From: Dave Burt <dave@...>
Date: 2006-03-22T20:13:50+09:00
Subject: Re: Round floats to N decimal places?

Joel VanderWerf wrote:
> Pat Maddox wrote:
>> I'm doing some math that results in floats with ~10 decimal places,
>> but I'd like to round them to 2 places.  Is there a built in way of
>> doing this?  Right now I'm doing format("%0.2f", the_float).to_f,
>> which seems to work fine but it seems like an ugly way of doing it.
> ...
> This might be good place to use a C extension, if you really need it to
> be fast.

Facets adds Numeric#round_at(d) and round_to(n).
http://facets.rubyforge.org/doc/api/classes/Float.html

Why not alter Ruby's core Numeric#round to accept an extra parameter, with 
the same function as Facets' round_at(d)? I've attached a patch (untested) 
below.

It seems to me it's fully backwards-compatible, hardly slower, and not at 
all confusing. Round is a good name for this function, and it isn't 
incongruent with the existing function of that method.

Cheers,
Dave

Untested patch against 1.8.2:
--- numeric.c~  Wed Mar 22 19:41:54 2006
+++ numeric.c   Wed Mar 22 22:01:47 2006
@@ -1274,17 +1274,35 @@
 flo_round(num)
     VALUE num;
 {
+    return flo_round2(0, NULL, num);
+}
+
+static VALUE
+flo_round2(argc, argv, num)
+    int argc;
+    VALUE *argv;
+    VALUE num;
+{
     double f = RFLOAT(num)->value;
     long val;
+    int dp;
+    double x;

-    if (f > 0.0) f = floor(f+0.5);
-    if (f < 0.0) f = ceil(f-0.5);
+    if (argc) {
+       rb_scan_args(argc, argv, "01", &dp);
+       x = pow(10.0, (double)dp);
+       return rb_float_new((int)(num * x) / x);
+    }
+    else {
+       if (f > 0.0) f = floor(f+0.5);
+       if (f < 0.0) f = ceil(f-0.5);

-    if (!FIXABLE(f)) {
-       return rb_dbl2big(f);
+       if (!FIXABLE(f)) {
+           return rb_dbl2big(f);
+       }
+       val = f;
+       return LONG2FIX(val);
     }
-    val = f;
-    return LONG2FIX(val);
 }

 /*
@@ -1372,6 +1390,15 @@
     return flo_round(rb_Float(num));
 }

+static VALUE
+num_round2(argc, argv, num)
+    int argc,
+       VALUE *argv,
+    VALUE num;
+{
+    return flo_round(argc, argv, rb_Float(num));
+}
+
 /*
  *  call-seq:
  *     num.truncate    => integer
@@ -2799,7 +2826,7 @@

     rb_define_method(rb_cNumeric, "floor", num_floor, 0);
     rb_define_method(rb_cNumeric, "ceil", num_ceil, 0);
-    rb_define_method(rb_cNumeric, "round", num_round, 0);
+    rb_define_method(rb_cNumeric, "round", num_round2, -1);
     rb_define_method(rb_cNumeric, "truncate", num_truncate, 0);
     rb_define_method(rb_cNumeric, "step", num_step, -1);

@@ -2819,7 +2846,6 @@
     rb_define_method(rb_cInteger, "to_int", int_to_i, 0);
     rb_define_method(rb_cInteger, "floor", int_to_i, 0);
     rb_define_method(rb_cInteger, "ceil", int_to_i, 0);
-    rb_define_method(rb_cInteger, "round", int_to_i, 0);
     rb_define_method(rb_cInteger, "truncate", int_to_i, 0);

     rb_cFixnum = rb_define_class("Fixnum", rb_cInteger);
@@ -2913,7 +2939,7 @@
     rb_define_method(rb_cFloat, "to_int", flo_truncate, 0);
     rb_define_method(rb_cFloat, "floor", flo_floor, 0);
     rb_define_method(rb_cFloat, "ceil", flo_ceil, 0);
-    rb_define_method(rb_cFloat, "round", flo_round, 0);
+    rb_define_method(rb_cFloat, "round", flo_round2, -1);
     rb_define_method(rb_cFloat, "truncate", flo_truncate, 0);

     rb_define_method(rb_cFloat, "nan?",      flo_is_nan_p, 0);