[erlang-questions] math:pow(X, Y).

Matthias Lang <>
Tue Mar 20 13:47:47 CET 2012


On Tuesday, March 20, Michael Turner wrote:
> "As these are the C library, the bugs are the same."

> I'm not sure what the IEEE floating point spec says

The 'math' module doesn't mention IEEE. There was a mention of it in a
thread about NaN a few weeks ago:

  http://erlang.org/pipermail/erlang-questions/2012-February/064713.html

But anyway, I'm interested in understanding, and, to do that, just follow
the clues:

Clue #1: The man page says it's "just the C library"

Clue #2: The C library on your machine probably has documentation for
         what pow() does. On mine, it says:

   RETURN VALUE
       On success, these functions return the value of x to the power of y.

       If x is a finite value less than 0, and y is  a  finite  noninteger,  a
       domain error occurs, and a NaN is returned.

       If the result overflows, a range error occurs, and the functions return
       HUGE_VAL, HUGE_VALF, or HUGE_VALL, respectively,  with  the  mathematiâ
       cally correct sign.

       If  result  underflows, and is not representable, a range error occurs,
       and 0.0 is returned.

Clue #3: Take a look at the Erlang source. It's in erl_math.c, which
         is just 233 lines long, so nothing hard. Looking at it, it
         funnels all functions through math_call_2() or math_call_1(),
         and that checks that the output is finite (in the macro
         ERTS_FP_ERROR_THOROUGH).

Finished. If the underlying C function returns NaN or INF, then Erlang
is going to throw a badarith, and that's true for all the functions in
the math module.

There's one dangling thing: I expect to see badarith, but the pretty printed
error suggests a badarg:

  8> math:pow(20,900).
  ** exception error: bad argument in an arithmetic expression
       in function  math:pow/2
          called as math:pow(20,900)

The non-pretty one doesn't:

  9> catch math:pow(20,900).
  {'EXIT',{badarith,[{math,pow,[20,900],[]},

The pretty error comes from stdlib/src/lib.erl. Maybe "error evaluating
arithmetic expression" would cover things better.

Matt



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