Integer subtraction problem, help!

[A. Sinan Unur]

Re: Integer subtraction problem, help!

bruce.james.lee @gmail.com wrote in news:1112571648 .404827.293590
@f14g2000cwb.go oglegroups.com:

dot@dot.dot wrote in news:geu051p9sl vauaj27vrohefv1 1sos1qb8c@4ax.c om:
[color=blue]
> Also look in your headers, if you have a limits.h header, it should
> provide a constant INT_MIN which will be the lowest legal value for
> your compiler. Use it instead of trying to define a new one...
>
> Yes, they are 32 bits. So, I calculated -2^31 as the minimum possible
> number.
>
> I just saw the header file you mentioned, it reads
>
> #define INT32_MIN (-2147483647-1)[/color]

The recommendation was to check the value of INT_MIN, not INT32_MIN.
INT_MIN should be in limits.h.

Sinan

--
A. Sinan Unur <1usa@llenroc.u de.invalid>
(reverse each component and remove .invalid for email address)

[bruce.james.lee@gmail.com]

Re: Integer subtraction problem, help!

Sinan
There is no INT_MIN defined in my limits.h

Bruce

[Peter Nilsson]

Re: Integer subtraction problem, help!

bruce.james.lee @gmail.com wrote:[color=blue]
> Sinan[/color]

[Please preserve the relevant context of what you're
replying to.]
[color=blue]
> There is no INT_MIN defined in my limits.h[/color]

Then either your implementation is non-conforming, or you
haven't invoked the options needed for conformance.

If the former, then you should consult a newsgroup
specific to your development tools.

That said, you can always define it yourself...

#define MINUS_0x8000000 0 (-0x7FFFFFFF-1)

int32_t x = MINUS_0x8000000 0;

--
Peter

[bruce.james.lee@gmail.com]

Re: Integer subtraction problem, help!

Peter,
I did define it myself
#define MINUS_0x8000000 0 (-0x7FFFFFFF-1)

and I directly used it in my code without doing the assignment you
mentioned (int32_t x = MINUS_0x8000000 0;).

That is working.

Thanks
Bruce

Peter Nilsson wrote:[color=blue]
> bruce.james.lee @gmail.com wrote:[color=green]
> > Sinan[/color]
>
> [Please preserve the relevant context of what you're
> replying to.]
>[color=green]
> > There is no INT_MIN defined in my limits.h[/color]
>
> Then either your implementation is non-conforming, or you
> haven't invoked the options needed for conformance.
>
> If the former, then you should consult a newsgroup
> specific to your development tools.
>
> That said, you can always define it yourself...
>
> #define MINUS_0x8000000 0 (-0x7FFFFFFF-1)
>
> int32_t x = MINUS_0x8000000 0;
>
> --
> Peter[/color]

[Mark McIntyre]

Re: Integer subtraction problem, help!

On 3 Apr 2005 15:54:51 -0700, in comp.lang.c ,
bruce.james.lee @gmail.com wrote:
[color=blue]
>Sinan,
>I dont think u got the problem. The value I want in 'a' is -2147483648,
>not -1.
>
>int a = -2147483648;
>will generate a warning.
>
>int a = 0x80000000;
>will also generate a warning.
>
>how to eliminate the warning, is my query.[/color]

You fail to tell us the error (please re-read Eric's advice, and post
your /actual/ code and /actual/ error messages) , but its probably
something like

warning C4146: unary minus operator applied to unsigned type, result
still unsigned

This problem has already been answered. USE A TYPE THAT IS LARGE
ENOUGH. You can't fit that number into an int (or even into a long,
probably) on your computer.

--
Mark McIntyre
CLC FAQ <http://www.eskimo.com/~scs/C-faq/top.html>
CLC readme: <http://www.ungerhu.com/jxh/clc.welcome.txt >

[Keith Thompson]

Re: Integer subtraction problem, help!

bruce.james.lee @gmail.com writes:[color=blue]
> There is no INT_MIN defined in my limits.h[/color]

Unlikely.

Did you look for the string "INT_MIN" in your limits.h file (perhaps
in /usr/include), or did you try to use it in a C program?

If the limits.h file itself doesn't define INT_MIN, it's probably
defined in another system-specific header file that limits.h includes.

Try this program:

#include <stdio.h>
#include <limits.h>

int main()
{
printf("INT_MIN = %d\n", INT_MIN);
return 0;
}

If it compiles and prints a value (most likely -2147483648, but
possibly -32768), then <limit.h> defines INT_MIN properly. Don't
worry about what's in the file. If the program doesn't compile,
something in your system is configured incorrectly.

