Mutually-Recursive Functions

Ruby doesn’t seem to do mutually-recursive functions. Or is it some
particular idiom I know not about? I am using an array to implement
them, and it stinks a bit. Any way to do it?

Thanks.

Revence

Revence K. wrote:

Ruby doesn’t seem to do mutually-recursive functions. Or is it some
particular idiom I know not about? I am using an array to implement
them, and it stinks a bit. Any way to do it?

Thanks.

Revence

Could you show us what you mean? I for one do not know what a mutually
recursive function is.

Dan

Dan Z. wrote:

Revence K. wrote:

Ruby doesn’t seem to do mutually-recursive functions. Or is it some
particular idiom I know not about? I am using an array to implement
them, and it stinks a bit. Any way to do it?

Could you show us what you mean? I for one do not know what a mutually
recursive function is.
I have written it all out. Now, since I’m fresh to this list, I dunno if
attachments show. Indulge me, please. I’ll paste all the code (it’s a
bit literate’) and also attach it (as mutrec.rb). :slight_smile:
By the way, ruby -v is: 1.8.5

#! /usr/bin/ruby

Two examples of a mutually-recursive function.

These are functions that depend on each other - recursively.

even and odd:

If a number is zero, then it is even. Otherwise, it is even if the

one before it is odd.

A number is odd if it is not zero. Otherwise, it is odd if it the one

before it is even. :o)

This is the canonical example.

def my_even(x)
x == 0 or odd(x - 1)
end

def my_odd(x)
x != 0 and even(x - 1)
end

That is, of course, a silly way to implemet even and odd. So, let me

give you the problem

I was trying to solve.

To generate the lettering similar to that above the spreadsheet

columns, where the first

column has ‘A’, the second ‘B’ … the twenty-sixth ‘Z’. Then, the

twenty-seventh has

‘AA’, the twenty-eight has ‘AB’, and so on. This is recursive by

nature. Here is what

should work:

Letters = [‘A’, ‘B’, ‘C’, ‘D’, ‘E’, ‘F’, ‘G’, ‘H’, ‘I’, ‘J’,
‘K’, ‘L’, ‘M’, ‘N’, ‘O’, ‘P’, ‘Q’, ‘R’, ‘S’, ‘T’,
‘U’, ‘V’, ‘W’, ‘X’, ‘Y’, ‘Z’]

def alphabetise(pos)
pile_letters(0, pos) # LABEL: 1
end
def pile_chars(thus_far, pos)
len = Letters.length
if pos < len then
Letters[pos]
else
alphabeticise(thus_far) + pile_chars(thus_far + 1, pos - len)
end

end

Labels:

1. That’s where it fails. I guess 'tis because the scope is searched

backwards

from that point.

OCaml, for example, requires me to put the `and’ keyword between

mutually-recursive

functions. I’d not mind doing that in Ruby, but I don’t know how.

n00b. :wink:

Also, it works in That Other Language.

Now, the (dirty) solution I found. I think you will help me with how

to get it done

without all this voodoo:

Funcs = [lambda do |pos|
Funcs[1].call(0, pos)
end,
lambda do |thus_far, pos|
len = Letters.length
if pos < len then
Letters[pos]
else
Funcs[0].call(thus_far) + Funcs[1].call(thus_far + 1, pos - len)
end
end]

def main(args)
puts Funcs[0].call((26 * 26) - 1) # Prints A to Z. But see how
un-Rubyistic it is!

puts alphabetise(675) # This fails.

if my_even 4 and my_odd 5 then 0 else 1 end # This also fails

0
end

exit(main(ARGV))

For the even and odd example, it works if you remplace odd by my_odd in
the my_even function, and even by my_even in the my_odd function. Like
this:

def my_even(x)
x == 0 or my_odd(x - 1)
end

def my_odd(x)
x != 0 and my_even(x - 1)
end

my_even(10) #=> true

Hope this helps.

Bruno M. wrote:

end

my_even(10) #=> true

Hope this helps.

