Get element number of array when using pointers?

[Noah Spitzer-Williams]

Hello guys,

I'm itinerating through my array using pointers in this fashion:

image is unsigned char image[]

do {

cout << "image byte is: " << *image << endl;

while (image++);
Now my problem is sometimes i don't just want to do image++, i want
to jump around in image but to know where i want to jump to, i need to
know which element i'm on (the 1st element image[0], 3rd, 54th?)

Is there anyway I can do this? Basically what I'm trying to do is
this:

while (image = (image + skip) % some int's)

So as you see, the location where I want to jump relies on where I
currently am.

Thanks!

- Noah

[Kevin Goodsell]

Noah Spitzer-Williams wrote:

Hello guys,

I'm itinerating through my array using pointers in this fashion:

image is unsigned char image[]

do {

cout << "image byte is: " << *image << endl;

while (image++);
You said image is an array of unsigned char. Arrays are non-modifiable
lvalues, so you can't apply '++' to them. This is an error.


Now my problem is sometimes i don't just want to do image++, i want
to jump around in image but to know where i want to jump to, i need to
know which element i'm on (the 1st element image[0], 3rd, 54th?)
Given a pointer 'ptr' to some element in 'image', the index of the
element is given by (ptr - image).

Is there anyway I can do this? Basically what I'm trying to do is
this:

while (image = (image + skip) % some int's)

image + skip yields a pointer, which you cannot apply '%' to. You also
can't assign to image, since it is an array.

Are you sure you don't want to use a simple index? Seems like that would
make this much easier.

-Kevin
--
My email address is valid, but changes periodically.
To contact me please use the address from a recent posting.

[lallous]

Hi,

Why not directly access the image array as:
for (int i=0;image[i];i++)
{
cout << image[i];
}

"Noah Spitzer-Williams" <no****@cyberdu de.com> wrote in message
news:3d******** *************** **@posting.goog le.com...

Hello guys,

I'm itinerating through my array using pointers in this fashion:

image is unsigned char image[]

do {

cout << "image byte is: " << *image << endl;

while (image++);
Now my problem is sometimes i don't just want to do image++, i want
to jump around in image but to know where i want to jump to, i need to
know which element i'm on (the 1st element image[0], 3rd, 54th?)

Is there anyway I can do this? Basically what I'm trying to do is
this:

while (image = (image + skip) % some int's)

So as you see, the location where I want to jump relies on where I
currently am.

Thanks!

- Noah

[Kevin Goodsell]

lallous wrote:

Hi,

Please stop top-posting.

http://www.parashift.com/c++-faq-lit...t.html#faq-5.4

-Kevin
--
My email address is valid, but changes periodically.
To contact me please use the address from a recent posting.

[Michael Winter]

"Noah Spitzer-Williams" wrote on 23 Sept 03:

Hello guys,

I'm itinerating through my array using pointers in this fashion:

image is unsigned char image[]

do {

cout << "image byte is: " << *image << endl;

while (image++);
Now my problem is sometimes i don't just want to do image++, i want to jump around in image but to know where i want to jump to, i need to know which element i'm on (the 1st element image[0], 3rd, 54th?)

Is there anyway I can do this? Basically what I'm trying to do is
this:

while (image = (image + skip) % some int's)

So as you see, the location where I want to jump relies on where I currently am.

Thanks!

- Noah

As long as I understand you're situation correctly, it's simple.

// your array with 'n' elements:
unsigned char image[ n ];
// pointer you use to access the elements of 'image':
unsigned char* p = image;

// current location in array:
unsigned long i = 0;
p[ i ] = some_new_value;
// move to a different location by a
// fixed amount (limited to array bounds)
i = (i+skip_size) % n;
p[ i ] = some_other_valu e;
// call function that requires address
// of current array element
useAddress( p );

If you really don't need the pointers, or &image[ i ] will suffice
when you need an address, you could make the above cleaner by losing
the pointer and subscripting the array directly:

// your array with 'n' elements:
unsigned char image[ n ];

// current location in array:
unsigned long i = 0;
image[ i ] = some_new_value;
// move to a different location by a
// fixed amount (limited to array bounds)
i = (i+skip_size) % n;
image[ i ] = some_other_valu e;
// call function that requires address
// of current array element
useAddress( &image[ i ] );

I hope I understood the situation properly so this is actually
relevant,
Mike

--
Michael Winter
M.Winter@[no-spam]blueyonder.co.u k (remove [no-spam] to reply)

[lallous]

"Kevin Goodsell" <us************ *********@never box.com> wrote in message
news:qT******** *****@newsread4 .news.pas.earth link.net...

lallous wrote:

Hi,

Please stop top-posting.

http://www.parashift.com/c++-faq-lit...t.html#faq-5.4

-Kevin
--
My email address is valid, but changes periodically.
To contact me please use the address from a recent posting.

