Velocity Reviews > Convert a binary value to unsigned decimal value

# Convert a binary value to unsigned decimal value

Golan
Guest
Posts: n/a

 12-07-2003
Hi,
I need to convert a Binary value to Decimal. I've been told that the
value is an unsigned one. How can I do this?
I use memcpy into an unsigned char variable, but when I print the
value I got a negative value.
For example if I'm using the xd -c (Unix) on the file, I can see the
value FFFFFFFFFFFFFFA2 which using the memcpy as described above I get
-94.
But the real value that I'd expect to get is a positive one.

Thanks

Malcolm
Guest
Posts: n/a

 12-07-2003

"Golan" <(E-Mail Removed)> wrote in message
> I need to convert a Binary value to Decimal. I've been told that the
> value is an unsigned one. How can I do this?
>

Presumably you mean convert a machine representation value into something
The normal way to do this is
sprintf("%d", (int) x);
if the value is unsigned
sprintf("%u", (unsigned int) x);
However generally unsigned integer values are a BAD THING, use normal int
unless you really do need that extra bit.
>
> I use memcpy into an unsigned char variable, but when I print the
> value I got a negative value.
> For example if I'm using the xd -c (Unix) on the file, I can see the
> value FFFFFFFFFFFFFFA2 which using the memcpy as described
> above I get -94.
> But the real value that I'd expect to get is a positive one.
>

OK. It's almost certain that char on your machine is 8 bits. Something is
sign-extending the value to 64 bits. Since your value (0xA2) is greater than
127, a signed char with the same bit pattern would be negative.

memcpy() ing a value into an unsigned char doesn't sound like the way to go.
What is the data originally?

Another problem, I've told you to use sprintf(), but how does sprintf() work
internally? It's something like this.

void bintoascii(char *out, int x)
{
char *save;
char temp;

if(x < 0)
{
*out++ = '-';
x = -x;
}
/* handle 0 specially */
if(x == 0)
{
*out++ = '0';
*out = 0;
return;
}

save = out;
while(x)
{
*out++ = (x % 10) + '0';
x /= 10;
}
*out = 0;

out--;

/* reverse the number */
while(out > save)
{
temp = *out;
*out = *save;
*save = temp;
save++;
out--;
}

}

Mark McIntyre
Guest
Posts: n/a

 12-07-2003
On 7 Dec 2003 01:17:36 -0800, in comp.lang.c , http://www.velocityreviews.com/forums/(E-Mail Removed) (Golan)
wrote:

>Hi,
>I need to convert a Binary value to Decimal.

in computers,. all numbers are binary. If your "binary number" is
stored in a numerical data type then you don't need to do anything..

>I've been told that the
>value is an unsigned one. How can I do this?

is your "binary" data in a string? Or in a numeric type? Sounds like
the latter. So its already a number, you only need to printf it using
the right format specifier.

>I use memcpy into an unsigned char variable, but when I print the
>value I got a negative value.

if you printf an unsigned char array, you should get a string, not a
number !! What /are/ you doing?

>For example if I'm using the xd -c (Unix) on the file, I can see the
>value FFFFFFFFFFFFFFA2 which using the memcpy as described above I get
>-94.

Show us your code for goodness sake !
--
Mark McIntyre
CLC FAQ <http://www.eskimo.com/~scs/C-faq/top.html>

----== Posted via Newsfeed.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeed.com The #1 Newsgroup Service in the World! >100,000 Newsgroups
---= 19 East/West-Coast Specialized Servers - Total Privacy via Encryption =---

Guest
Posts: n/a

 12-07-2003
Mark McIntyre wrote:

> in computers,. all numbers are binary. [...]

<nit attribute="totally_useless_fact">

Not necessarily. The THROBAC design (as described in an article by C.E.
Shannon) uses Roman numerals internally. For same reason, nobody
attempted to target a C compiler at the platform

Which reminds me.... Did I already mention my desire for a Roman
numerals printf/scanf format specifier?

</nit>

Best regards,

Sidney

Nejat AYDIN
Guest
Posts: n/a

 12-07-2003
Malcolm wrote:
>
> "Golan" <(E-Mail Removed)> wrote in message
> > I need to convert a Binary value to Decimal. I've been told that the
> > value is an unsigned one. How can I do this?
> >

> Presumably you mean convert a machine representation value into something
> The normal way to do this is
> sprintf("%d", (int) x);

Where will the sprintf write to ? It must have been
sprintf(buf, "%d", (int)x);
where buf is an array of characters having enough space to hold decimal
string represantation of an int, plus the '\0'.