Yeah. Solved it. Lots of typos. That happens under stress. Thanks a lot,
everyone. :slight_smile:

Well, actually as far as I can tell your even / odd functions principle
works a treat. There’s just a small bug :
def my_even(x)
x == 0 or odd(x - 1)
end

def my_odd(x)
x != 0 and even(x - 1)
end

The name my_even and even doesn’t match, as my_odd and odd, so if u do
something like that :
def even?(x)
x == 0 or odd?(x - 1)
end

def odd?(x)
x != 0 and even?(x - 1)
end

It should work.

On 6/7/07, Revence K. [email protected] wrote:

x != 0 and my_even(x - 1)
end

my_even(10) #=> true

Hope this helps.

Yeah. Solved it. Lots of typos. That happens under stress. Thanks a lot,
everyone. :slight_smile:

For this very case, String#succ and String.succ! might be helpful
(‘A’.succ -> ‘B’, ‘Z’.succ -> ‘AA’).

Hi –

On Thu, 7 Jun 2007, Harry K. wrote:

‘AA’, the twenty-eight has ‘AB’, and so on. This is recursive by

arr = []
(“a”…“ff”).each {|x| arr << x}
p arr

Work-saving tip for the day:

p [*“a”…“ff”]

:slight_smile:

David

On 6/7/07, Revence K. [email protected] wrote:

I may be missing something, but…

If you just want to do that then try something like this.

arr = []
(“a”…“ff”).each {|x| arr << x}
p arr

Harry

A Look into Japanese Ruby List in English
http://www.kakueki.com/

On 6/7/07, [email protected] [email protected] wrote:

(“a”…“ff”).each {|x| arr << x}


Q. What is THE Ruby book for Rails developers?
A. RUBY FOR RAILS by David A. Black (http://www.manning.com/black)
(See what readers are saying! http://www.rubypal.com/r4rrevs.pdf)
Q. Where can I get Ruby/Rails on-site training, consulting, coaching?
A. Ruby Power and Light, LLC (http://www.rubypal.com)

I was going to do this.

p (“a”…“ff”).each {|x| p x}

But I didn’t want it to run off the screen.
But you did the same thing in a better way.
Thanks for the tip.

Harry

A Look into Japanese Ruby List in English
http://www.kakueki.com/

On Jun 7, 2007, at 5:06 AM, [email protected] wrote:

spreadsheet columns, where the first
arr = []
(“a”…“ff”).each {|x| arr << x}
p arr

Work-saving tip for the day:

p [*“a”…“ff”]

:slight_smile:

seems a bit perlish no?

cfp:~ > cat a.rb
p (‘a’ … ‘ff’).to_a

cfp:~ > ruby a.rb
[“a”, “b”, “c”, “d”, “e”, “f”, “g”, “h”, “i”, “j”, “k”, “l”, “m”,
“n”, “o”, “p”, “q”, “r”, “s”, “t”, “u”, “v”, “w”, “x”, “y”, “z”,
“aa”, “ab”, “ac”, “ad”, “ae”, “af”, “ag”, “ah”, “ai”, “aj”, “ak”,
“al”, “am”, “an”, “ao”, “ap”, “aq”, “ar”, “as”, “at”, “au”, “av”,
“aw”, “ax”, “ay”, “az”, “ba”, “bb”, “bc”, “bd”, “be”, “bf”, “bg”,
“bh”, “bi”, “bj”, “bk”, “bl”, “bm”, “bn”, “bo”, “bp”, “bq”, “br”,
“bs”, “bt”, “bu”, “bv”, “bw”, “bx”, “by”, “bz”, “ca”, “cb”, “cc”,
“cd”, “ce”, “cf”, “cg”, “ch”, “ci”, “cj”, “ck”, “cl”, “cm”, “cn”,
“co”, “cp”, “cq”, “cr”, “cs”, “ct”, “cu”, “cv”, “cw”, “cx”, “cy”,
“cz”, “da”, “db”, “dc”, “dd”, “de”, “df”, “dg”, “dh”, “di”, “dj”,
“dk”, “dl”, “dm”, “dn”, “do”, “dp”, “dq”, “dr”, “ds”, “dt”, “du”,
“dv”, “dw”, “dx”, “dy”, “dz”, “ea”, “eb”, “ec”, “ed”, “ee”, “ef”,
“eg”, “eh”, “ei”, “ej”, “ek”, “el”, “em”, “en”, “eo”, “ep”, “eq”,
“er”, “es”, “et”, “eu”, “ev”, “ew”, “ex”, “ey”, “ez”, “fa”, “fb”,
“fc”, “fd”, “fe”, “ff”]

