Calculating the maximum value of integer

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January 26th, 2006, 07:56 AM

ravisinghhoney

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Calculating the maximum value of integer

This seems to be a simple question but i would like to know ideas to make a program to calculate the largest value of integer.There is the in built function max_int,but i would like to calculate this value without using max_int.

One way is just add on 1 to integer till we get a negative value but it is time consuming,i would like some ideas to do it more easily..
I hope the great minds would help

January 26th, 2006, 10:53 AM

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I am not sure why you want this but a possible way could be:

c Code:

Original - c Code
#include <stdio.h> #include <limits.h> int main() { int i,j; i = j = 0; for( ; j<(sizeof(int)*8)-1; j++ ) { i <<= 1; i \|= 1; } printf("%d\n%d\n",i,INT_MAX); return 0; }

#include <stdio.h>
#include <limits.h>
 
int main()
{
  int i,j;
 
  i = j = 0;
 
  for( ; j<(sizeof(int)*8)-1; j++ )
  {
    i <<= 1;
    i |= 1;
  }
 
  printf("%d\n%d\n",i,INT_MAX);
 
  return 0;
}

But this only works for 2's complement (and 1's complement), for details about that I have to refer you to Itsacon, he's the bit king in my opinion ;-)

January 26th, 2006, 02:47 PM

Itsacon

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Indeed we can do better

Note that the following only works on 2's complement systems, but you'll be hard pressed to find anything that isn't these days :-)

c Code:

Original - c Code

#include <stdio.h>
#include <limits.h>
 
int main()
{
    /*  Declarations  */
    int i;
    unsigned int j;
 
    /*  Do magic  */
    i = -1;   /*   First we fill i with all 1's   */
    j = 0 | i;    /*    we transfer this to j. Don't use j = i, since this might **** up the sign bit on some systems.  */
    j >>= 1;    /*  we make the highest bit zero. Has to be done on unsigned, otherwise a 1 gets shifted in   */
 
    /*  Output result */
    printf("%u\n%d\n",j ,INT_MAX);
 
    /*  Return succes */
    return 0;
}

Note that line 12 COULD be replaced with

Code:

j = (unsigned int) i;

But the behaviour for isn't documented, it might screw up the sign bit. By doing it this way, you're talking bits only and the system will surely not **** anything up.

Note that all can be done even shorter, setting j to zero and substracting 1 for example, but it's less neat (overflows and such) and if you wanted simple, you could just use MAX_INT :-)

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January 26th, 2006, 02:56 PM

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Hmm it seems the shift operator is already zero-filled.. So this also seems to work (at least on my box):

C Code:

Original - C Code
#include <stdio.h> #include <limits.h> int main() { /* Declarations / unsigned int j; / Do magic / j = -1; j >>= 1; / Output result / printf("%u\n%d\n",j ,INT_MAX); / Return succes */ return 0; }

#include <stdio.h>
#include <limits.h>
 
int main()
{
        /*      Declarations    */
        unsigned int j;
 
        /*      Do magic        */
        j = -1;
        j >>= 1;
 
        /*      Output result   */
        printf("%u\n%d\n",j ,INT_MAX);
 
        /*      Return succes   */
        return 0;
}

January 26th, 2006, 02:59 PM

Itsacon

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Because you're using an unsigned int.
Like I said, assigning -1 to an unsigned int works, but is undefined behaviour, and MIGHT go wrong. If you want to do weird stuff, be safe, right?

Comments on this post

Icon agrees: You're still the reigning bit king here

January 26th, 2006, 03:05 PM

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This is safe I think, it is the opposite of your example:

C Code:

Original - C Code
#include <stdio.h> #include <limits.h> int main() { /* Declarations / int j; / Do magic / j = 1; j <<= (sizeof(int)8-1); j = ~j; /* Output result / printf("%u\n%d\n",j ,INT_MAX); / Return succes */ return 0; }

#include <stdio.h>
#include <limits.h>
 
int main()
{
  /*  Declarations  */
  int j;
 
  /*  Do magic  */
  j = 1;
  j <<= (sizeof(int)*8-1);
  j = ~j;
 
  /*  Output result */
  printf("%u\n%d\n",j ,INT_MAX);
 
  /*  Return succes */
  return 0;
}

Comments on this post

Itsacon agrees: Well done, young padawan! :-D

January 26th, 2006, 05:29 PM

Itsacon

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Yeah, works too, though IMHO, using sizeof is the same as using MAX_INT
otherwise you could just do pow(2, (sizeof(int) * 8)) - 1 (which is the same as pow(256, sizeof(int)) - 1....

Getting bored yet?

January 27th, 2006, 01:30 PM

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Offcourse... Simple rewrites don't count..

Besides that, pow is a _much_ more expensive operation and you'll have to include math..

February 15th, 2006, 01:07 PM

UnderWaterman

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0111 1111
:-d

i would just subtract one from unsigned int

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