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converting fron unsiged char to int

P: n/a
I have an array of unsigned chars and i would like them converted to
an array of ints. What is the best way to do this? Using RedHat 7.3
on an Intel Pentium 4 machine. Having trouble here, hope someone can
help
Thanks
Jul 19 '05 #1
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4 Replies


P: n/a

"Joseph Suprenant" <la******@yahoo.com> wrote in message
news:28**************************@posting.google.c om...
I have an array of unsigned chars and i would like them converted to
an array of ints. What is the best way to do this? Using RedHat 7.3
on an Intel Pentium 4 machine. Having trouble here, hope someone can
help
Thanks


Have you considered a for loop?

unsigned char a[10];
int b[10];
for (int i = 0; i < 10; ++i)
b[i] = a[i];

john
Jul 19 '05 #2

P: n/a
J. Campbell wrote:

I'm no expert, so...listen to John Harrison. However, if you want the
bit pattern of the first 4-bytes of your char array to represent your
first int, then the next 4-bytes to represent the next int, the
following will work.

.
.
.
char* inBYTES;
.
//define your char array
.
unsigned long* inWORDS = (unsigned long*)inBYTES;


This is extremely dangerous and likely to fail. You have no guarantee
that a char* points to an address that is properly aligned for an
unsigned long. Also, this C-style cast is a bad idea. reinterpret_cast
would be slightly better.

There's also no guarantee that unsigned long is 4 bytes wide.

If this is, in fact, what the OP wants to do, the correct method would
look something like this:

char *bytes;
// ...
long val = 0;
for (int i=0; i<sizeof(long); ++i)
{
val = (val << CHAR_BIT) | bytes[i];
}

Of course, how this is done depends on exactly how the value is
represented in the array pointed to by 'bytes'. For example, the order
of the bytes, how many bytes are used to represent the value, etc.

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

Jul 19 '05 #3

P: n/a
J. Campbell wrote:
Kevin Goodsell <us*********************@neverbox.com> wrote in message news:<3f4148c4@shknews01>...
J. Campbell wrote:

I'm no expert, so...listen to John Harrison. However, if you want the
bit pattern of the first 4-bytes of your char array to represent your
first int, then the next 4-bytes to represent the next int, the
following will work.

.
.
.
char* inBYTES;
.
//define your char array
.
unsigned long* inWORDS = (unsigned long*)inBYTES;
This is extremely dangerous and likely to fail. You have no guarantee
that a char* points to an address that is properly aligned for an
unsigned long. Also, this C-style cast is a bad idea. reinterpret_cast
would be slightly better.

There's also no guarantee that unsigned long is 4 bytes wide.

If this is, in fact, what the OP wants to do, the correct method would
look something like this:

char *bytes;
// ...
long val = 0;
for (int i=0; i<sizeof(long); ++i)
{
val = (val << CHAR_BIT) | bytes[i];
}

Of course, how this is done depends on exactly how the value is
represented in the array pointed to by 'bytes'. For example, the order
of the bytes, how many bytes are used to represent the value, etc.

-Kevin


Kevin, thanks for the reply. This post is a bit off topic to the
original thread, but perhaps you can help me think about programming
in c++ terms.

I'm a physical scientist, not a computer programmer. I've been
writing programs for years to solve very narrowly-defined problems.
My old language was a compiled BASIC, QB 4.5. An example of the types
of things I use programming for, I once modified (hardware) an
instrument to take a different sort of measurement than it was
originally intended to take. The file that held the measurements
contained the data I was after, but it was buried in a bunch of binary
data that I didn't need, and the instrument-driving software wouldn't
allow me to extract the data I needed. I could get the data by hand
using a hex-editor, but it was extremely tedious and I had hundereds
of files to scour. To solve this problem, I made a program to read
the file scan the text header for the proper offsets, then go to these
offsets, get the binary data, convert it to ascii and output
deliminated data to a file that I could then use in plotting-type
software. I recently decided to learn C++ in order to avoid the slow
(on my old compiler) conversions like you outlined above where you
take each byte and shift it to the proper place in the integer then
add to the underlying value. As such, I thought "C++...groovy, I can
load the data, then create a pointer to whatever position I choose,
select the type of data I want, create a second pointer at that
location, then grab the data without performing *any* conversions."

I realize that this is non-portable, and that it's poor practice to
have 2 pointers to the same memory space, and I'm not trying to argue
that my method has any merit. I'm so used to thinking in terms of
"how to most efficiently get the results on the platform at hand" that
I feel like I'm missing the point of C++. Perhaps C++ is the wrong
language for me, since it was designed for large projects that need to
be maintained over time. Anyway, C++ is so much faster, that I can
probablly discard the notion that I need to look for efficiency
shortcuts.

=-+-=-+-=-+-=-+-=-+-=-+-=-+-=-+-=-+-=-+-=-+-=-+-=-+-=-+-=-+-=-+-=-+-=
One of the benefits of C and C++ is that you can cast a pointer
{or anytype) into almost any other type.
For example, read the data into a large unsigned char array.
Using a cast, you could convert the value at offset 4 into an integer:
int value = (int)(*ptr_to_buffer);
One issue to watch out for is byte ordering for multibyte quantities
otherwise known as Endianess.
=-+-=-+-=-+-=-+-=-+-=-+-=-+-=-+-=-+-=-+-=-+-=-+-=-+-=-+-=-+-=-+-=-+-=


My question is this. If you were writing code for your own use, that
would be unlikely to be used by anyone else, would you bother with
properly converting your bytes to ints in a platform independent
manner, or would you take the shortcut of simply loading the file,
then accessing it in it's native format? No, I don't worry about the conversions or sizes for most private
utilities that I write.