The standard requires an implementation to provide a <limits.h>
header. It doesn't require it to be a file, and if it is a file it
doesn't have to be legible. It just has to work.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[Mark McIntyre]

Re: Integer subtraction problem, help!

On 3 Apr 2005 16:09:49 -0700, in comp.lang.c ,
bruce.james.lee @gmail.com wrote:
[color=blue]
>Its not a guess, I did enough googling and searched in google groups
>before asking here, and most said it is a 'split as an operator and
>positive number'. And it made sense, so I think it is right.[/color]

Okay, I see what it means. Yes, in that sense it is treated thus.

There's no way to stop it, this is how the C parser works, the "-" is
a unary operator and operates on its operand, in this case 2147483648.
Since that won't fit into an int, you can't use it.

But now we're back to what I said - you're trying to fit a quart into
a pint pot. Choose a variable thats the right size.

[color=blue][color=green][color=darkred]
>> >How to remove this warning? (a = -2147483647 - 1 looks dirty)[/color]
>>
>> use the right size variable?[/color][/color]



--
Mark McIntyre
CLC FAQ <http://www.eskimo.com/~scs/C-faq/top.html>
CLC readme: <http://www.ungerhu.com/jxh/clc.welcome.txt >

[Keith Thompson]

Re: Integer subtraction problem, help!

Mark McIntyre <markmcintyre@s pamcop.net> writes:[color=blue]
> On 3 Apr 2005 16:09:49 -0700, in comp.lang.c ,
> bruce.james.lee @gmail.com wrote:
>[color=green]
>>Its not a guess, I did enough googling and searched in google groups
>>before asking here, and most said it is a 'split as an operator and
>>positive number'. And it made sense, so I think it is right.[/color][/color]

What exactly is "it"? What is "split as an operator and positive number"?

A value stored in a variable is not split in this way; it's just a
value. I think what you're referring to is how to represent the value
as a C expression.
[color=blue]
> Okay, I see what it means. Yes, in that sense it is treated thus.
>
> There's no way to stop it, this is how the C parser works, the "-" is
> a unary operator and operates on its operand, in this case 2147483648.
> Since that won't fit into an int, you can't use it.
>
> But now we're back to what I said - you're trying to fit a quart into
> a pint pot. Choose a variable thats the right size.[/color]

In the following I'll assume that "int" is a 32-bit 2's-complement
type with no padding bits or trap repsentations. When I use the
phrase "numeric value", I'm talking about a mathematical integer
value, not a C expression or a value of any C type. Finally, I'll use
the term "integer literal" where the standard talks about "integer
constants", just to avoid confusion with the unrelated "const"
feature.

I think the point is that the OP wants to store the numeric value
-2147483648 in an int. Since this happens to be the value of INT_MIN,
it should be possible. Unfortunately, it's not as easy as you might
expect.

C has no negative integer literals, only non-negative ones. Something
like -42 is not a literal, it's a literal with a unary "-" operator
applied to it. In many contexts, this difference doesn't matter, but
you've found one where it does.

If -2147483648 were treated as a negative literal, then this:

int a = -2147483648;

would be ok. Instead, 2147483648 is a literal, and it can't be
represented as an int. Let's assume type "long int" is 64 bits,
making 2147483648 a literal of type long int with the obvious value.
Applying the "-" to this value yields a long int with the numeric
value -2147483648. Using this to initialize an int causes an implicit
conversion, and stores the expected value in a.

But in a C90 implementation, the longest integer might be only 32
bits. This means that 2147483648 is not a value of *any* integer
type. Attempting to use it in a program results in a warning.

The declaration

int a = -2147483648;

will still *probably* do what you expect, but there's no guarantee,
and you can expect the compiler to warn you that you're doing
something questionable.

An easy workaround is to use a slightly more complicated expression:

int a = -2147483647-1;

Both literals (with positive values) are within the range of type int,
and none of the intermediate expressions cause an overflow.

Out of the 4294967296 distinct values of a 32-bit 2's-complement
integer type, 42949672965 can be easily represented in C by either a
single integer literal, or by an integer literal with a unary "-"
operator; only one value cannot be represented this way. This is
arguably a hole in the language, but given that there's an easy
workaround, it's probably not worth adding a special-case feature to
correct it.

Some more things to keep in mind:

The standard doesn't guarantee that int is 32 bits; the minimum
guaranteed range is only -32767 .. +32767.

If you're assuming a 32-bit 2's-complement representation for int, you
can just use INT_MIN, defined in <limits.h>.

--
Keith Thompson (The_Other_Keit h) kst-u@mib.org <http://www.ghoti.net/~kst>
San Diego Supercomputer Center <*> <http://users.sdsc.edu/~kst>
We must do something. This is something. Therefore, we must do this.

