BitWise Newbie - Needs Urgent Help

So how do you exactly do this ????

A function that zeroes the upper 4 bits of an unsigned char x and inverts its lower 4
bits.

EG : 170 binary is 10101010

Then result : 00000101

I managed until 00001010, but i unable to do the inversion to 00000101. Need urgent help, i'm going crazy with this lab tutorial ....

Code:

#include <stdio.h>

main()
{
      int i,c;
      unsigned int a, b[8];
      
      printf("\nEnter a decimal value: ");
      scanf("%d",&a);
      c=a;
      
      for(i=0;i<=7;i++)
      {
            b[i]=c%2;
            c=c/2;
      }

      printf("\nInput %d is ",a);
      for(i=7;i>=0;i--)
      {
            printf("%d",b[i]);
      }
      printf(" in binary.\n\n");
      for(i=7;i>=4;i--)
      {
          b[i] = b[i] >> 1;
      }      
      printf("Result :");
      for(i=7;i>=0;i--)
      {
            
            printf("%d", b[i]);
      }
      getchar();
      getchar();
}

It's easy - first do the invert, then the zeroing. Instead of inverting just the lower 4 bits, you can invert all of them, since the result is the same. BTW, an unsigned char is guaranteed only to have at least 8 bits, not exactly 8.

Study the ^ operator.

Originally Posted by robatino

It's easy - first do the invert, then the zeroing. Instead of inverting just the lower 4 bits, you can invert all of them, since the result is the same. BTW, an unsigned char is guaranteed only to have at least 8 bits, not exactly 8.

i have did the invert function but my output is mumbo jumbo...

i've tried this with b[i] = ~b[i]

it just comes out -1. i don know why....

is there any part of my program where i declared it wrong.

Originally Posted by Salem

Study the ^ operator.

i read 2 books on it already and i'm going crazy

can someone post me on the code on the inversion part based on my program...???

> it just comes out -1. i don know why....

You need to use %u to correctly print the value of an unsigned int, not %d which is for (signed) int. Try that.

Edit: The same goes for scanf(). Study carefully the conversion characters for printf() and scanf() for unsigned vs. signed types.

Originally Posted by robatino

> it just comes out -1. i don know why....

You need to use %u to correctly print the value of an unsigned int, not %d which is for (signed) int. Try that.

Edit: The same goes for scanf(). Study carefully the conversion characters for printf() and scanf() for unsigned vs. signed types.

i have used %u for print f and scanf still does not work.

when i tried the b[i] = ~b[i]

my output is 42531145614879798456 bla bla....

The maximum value of a 32-bit unsigned int (with no padding bits, don't worry about what those are right now) is 2^32 - 1 == 4294967295, which is what ~0U should be equal to in that case.

Originally Posted by robatino

The maximum value of a 32-bit unsigned int (with no padding bits, don't worry about what those are right now) is 2^32 - 1 == 4294967295, which is what ~0U should be equal to in that case.

i see your point on why i getting that very big number, but i don see where i did wrong.

printf will treat an unsigned char as a unsigned int, and if you do something like "printf("%d", ~b);", then you may get the unsigned int version of ~b, which would be a rather large number.

You can do the required stuff with one invert and one and operation, by the way.

--
Mats

a = a ^ 0x0f;
Rather giving the game away I'm afraid.

Originally Posted by Salem

a = a ^ 0x0f;
Rather giving the game away I'm afraid.

That inverts the lower 4 bits, but leaves the higher bits unchanged, instead of zeroing them.

have a try with the following sequence

Code:

unsigned char p=170;
p=~p;
p=p<<4;
p=p>>4;
printf("&#37;d \n",p);

or

Code:

p=~p;
p=p & 0x0f;

There's a simpler one-line solution involving ~, &, and 0x0f.

Edit: Actually, your original solution can be written on one line too. The OP was assuming that an unsigned char has exactly 8 bits, when it only has to be >= 8. Your solution inverts the lowest 4 bits and zeros the highest 4, while mine inverts the lowest 4 bits and zeros all the higher bits. If the number of bits is 8 they are equivalent.