0004345: Reduced precision when converting floats to strings

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Date Submitted

Last Update

0004345

OCaml

Camlp4

public

2007-07-17 11:46

2012-01-20 14:16

Reporter

brunodefraine

Assigned To

xclerc

Priority

normal

Severity

minor

Reproducibility

always

Status

resolved

Resolution

fixed

Platform

OS Version

Product Version

3.10.0

Target Version

Fixed in Version

3.12.1+dev

Summary

0004345: Reduced precision when converting floats to strings

Description

First, consider the top-level printing of values:

$ ocaml
# let pi = 3.14159265358979312 ;;
val pi : float = 3.14159265358979312
# float_of_string "3.14159265358979312" = pi ;;
- : bool = true

While:

$ ocaml camlp4rf.cma
# value pi = 3.14159265358979312 ;
value pi : float = 3.14159265359
# float_of_string "3.14159265359" = pi ;
- : bool = False

Similarly, consider `flo antiquotations:

$ ocaml camlp4rf.cma
# open Camlp4.PreCast;
# value _loc = Loc.ghost;
# value pi = 3.14159265358979312;
# <:expr< $`flo:pi$ >> ;
- : Camlp4.PreCast.Ast.expr =
Camlp4.PreCast.Ast.ExFlo <abstr> "3.14159265359"

While it might not be possible to produce string versions whose float_of_string value always exactly equals the original float value, the precision of the currently produced versions is far below the precision of Ocaml's float type.

Additional Information

This is probably related to the usage of string_of_float to produce the string versions. Consider the following alternative, located inside typing/oprint.ml, and probably used by OCaml's standard toplevel:

let float_repres f =
  match classify_float f with
    FP_nan -> "nan"
  | FP_infinite ->
      if f < 0.0 then "neg_infinity" else "infinity"
  | _ ->
      let s1 = Printf.sprintf "%.12g" f in
      if f = float_of_string s1 then valid_float_lexeme s1 else
      let s2 = Printf.sprintf "%.15g" f in
      if f = float_of_string s2 then valid_float_lexeme s2 else
      Printf.sprintf "%.18g" f

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Attached Files

Relationships

related to	0004767	closed	xclerc	Camlp4 prints infinity
related to	0001154	resolved		string_of_float loosing precision

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Issue History
Date Modified	Username	Field	Change
2007-07-17 11:46	brunodefraine	New Issue
2007-11-10 15:00	xleroy	Status	new => assigned
2007-11-10 15:00	xleroy	Assigned To	=> ertai
2011-02-08 15:05	xclerc	Assigned To	ertai => xclerc
2011-02-08 15:05	xclerc	Relationship added	related to 0004767
2011-02-08 15:08	xclerc	Status	assigned => resolved
2011-02-08 15:08	xclerc	Fixed in Version	=> 3.12.1+dev
2011-02-08 15:08	xclerc	Resolution	open => fixed
2012-01-20 14:16	doligez	Relationship added	related to 0001154