Unexpected result with right shift after bitwise negation





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18















I expected that below code will output 10 because (~port) equal to 10100101
So, when we right shift it by 4 we get 00001010 which is 10.
But the output is 250! Why?



int main()
{
uint8_t port = 0x5a;
uint8_t result_8 = (~port) >> 4;
//result_8 = result_8 >> 4;

printf("%i", result_8);

return 0;
}









share|improve this question































    18















    I expected that below code will output 10 because (~port) equal to 10100101
    So, when we right shift it by 4 we get 00001010 which is 10.
    But the output is 250! Why?



    int main()
    {
    uint8_t port = 0x5a;
    uint8_t result_8 = (~port) >> 4;
    //result_8 = result_8 >> 4;

    printf("%i", result_8);

    return 0;
    }









    share|improve this question



























      18












      18








      18


      4






      I expected that below code will output 10 because (~port) equal to 10100101
      So, when we right shift it by 4 we get 00001010 which is 10.
      But the output is 250! Why?



      int main()
      {
      uint8_t port = 0x5a;
      uint8_t result_8 = (~port) >> 4;
      //result_8 = result_8 >> 4;

      printf("%i", result_8);

      return 0;
      }









      share|improve this question
















      I expected that below code will output 10 because (~port) equal to 10100101
      So, when we right shift it by 4 we get 00001010 which is 10.
      But the output is 250! Why?



      int main()
      {
      uint8_t port = 0x5a;
      uint8_t result_8 = (~port) >> 4;
      //result_8 = result_8 >> 4;

      printf("%i", result_8);

      return 0;
      }






      c bit-manipulation






      share|improve this question















      share|improve this question













      share|improve this question




      share|improve this question








      edited Apr 15 at 1:02









      John Kugelman

      249k54407460




      249k54407460










      asked Apr 15 at 0:46









      IslamIslam

      1146




      1146
























          1 Answer
          1






          active

          oldest

          votes


















          27














          C promotes uint8_t to int before doing operations on it. So:





          1. port is promoted to signed integer 0x0000005a.


          2. ~ inverts it giving 0xffffffa5.

          3. An arithmetic shift returns 0xfffffffa.

          4. It's truncated back into a uint8_t giving 0xfa == 250.


          To fix that, either truncate the temporary result:



          uint8_t result_8 = (uint8_t)(~port) >> 4;


          mask it:



          uint8_t result_8 = (~port & 0xff) >> 4;


          or xor it (thanks @Nayuki!):



          uint8_t result_8 = (port ^ 0xff) >> 4;





          share|improve this answer


























          • you're right but i think C doesn't promote only uint8_t but also unsigned char because i tested it with unsigned char too and got the same result! Am i right?

            – Islam
            Apr 15 at 1:04






          • 12





            uint8_t is, very likely, a synonym of unsigned char on your system. The promotion rules apply to all integral types smaller than int.

            – ybungalobill
            Apr 15 at 1:07






          • 7





            Or explicitly only flip the low 8 bits: result = (port ^ 0xFF) >> 4;

            – Nayuki
            Apr 15 at 2:16











          • @Nayuki: that's a good one too!

            – ybungalobill
            Apr 15 at 2:26






          • 2





            @TomášZato: Yes. See en.cppreference.com/w/cpp/language/…

            – ybungalobill
            Apr 15 at 9:02














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          1 Answer
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          1 Answer
          1






          active

          oldest

          votes









          active

          oldest

          votes






          active

          oldest

          votes









          27














          C promotes uint8_t to int before doing operations on it. So:





          1. port is promoted to signed integer 0x0000005a.


          2. ~ inverts it giving 0xffffffa5.

          3. An arithmetic shift returns 0xfffffffa.

          4. It's truncated back into a uint8_t giving 0xfa == 250.


          To fix that, either truncate the temporary result:



          uint8_t result_8 = (uint8_t)(~port) >> 4;


          mask it:



          uint8_t result_8 = (~port & 0xff) >> 4;


          or xor it (thanks @Nayuki!):



          uint8_t result_8 = (port ^ 0xff) >> 4;





          share|improve this answer


























          • you're right but i think C doesn't promote only uint8_t but also unsigned char because i tested it with unsigned char too and got the same result! Am i right?

            – Islam
            Apr 15 at 1:04






          • 12





            uint8_t is, very likely, a synonym of unsigned char on your system. The promotion rules apply to all integral types smaller than int.

            – ybungalobill
            Apr 15 at 1:07






          • 7





            Or explicitly only flip the low 8 bits: result = (port ^ 0xFF) >> 4;

            – Nayuki
            Apr 15 at 2:16











          • @Nayuki: that's a good one too!

            – ybungalobill
            Apr 15 at 2:26






          • 2





            @TomášZato: Yes. See en.cppreference.com/w/cpp/language/…

            – ybungalobill
            Apr 15 at 9:02


















          27














          C promotes uint8_t to int before doing operations on it. So:





          1. port is promoted to signed integer 0x0000005a.


          2. ~ inverts it giving 0xffffffa5.

          3. An arithmetic shift returns 0xfffffffa.

          4. It's truncated back into a uint8_t giving 0xfa == 250.


          To fix that, either truncate the temporary result:



          uint8_t result_8 = (uint8_t)(~port) >> 4;


          mask it:



          uint8_t result_8 = (~port & 0xff) >> 4;


          or xor it (thanks @Nayuki!):



          uint8_t result_8 = (port ^ 0xff) >> 4;





          share|improve this answer


























          • you're right but i think C doesn't promote only uint8_t but also unsigned char because i tested it with unsigned char too and got the same result! Am i right?

