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cmultiplication

Multiplying two long numbers

I have tried to multiply to numbers i.e. 10000 and 10000 + 1 through C program. But I am not getting the correct output.

printf("%lld",(100000)*(100001));

I have tried the above code on different compilers but I am getting same 1410165408 instead of 10000100000.

Solution

  • Well, let's multiply

      int64_t a = 100000;
  int64_t b = 100001;
  int64_t c = a * b;

    And we'll get (binary)

         1001010100000011010110101010100000 /* 10000100000 decimal */

    but if you convert it to int32_t 

      int32_t d = (int32_t) c;

    you'll get the last 32 bits only (and throw away the top 10):

           01010100000011010110101010100000 /* 1410165408 decimal */

    A simplest way out, probably, is to declare both constants as 64-bit values (LL suffix stands for long long):

      printf("%lld",(100000LL)*(100001LL));