In a world of humans and finance, it's quite common to need base-10 logarithms. (Yay for Math.log10!) In a world of computers, it's also quite common to need base-2 logarithms. Ruby only gives you 'natural' logarithms in the form of Math.log. (Bummer.) It has probably been more than 5 times that I've had to look up the formula to discover than log-base-n(x) == log-base-whatever(x) / log-base-whatever(n) I was reminded by it when Martin had to write his own log2 function to answer my previous question. Is there any objection to me proposing to add both: Math.log2(x) - base-2 logarithm Math.logn(x,n) - base-n logarithm directly to the core of Ruby? Ideally they'd be implemented in C, but of course the simple Ruby implementation is just: def Math.log2( x ); Math.log( x ) / Math.log( 2 ); end def Math.logn( x, n ); Math.log( x ) / Math.log( n ); end Since they're so simple, I'm almost hesitant to ask for them in core...except that they've cropped up for me again and again, and it always takes me a while to hunt them down. Languages like Lua thrive on simplicity of implementation. I think Ruby thrives on the richness of the immediately-available methods in core and stdlib. I feel these are common enough to deserve general inclusion.

on 2007-02-07 21:50

on 2007-02-07 23:57

perhaps it would be wiser to add def log_(n, x) log x / log n end so that any log base may be used. .... or you could just google the formula like any one else who didn't pay attention to math class.

on 2007-02-08 00:58

On Feb 7, 2007, at 2:50 PM, Phrogz wrote: > Ideally they'd be implemented in C, but of course the simple Ruby > implementation is just: > def Math.log2( x ); Math.log( x ) / Math.log( 2 ); end > def Math.logn( x, n ); Math.log( x ) / Math.log( n ); end I second the motion. The log2 function is now in the C math library, so I think it would easy to add it the Ruby Math library. I would like see log for arbitrary base added too. But here I would suggest a slightly different name and argument order: def Math.logb(b, x); Math.log(x) / Math.log(b); end This, I believe, would be more in accord with other programming languages that already offer this function. Regards, Morton

on 2007-02-08 01:07

```
On 2/8/07, Morton G. <removed_email_address@domain.invalid> wrote:
> easy to add it the Ruby Math library. I would like see log for
And while we're at it, I'd really like to see log aliased to ln
martin
```

on 2007-02-08 01:16

On Feb 7, 4:07 pm, "Martin DeMello" <removed_email_address@domain.invalid> wrote: > > > The log2 function is now in the C math library, so I think it would > > easy to add it the Ruby Math library. I would like see log for > > And while we're at it, I'd really like to see log aliased to ln Thank you, yes. I thought I was the only one for whom "log" implied base 2 or 10.

on 2007-02-08 01:21

On Feb 7, 2:57 pm, Keynan P. <removed_email_address@domain.invalid> wrote: > perhaps it would be wiser to add > > def log_(n, x) > log x / log n > end > > so that any log base may be used. Other than changing the order of parameters (which I'm personally ambivalent about, and would certainly accept this order if that's how most other libraries do it) and changing the name to end in an underscore (which I'm not sure I like), what are you proposing that I wasn't? Simply that we ONLY add the log-base-n capability that I described, without a specific method for log-base-2 calculations?

on 2007-02-08 02:09

On Feb 7, 2007, at 6:20 PM, Phrogz wrote: > ambivalent about, and would certainly accept this order if that's how > most other libraries do it) and changing the name to end in an > underscore (which I'm not sure I like), what are you proposing that I > wasn't? Simply that we ONLY add the log-base-n capability that I > described, without a specific method for log-base-2 calculations? Well, I can find examples with the base first or second. Perhaps for Ruby it would most idiomatic to define log as follows def Math.log(x, b=Math::E); ... ; end as the core method with log10 and log2 handled (for performance) as special cases within this method. Given that, I would still want Math to implement log10 and log2 methods for convenience. If we were to go with a two argument log, I prefer the "logb" name to "logn" because "n" suggests an integer and the base doesn't have to be an integer (e.g., it could be Math::E). Regards, Morton

on 2007-02-08 04:01

On Feb 7, 7:08 pm, Morton G. <removed_email_address@domain.invalid> wrote: > Well, I can find examples with the base first or second. Perhaps for > Ruby it would most idiomatic to define log as follows > > def Math.log(x, b=Math::E); ... ; end > > as the core method with log10 and log2 handled (for performance) as > special cases within this method. Given that, I would still want Math > to implement log10 and log2 methods for convenience. Math.log calls the log in the standard math library, which I suspect implements the natural log calculation pretty efficiently. If you make the second parameter the base, you'll either have to check it against Math::E and treat it as a special case or do two log calculations for the natural log case. I don't like either of those options. Eric

on 2007-02-08 07:01

On Feb 7, 2007, at 9:00 PM, Eric I. wrote: > Math.log calls the log in the standard math library, which I suspect > implements the natural log calculation pretty efficiently. If you > make the second parameter the base, you'll either have to check it > against Math::E and treat it as a special case or do two log > calculations for the natural log case. I don't like either of those > options. If there's too much of a performance penalty for this, I would have no problem with the general case being implemented under a different name, and I would suggest "logb" -- that is, def Math.logb(x, b=Math::E); ... ; end My main point is that I think the general case should default its base to Math::E. If we were to end up with Math.log (alias Math.ln), Math.log10, Math.log2, and Math.logb (the general case) that would be fine. Regards, Morton

on 2007-02-08 12:10

Hi, In message "Re: Adding Math.log2 and Math.logn to the core library." on Thu, 8 Feb 2007 09:08:38 +0900, Morton G. <removed_email_address@domain.invalid> writes: |Well, I can find examples with the base first or second. Perhaps for |Ruby it would most idiomatic to define log as follows | | def Math.log(x, b=Math::E); ... ; end 1.9 Math.log does work so. matz.