            – Islam
            Apr 15 at 1:04






          • 12





            uint8_t is, very likely, a synonym of unsigned char on your system. The promotion rules apply to all integral types smaller than int.

            – ybungalobill
            Apr 15 at 1:07






          • 7





            Or explicitly only flip the low 8 bits: result = (port ^ 0xFF) >> 4;

            – Nayuki
            Apr 15 at 2:16











          • @Nayuki: that's a good one too!

            – ybungalobill
            Apr 15 at 2:26






          • 2





            @TomášZato: Yes. See en.cppreference.com/w/cpp/language/…

            – ybungalobill
            Apr 15 at 9:02
















          27












          27








          27







          C promotes uint8_t to int before doing operations on it. So:





          1. port is promoted to signed integer 0x0000005a.


          2. ~ inverts it giving 0xffffffa5.

          3. An arithmetic shift returns 0xfffffffa.

          4. It's truncated back into a uint8_t giving 0xfa == 250.


          To fix that, either truncate the temporary result:



          uint8_t result_8 = (uint8_t)(~port) >> 4;


          mask it:



          uint8_t result_8 = (~port & 0xff) >> 4;


          or xor it (thanks @Nayuki!):



          uint8_t result_8 = (port ^ 0xff) >> 4;





          share|improve this answer















          C promotes uint8_t to int before doing operations on it. So:





          1. port is promoted to signed integer 0x0000005a.


          2. ~ inverts it giving 0xffffffa5.

          3. An arithmetic shift returns 0xfffffffa.

          4. It's truncated back into a uint8_t giving 0xfa == 250.


          To fix that, either truncate the temporary result:



          uint8_t result_8 = (uint8_t)(~port) >> 4;


          mask it:



          uint8_t result_8 = (~port & 0xff) >> 4;


          or xor it (thanks @Nayuki!):



          uint8_t result_8 = (port ^ 0xff) >> 4;






          share|improve this answer














          share|improve this answer



          share|improve this answer








          edited Apr 15 at 5:37

























          answered Apr 15 at 0:50









          ybungalobillybungalobill

          46.6k1397163




          46.6k1397163













          • you're right but i think C doesn't promote only uint8_t but also unsigned char because i tested it with unsigned char too and got the same result! Am i right?

            – Islam
            Apr 15 at 1:04






          • 12





            uint8_t is, very likely, a synonym of unsigned char on your system. The promotion rules apply to all integral types smaller than int.

            – ybungalobill
            Apr 15 at 1:07






          • 7





            Or explicitly only flip the low 8 bits: result = (port ^ 0xFF) >> 4;

            – Nayuki
            Apr 15 at 2:16











          • @Nayuki: that's a good one too!

            – ybungalobill
            Apr 15 at 2:26






          • 2





            @TomášZato: Yes. See en.cppreference.com/w/cpp/language/…

            – ybungalobill
            Apr 15 at 9:02





















          • you're right but i think C doesn't promote only uint8_t but also unsigned char because i tested it with unsigned char too and got the same result! Am i right?

            – Islam
            Apr 15 at 1:04






          • 12





            uint8_t is, very likely, a synonym of unsigned char on your system. The promotion rules apply to all integral types smaller than int.

            – ybungalobill
            Apr 15 at 1:07






          • 7





            Or explicitly only flip the low 8 bits: result = (port ^ 0xFF) >> 4;

            – Nayuki
            Apr 15 at 2:16











          • @Nayuki: that's a good one too!

            – ybungalobill
            Apr 15 at 2:26






          • 2





            @TomášZato: Yes. See en.cppreference.com/w/cpp/language/…

            – ybungalobill
            Apr 15 at 9:02



















          you're right but i think C doesn't promote only uint8_t but also unsigned char because i tested it with unsigned char too and got the same result! Am i right?

          – Islam
          Apr 15 at 1:04





          you're right but i think C doesn't promote only uint8_t but also unsigned char because i tested it with unsigned char too and got the same result! Am i right?

          – Islam
          Apr 15 at 1:04




          12




          12





          uint8_t is, very likely, a synonym of unsigned char on your system. The promotion rules apply to all integral types smaller than int.

          – ybungalobill
          Apr 15 at 1:07





          uint8_t is, very likely, a synonym of unsigned char on your system. The promotion rules apply to all integral types smaller than int.

          – ybungalobill
          Apr 15 at 1:07




          7




          7





          Or explicitly only flip the low 8 bits: result = (port ^ 0xFF) >> 4;

          – Nayuki
          Apr 15 at 2:16





          Or explicitly only flip the low 8 bits: result = (port ^ 0xFF) >> 4;

          – Nayuki
          Apr 15 at 2:16













          @Nayuki: that's a good one too!

          – ybungalobill
          Apr 15 at 2:26





          @Nayuki: that's a good one too!

          – ybungalobill
          Apr 15 at 2:26




          2




          2





          @TomášZato: Yes. See en.cppreference.com/w/cpp/language/…

          – ybungalobill
          Apr 15 at 9:02







          @TomášZato: Yes. See en.cppreference.com/w/cpp/language/…

          – ybungalobill
          Apr 15 at 9:02






















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