> if the value is unsigned
> sprintf("%u", (unsigned int) x);

Ditto

> However generally unsigned integer values are a BAD THING,

Why ?

> use normal int unless you really do need that extra bit.

> >
> > I use memcpy into an unsigned char variable, but when I print the
> > value I got a negative value.
> > For example if I'm using the xd -c (Unix) on the file, I can see the
> > value FFFFFFFFFFFFFFA2 which using the memcpy as described
> > above I get -94.
> > But the real value that I'd expect to get is a positive one.
> >

> OK. It's almost certain that char on your machine is 8 bits. Something is
> sign-extending the value to 64 bits. Since your value (0xA2) is greater than
> 127, a signed char with the same bit pattern would be negative.
>
> memcpy() ing a value into an unsigned char doesn't sound like the way to go.
> What is the data originally?
>
> Another problem, I've told you to use sprintf(), but how does sprintf() work
> internally? It's something like this.
>
> void bintoascii(char *out, int x)

The function has nothing special to ASCII, so the name of the function

> {
> char *save;
> char temp;
>
> /* add the minus */
> if(x < 0)
> {
> *out++ = '-';
> x = -x;

In a two's complement implementation, that does not work if x is equal
to INT_MIN. So this case must be handled specially.

> }
> /* handle 0 specially */
> if(x == 0)
> {
> *out++ = '0';
> *out = 0;
> return;
> }
>
> save = out;
> while(x)
> {
> *out++ = (x % 10) + '0';
> x /= 10;
> }
> *out = 0;
>
> out--;
>
> /* reverse the number */
> while(out > save)
> {
> temp = *out;
> *out = *save;
> *save = temp;
> save++;
> out--;
> }
>
> }

Mark McIntyre
Guest
Posts: n/a

 12-08-2003
On Sun, 07 Dec 2003 12:59:14 +0100, in comp.lang.c , Sidney Cadot
<(E-Mail Removed)> wrote:

>Mark McIntyre wrote:
>
>> in computers,. all numbers are binary. [...]

>
>Not necessarily. The THROBAC design (as described in an article by C.E.
>Shannon) uses Roman numerals internally. For same reason, nobody
>attempted to target a C compiler at the platform

I'll settle for "in computers of practical interest to C
programmers"...
>
>Which reminds me.... Did I already mention my desire for a Roman
>numerals printf/scanf format specifier?

I have discovered one, but there's not enough room in this margin to
write it all down.

></nit>

--
Mark McIntyre
CLC FAQ <http://www.eskimo.com/~scs/C-faq/top.html>

----== Posted via Newsfeed.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeed.com The #1 Newsgroup Service in the World! >100,000 Newsgroups
---= 19 East/West-Coast Specialized Servers - Total Privacy via Encryption =---

Golan
Guest
Posts: n/a

 12-08-2003
Thank you all for your help.
I think now I understand.

Malcolm
Guest
Posts: n/a

 12-08-2003

"Nejat AYDIN" <(E-Mail Removed)> wrote in message
>
> > However generally unsigned integer values are a BAD THING,

>
> Why ?
>

Imagine we are writing a program that contains a number of employees. It is
a typical novice move to think "The number of employees in a company cannot
be negative, so I'll use an unsigned int".

So we have
unsigned int N; /* number of employees */

Now it is very likely that we will want to iterate over the employee array.
Since N is unsigned, i must also be unsigned

unsigned int i;

for(i=0;i<N;i++)
employee[i].salary += 100;

Now the fun comes when we need to do the iteration in reverse

for(i=N-1;i>=0;i--)

whoops won't work.

This is just one of the niggly little problems you get when you allow
unsigned values. Of course it isn't a total disaster - an experienced
programmer would write the loop so that it works.

You also get problems when you mix singed and unsigned arithmetic, and it is
very difficult to avoid doing this if you use unsigned values regularly. Of
course you can use casts to suppress the compiler warnings, at risk of
casting away those warnings that point to real weaknesses in the code.
>
> > void bintoascii(char *out, int x)

>
> The function has nothing special to ASCII, so the name of the function
>

Like atoi().

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are Off Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post pozz C Programming 12 03-20-2011 11:32 PM Neal Software 0 03-20-2011 03:08 AM valpa Python 11 03-24-2009 07:11 AM Vitaliy Python 1 05-29-2008 10:36 AM makok VHDL 1 02-23-2004 06:04 PM