[pete]

Re: Integer subtraction problem, help!

Keith Thompson wrote:[color=blue]
>
> bruce.james.lee @gmail.com writes:[color=green]
> > There is no INT_MIN defined in my limits.h[/color]
>
> Unlikely.
>
> Did you look for the string "INT_MIN" in your limits.h file (perhaps
> in /usr/include), or did you try to use it in a C program?
>
> If the limits.h file itself doesn't define INT_MIN, it's probably
> defined in another system-specific header file that limits.h includes.
>
> Try this program:
>
> #include <stdio.h>
> #include <limits.h>
>
> int main()
> {
> printf("INT_MIN = %d\n", INT_MIN);
> return 0;
> }[/color]

or this one:

/* BEGIN new.c */

#include <stdio.h>
#include <limits.h>

#define str(s) # s
#define xstr(s) str(s)

int main(void)
{
puts("INT_MIN = " xstr(INT_MIN));
return 0;
}

/* END new.c */


--
pete

[Joe Wright]

Re: Integer subtraction problem, help!

pete wrote:[color=blue]
> Keith Thompson wrote:
>[color=green]
>>bruce.james.l ee@gmail.com writes:
>>[color=darkred]
>>>There is no INT_MIN defined in my limits.h[/color]
>>
>>Unlikely.
>>
>>Did you look for the string "INT_MIN" in your limits.h file (perhaps
>>in /usr/include), or did you try to use it in a C program?
>>
>>If the limits.h file itself doesn't define INT_MIN, it's probably
>>defined in another system-specific header file that limits.h includes.
>>
>>Try this program:
>>
>>#include <stdio.h>
>>#include <limits.h>
>>
>>int main()
>>{
>> printf("INT_MIN = %d\n", INT_MIN);
>> return 0;
>>}[/color]
>
>
> or this one:
>
> /* BEGIN new.c */
>
> #include <stdio.h>
> #include <limits.h>
>
> #define str(s) # s
> #define xstr(s) str(s)
>
> int main(void)
> {
> puts("INT_MIN = " xstr(INT_MIN));
> return 0;
> }
>
> /* END new.c */
>
>[/color]
Neat. This yields ..

INT_MIN = (-2147483647-1)

... at my house. I'm gonna start looking at that # and ## stuff.

Thanks pete.
--
Joe Wright mailto:joewwrig ht@comcast.net
"Everything should be made as simple as possible, but not simpler."
--- Albert Einstein ---

[pete]

Re: Integer subtraction problem, help!

Joe Wright wrote:[color=blue]
>
> pete wrote:[color=green]
> > Keith Thompson wrote:
> >[color=darkred]
> >>bruce.james.l ee@gmail.com writes:
> >>
> >>>There is no INT_MIN defined in my limits.h
> >>
> >>Unlikely.
> >>
> >>Did you look for the string "INT_MIN" in your limits.h file (perhaps
> >>in /usr/include), or did you try to use it in a C program?
> >>
> >>If the limits.h file itself doesn't define INT_MIN, it's probably
> >>defined in another system-specific header file that
> >>limits.h includes.
> >>
> >>Try this program:
> >>
> >>#include <stdio.h>
> >>#include <limits.h>
> >>
> >>int main()
> >>{
> >> printf("INT_MIN = %d\n", INT_MIN);
> >> return 0;
> >>}[/color]
> >
> >
> > or this one:
> >
> > /* BEGIN new.c */
> >
> > #include <stdio.h>
> > #include <limits.h>
> >
> > #define str(s) # s
> > #define xstr(s) str(s)
> >
> > int main(void)
> > {
> > puts("INT_MIN = " xstr(INT_MIN));
> > return 0;
> > }
> >
> > /* END new.c */
> >
> >[/color]
> Neat. This yields ..
>
> INT_MIN = (-2147483647-1)
>
> .. at my house. I'm gonna start looking at that # and ## stuff.
>
> Thanks pete.[/color]

I'm glad you liked it.
It comes in handy if you're implementing something like this:

#define LENGTH 3
#define str(x) # x
#define xstr(x) str(x)

int rc;
char array[LENGTH + 1];

rc = scanf("%" xstr(LENGTH) "[^\n]%*[^\n]", array);
if (!feof(stdin)) {
getchar();
}

rc is 1 if any characters are read before the newline.
The text line is converted to a string in the array.
rc is 0 if the line consists of a newline character only.
Nothing is converted.
rc is EOF if you use fscanf instead of scanf
and read the end of a text file.

I think that converting strings to lines and lines to strings,
is a good way for a program to interact with a text file.
I used lower case xstr for the macro
because I copied it from the standard.

--
pete