The only time I worry about the size of an integer variable is
when I'm dealing with the I/O of data or writing/reading to/from
hardware devices.

I have better things to do with my time than worrying about the
size of an integral or floating point variable. One of those
items is finishing the program on time, robust and of good
quality. If time or space is an issue (after the program works)
then I'll go back with a profiler and adjust the program as
necessary.

In your case, get your data parsing working correctly first.
Use simple techniques. After you have it working and need to
speed it up, you can use techniques such as buffer large
amounts of data.

Anyway, sorry for the rambling post, and thanks for the input.

No problem. Better entertainment than those people complaining
about being redirected.

--
Thomas Matthews

C++ newsgroup welcome message:
http://www.slack.net/~shiva/welcome.txt
C++ Faq: http://www.parashift.com/c++-faq-lite
C Faq: http://www.eskimo.com/~scs/c-faq/top.html
alt.comp.lang.learn.c-c++ faq:
http://www.raos.demon.uk/acllc-c++/faq.html
Other sites:
http://www.josuttis.com -- C++ STL Library book

Jul 19 '05 #4

P: n/a
J. Campbell wrote:

<snip>
I recently decided to learn C++ in order to avoid the slow
(on my old compiler) conversions like you outlined above where you
take each byte and shift it to the proper place in the integer then
add to the underlying value.
It really shouldn't be very slow. Besides that the "search for
efficiency" that many programmers feel is a core component of
programming is often misguided. There are many reasons for this. First,
speed simply isn't that important in most case - at least, not compared
to other factors such as correctness, portability, maintainability, and
getting the program done on time. Attempts to speed up code are often at
odds with these other goals. Second, most code is executed infrequently
enough that optimizing it down to nothing would not significantly
improve overall program performance - to be worth while, optimizations
have to be carefully targeted at the parts of the program that really
need it. Third, the efficiency bottlenecks in a program tend to be
things like I/O accesses, not the actual code itself. Fourth,
algorithm-level optimizations almost always give much, much more
dramatic improvements than micro-tuning code, so worrying about things
like a few shift operations is rather foolish.

Don't get me wrong - I don't like slow programs. But it's much more
important to address the overall design than to worry about the
efficiency of any particular section of code, particularly because you
don't know from the start which sections are going to be taking up the
program's execution time. By addressing design first, you can ensure
correctness, get the program running, and pave the way for optimizations
later on if the program is deemed too slow - if it's fast enough, you've
saved yourself the effort. Besides that, good design tends to lead to
reasonably efficient code in the first place.

Sorry about the barely-topical (for the thread) rant. It's one of those
things I'm always going off on.
As such, I thought "C++...groovy, I can
load the data, then create a pointer to whatever position I choose,
select the type of data I want, create a second pointer at that
location, then grab the data without performing *any* conversions."
I used to think that also. There are a number of problems with it,
however. First and foremost, different types may require different
memory alignment. A long might need to be on a 4-byte boundary, for
example, so if I try to access a char array as a long, and that char
array is not on a 4-byte boundary, the program's behavior is undefined.
This particular error results in a bus error (causing a crash) on some
systems. On Intel-bases systems I believe that improper alignment simply
causes your program to take a performance hit.

But that's just the first problem. You also have to worry about whether
the data in the array is the right format for the type you want to
interpret it as, if there's padding bytes, and things like that. Even if
all that checks out, the same data on a different platform won't work
the same way. Byte order can be different, data type sizes can vary, etc.

A few final notes about alignment: void * and char * are both capable of
representing any other object pointer type, and chars don't have
alignment requirements, so you can always access anything as an array of
chars without alignment problems (though unless you use unsigned chars
there is also a possible problem with invalid representations - unsigned
char is the only thing that is required to have none of those, and no
padding bits). Also, memory returned from malloc() is required to be
properly aligned for any type, so in theory it can be used as "common
ground" for any types, but this doesn't seem to be useful very often.

I realize that this is non-portable, and that it's poor practice to
have 2 pointers to the same memory space,
I don't know about 2 pointers to the same place being bad practice. I
can see how it could lead to problems in some cases, but I think such
problems are more a result of other things, such as not sufficiently
limiting the scope of objects, or poor memory management. Multiple
pointers to the same object is harmless by itself.
and I'm not trying to argue
that my method has any merit. I'm so used to thinking in terms of
"how to most efficiently get the results on the platform at hand" that
I feel like I'm missing the point of C++. Perhaps C++ is the wrong
language for me, since it was designed for large projects that need to
be maintained over time. Anyway, C++ is so much faster, that I can
probablly discard the notion that I need to look for efficiency
shortcuts.
In many cases, that is true. Shortcuts for efficiency are often
counter-productive anyway.

I think you'll have greater success with the language if you learn to
use the language itself, rather than learning "C++ for <insert system
name here>". Code relying on particular properties of a given system,
aside from being non-portable, tends to be more brittle as well.

My question is this. If you were writing code for your own use, that
would be unlikely to be used by anyone else, would you bother with
properly converting your bytes to ints in a platform independent
manner, or would you take the shortcut of simply loading the file,
then accessing it in it's native format?


It depends on how useful I expect the program to be. If it's a
quick-and-dirty program that will soon be discarded, it's likely that
I'd use the simplest method that I could think of, which may be
something like what you describe. For anything else, I'd do it the Right
Way, even if I'm only doing it for practice. I consider any and all
programming to be an opportunity to learn, helping me to become a better
programmer.

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

Jul 19 '05 #5

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