is: does p store the address of 'H' too?
No.

I know p is the name of the array that contains "Hello". Is array name
a pointer?

No, but it is treated like one most of the time. Exceptions: sizeof p and &p

In other words, is p exactly the same as &p[0]?

In a value context, yes. Consider that p and *(p + i) mean the same
thing, recognise that & and * are complementary (and therefore cancel), and
set i to 0:

p means the same as *(p + i)
&p means the same as &*(p + i)
&p means the same as (p + i)
&p[0] means the same as (p + 0)
&p[0] means the same as (p )
&p[0] means the same as p

p and q are the same if you want to print them out by %s.

If you mean printf will produce the same output for either when you pass
them as a match for the %s format specifier, you're right.

Is there a case where p and q (array name and pointer to the array)
can not be used interchangably? I knew there is, such as sizeof( ).

It's sizeof, not sizeof(). Yes, the other case is &

p.s. I remembered reading somewhere on the Net says that the statement
char *q = "Hello"
is not a good style of programming.

Right. Better: const char *q = "Hello";

coz you do not know whether q points to a valid address or not.
Is it true?

No, the reason is that you mustn't modify the string literal. The const
helps the compiler to remind you not to do that.

 

[Richard Heathfield]

Scott Fluhrer said:

Hello all:

What's the difference between p and q in the following statements?


char p[] = "Hello";

Here's a paraphrase of what this means: "Mr. Compiler Sir: Please create
an
array of 6 char's, which I will refer to as 'p'. Also, make its initial
contents the chars 'H', 'e', 'l', 'l', 'o', 0. Oh, and I might just write
new values in the array, if I feel like it"

SCOTT! Ltnc! Welcome back.

 

[Scott Fluhrer]

Scott Fluhrer said:

Hello all:

What's the difference between p and q in the following statements?


char p[] = "Hello";

Here's a paraphrase of what this means: "Mr. Compiler Sir: Please create
an
array of 6 char's, which I will refer to as 'p'. Also, make its initial
contents the chars 'H', 'e', 'l', 'l', 'o', 0. Oh, and I might just
write
new values in the array, if I feel like it"

SCOTT! Ltnc! Welcome back.

Thanks. I've been busy with other things, but I'll be lurking from time to
time.

 

[Keith Thompson]

Hello all:

What's the difference between p and q in the following statements?


char p[] = "Hello";
char *q = "Hello";

Have you read section 6 of the comp.lang.c FAQ <http://www.c-faq.com/>?