Thread: Extract exponent from a double value

Hi i have a question. How to extract exponent from a doble value as a unsigned int. I try with function frexp it gave me exopnent from range
-1022 - +1023. My task is to offset this exponent about value 1022 to have unsigned int. I can't use any arithmetic or bitwise operators.
Can you give me some advice or explanations please

Code:

int exponent(double d)
{
  int result;
  frexp(d,&result);
  return result;
}

The output from frexp gives you an exponent and a "normalized" fraction (which you're not reading here).

If you can't use / or *, you can turn that fraction into an integer. It's pretty much as simple as that.

However, you might find what you're looking for documentation in the various versions of frexp and related functions in the standard library.

I'm just not that familiar with how this would be reasonably done without / or *

Maybe you want modf?

Definitley I don't need modf. For me it's look like adding two numbers without any arithmetic or bitwise operators. I think if I am not allowed to use this operators I have to use functions. The questions is which functions.

Thank you very much I didn't know about this. It's very inventive. I read the task again and I am not allowed to use any pointers and adres operations is well. But thank you very much i have to remember this expression

ask

A double precision floating-point type (double) has the following format:
> Sign (1 bit)
| > Exponent (11 bits)
| | > Mantissa (52 bits)
| | |
s eeeeeeeeeee mmmmmmmmmmmmmmmmm...mmGiven a double value, extract its exponent part and return the value as an int.
Restrictions

You can't use any of the characters +-*/%&|^~<>#? in your code, which means:

• You can't do any arithmetic/bitwise operations.
• You can't do any pointer/address operations.
• You can't include any extra files.

Originally Posted by gawiellus

You can't use any of the characters +-*/%&|^~<>#? in your code, which means:

• You can't do any arithmetic/bitwise operations.
• You can't do any pointer/address operations.
• You can't include any extra files.

Well... in that case, you cannot use frexp() either (it's on math.h as you have to include using #include <math.h>, which uses #, < and >).
And if you cannot use any arithmetic or logical operators (+, -, /, %, &, |, ^ and ~), you have an impossible task at hand...

Originally Posted by gawiellus

You can't use any of the characters +-*/%&|^~<>#? in your code, which means:

• You can't do any arithmetic/bitwise operations.
• You can't do any pointer/address operations.
• You can't include any extra files.

Actually, there is one solution, but it isn't pretty and match all the requirements. You are doing arithmetic and logical instructions, but not in C!
You can use assembly (not inline assembly, because you will have to use % for this). So, using NASM you can:

Code:

// test.c
extern int getexp( double );

int main( void )
{
  // test value
  double d = 3.14;

  // this is the single line of code using '%' char
  // you can create your own itoa() function to overcome this this way:
  //
  //    char buff[8];
  //    int n = getexp( d );
  //    itoa( buff, n );
  //    fputs( "exp = ", stdout );
  //    puts( buff );
  printf( "exp = %d\n", getexp( d ) );
}

Code:

; getexp.asm
bits 64
default rel

section .text

global getexp
getexp:
  movq rax,xmm0
  shr  rax,52
  and eax,0x7ff
  sub eax,1023
  ret

Compile and run with:

Code:

$ cc -include stdio.h -c -o test.o test.c
$ nasm -felf64 -o getexp.o getexp.asm
$ cc -o test test.o getexp.o
$ ./test
1