:wink:

-a

Hi –

On Thu, 7 Jun 2007, ara.t.howard wrote:

I was trying to solve.

:slight_smile:

seems a bit perlish no?

No, since you asked :slight_smile: It looks like that thing in Ruby where you
use the unar[r]ay operator to unarray a range :slight_smile:

David

On 6/7/07, Harry K. [email protected] wrote:

p (“a”…“ff”).each {|x| p x}

Harry

Oops.
Don’t need 2 p’s.

(“a”…“ff”).each {|x| p x}

Harry

A Look into Japanese Ruby List in English
http://www.kakueki.com/

p [*“a”…“ff”]

David Black’s solution above looks like the best for Ruby (hey, he
wrote the Ruby for Rails book :slight_smile:

If Ruby didn’t have that ability to enumerate from “a” to “ff” (I know
it does), this one would also work (in Ruby as well as in other
languages like Python, with suitable changes for their different
syntax):

labels = []
[“a”…“z”].each { |c| labels << c }
[“a”…“z”].each do |c|
[“a”…“z”].each { |c2| labels << (c + c2) }
end

Now do whatever you want with the array ‘labels’.]

( I’m not at my PC right now, so can’t check if its quite right. )

Vasudev Ram
Dancing Bison Enterprises
http://www.dancingbison.com

On Jun 7, 8:07 am, “Harry K.” [email protected] wrote:

:slight_smile:
I was going to do this.
Harry

A Look into Japanese Ruby List in Englishhttp://www.kakueki.com/- Hide quoted text -

  • Show quoted text -

I am not exactly sure what the OP was trying to do … except that it
has to do with Columns in Excel. It does seems to me to be fairly
complicated. If this off topic, please excuse.

In Excel, columns may either have a numeric format or an alpha
format. Columns range for 1 … 255 (A … IV). Since I do quite a bit
of work using Excel from Ruby via WIN32OLE, I have written a couple of
helper functions to convert a numeric column value to an alpha column
value and vice versa. Note that alpha columns in Excel are all caps.

Here they are, I hope that someone might find them useful.

method to convert from numeric column to alpha col

accepts numeric column 1 through 256

returns alpha column A through IV

returns empty string on invalid input

def n2a_col(num_col)

verify that it in the range of 1 to 255

return nil if num_col < 1 || num_col > 256

mostSD = num_col / 26 # SD = Significant Digit
leastSD = num_col % 26

if leastSD == 0
mostSD -= 1
leastSD = 26
end

leastSA = (‘A’[0] + leastSD - 1 ).chr
mostSA = mostSD > 0 ? (‘A’[0] + mostSD - 1).chr : ‘’

mostSA + leastSA
end

method to convert from alpha column to numeric col

accepts alpha column A through IV

returns numeric column 1 through 255

returns 0 on invalid input

def a2n_col(alpha_col)

to uppercase

alpha_col.upcase

col_size = alpha_col.size

case col_size
when 1
return 0 if alpha_col < ‘A’ || alpha_col > ‘Z’
return alpha_col[0] - ‘A’[0] + 1
when 2
return 0 if alpha_col < ‘AA’ || alpha_col > ‘IV’
return (alpha_col[0] - ‘A’[0] + 1) * 26 + alpha_col[1] - ‘A’[0] + 1
else
return 0
end
end

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