Is that any better?

--
Elias

[Noah Spitzer-Williams]

Because if image[i] = 0, it stops the for loop prematurely... even if
the actual element does exist

I don't know the size of image either.

The reason I was doing image++ is because it would go to the next
element in the array... it was working...

Thanks!

- Noah

"lallous" <la*****@lgwm.o rg> wrote in message news:<bk******* *****@ID-161723.news.uni-berlin.de>...

Hi,

Why not directly access the image array as:
for (int i=0;image[i];i++)
{
cout << image[i];
}

"Noah Spitzer-Williams" <no****@cyberdu de.com> wrote in message
news:3d******** *************** **@posting.goog le.com...

Hello guys,

I'm itinerating through my array using pointers in this fashion:

image is unsigned char image[]

do {

cout << "image byte is: " << *image << endl;

while (image++);
Now my problem is sometimes i don't just want to do image++, i want
to jump around in image but to know where i want to jump to, i need to
know which element i'm on (the 1st element image[0], 3rd, 54th?)

Is there anyway I can do this? Basically what I'm trying to do is
this:

while (image = (image + skip) % some int's)

So as you see, the location where I want to jump relies on where I
currently am.

Thanks!

- Noah

[Default User]

lallous wrote:

"Kevin Goodsell" <us************ *********@never box.com> wrote in message
news:qT******** *****@newsread4 .news.pas.earth link.net...

lallous wrote:

Hi,

Please stop top-posting.

http://www.parashift.com/c++-faq-lit...t.html#faq-5.4

-Kevin
--
My email address is valid, but changes periodically.
To contact me please use the address from a recent posting.

Is that any better?

Somewhat, but please snip extraneous material, especially quoted .sig
files.

Brian Rodenborn

[Noah Spitzer-Williams]

Hello Michael,

That all does work, however my problem was when I was looping
through each element. For example if I do:

for (int i = 0; image[i]; i++) {
// if image[i] == 0, the for loop exits when I haven't reached
the end of the image array
}

That is my major problem... How do I tell if I'm at the end of my
array or just at an element who's value is 0?

- Noah

"Michael Winter" <M.Winter@[no-spam]blueyonder.co.u k> wrote in message news:<k3******* **************@ news-text.cableinet. net>...

"Noah Spitzer-Williams" wrote on 23 Sept 03:

Hello guys,

I'm itinerating through my array using pointers in this fashion:

image is unsigned char image[]

do {

cout << "image byte is: " << *image << endl;

while (image++);
Now my problem is sometimes i don't just want to do image++, i

want

to jump around in image but to know where i want to jump to, i need

to

know which element i'm on (the 1st element image[0], 3rd, 54th?)

Is there anyway I can do this? Basically what I'm trying to do is
this:

while (image = (image + skip) % some int's)

So as you see, the location where I want to jump relies on where

I

currently am.

Thanks!

- Noah

As long as I understand you're situation correctly, it's simple.

// your array with 'n' elements:
unsigned char image[ n ];
// pointer you use to access the elements of 'image':
unsigned char* p = image;

// current location in array:
unsigned long i = 0;
p[ i ] = some_new_value;
// move to a different location by a
// fixed amount (limited to array bounds)
i = (i+skip_size) % n;
p[ i ] = some_other_valu e;
// call function that requires address
// of current array element
useAddress( p );

If you really don't need the pointers, or &image[ i ] will suffice
when you need an address, you could make the above cleaner by losing
the pointer and subscripting the array directly:

// your array with 'n' elements:
unsigned char image[ n ];

// current location in array:
unsigned long i = 0;
image[ i ] = some_new_value;
// move to a different location by a
// fixed amount (limited to array bounds)
i = (i+skip_size) % n;
image[ i ] = some_other_valu e;
// call function that requires address
// of current array element
useAddress( &image[ i ] );

I hope I understood the situation properly so this is actually
relevant,
Mike

[Kevin Goodsell]

Noah Spitzer-Williams wrote:

Hello Michael,

<snip>

Please stop top-posting.

http://www.parashift.com/c++-faq-lite/how-to-post.html
http://www.parashift.com/c++-faq-lit...t.html#faq-5.4

-Kevin
--
My email address is valid, but changes periodically.
To contact me please use the address from a recent posting.