# Rand(-10..10)

Hi all,

wouldn’t it make sense to allow

rand -10…10

be aquivalent to

-10 + rand(20)

the first is by far more obvious.
Of course rand -10…10 should work too.
What do you think?

one solution could be

irb(main):210:0* class Range
irb(main):211:1> def rand
irb(main):212:2> return self.begin + Kernel.rand(self.end-self.begin)
irb(main):213:2> end
irb(main):214:1> end
=> nil
irb(main):215:0> (-10…10).rand
=> 2
irb(main):216:0> (-10…10).rand
=> 6
irb(main):217:0> (-10…10).rand
=> 4
irb(main):218:0> (-10…10).rand
=> -1

it’s a matter of style to write

rand(-10…10) or (-10…10).rand

but since 10.sin doesn’t work, as opposite tosin(10)
non object oriented rand(-10…10) is also a nice thing to have.

I had similar thoughts a while back. I decided that extending Kernel
and mimicing the behavior of rand was the way to go. My code follows:

Regards, Morton

------ code starts
#!/usr/bin/ruby

# Has the same behavior as rand for a single argument.

module Kernel
URAND_ARG_ERR = “Arguments not valid for urand”
# Return a pseudo-random number in the range 0.0…1.0, 0…m, or
m…n.
def urand(m=0, n=nil)
case m
when Range
m, n = m.begin, m.end
when Integer
return rand(m) if n.nil?
else
raise ArgumentError, URAND_ARG_ERR
end
raise ArgumentError, URAND_ARG_ERR if n < m
m + rand(n - m + 1)
end
end

# Now for a little testing …

def test(times, m=0, n=nil)
r = Array.new(times)
if n.nil? then
if m == 0 then r.collect! {urand}
else r.collect! {urand(m)}
end
else r.collect! {urand(m, n)}
end
p r
end
test(3) # => [0.0403243284672499, 0.875487065408379,
0.142408860381693]
test(3, 11, 11) # => [11, 11, 11]
test(10, 3) # => [0, 1, 2, 0, 1, 0, 1, 2, 0, 0]
test(10, -1…1) # => [0, 0, 1, -1, -1, 1, -1, 1, -1, 0]
test(10, 100, 200) # => [126, 148, 183, 140, 188, 175, 115, 157, 136,
179]
begin
test(1, 3, -1)
rescue ArgumentError => error
puts error.message
end # => Arguments not valid for urand

------ code ends

“the first is by far more obvious.”

I think it looks ugly.

(-10…10).rand
looks much nicer IMHO.

Marc H. wrote:

“the first is by far more obvious.”

I think it looks ugly.

(-10…10).rand
looks much nicer IMHO.

This should work with all enumerables:

module Enumerable
def rand
entries = entries
entries.at(Kernel.rand(entries.length))
end
end

I can’t seem to get around the `entries = entries’ part – I don’t want
to convert the enumerable to an array more than once. Any ideas?

Cheers,
Daniel

Daniel S. wrote:

to convert the enumerable to an array more than once. Any ideas?
Enumerable supports sort_by, so could just use:

module Enumerable
def rand
self.sort_by{Kernel.rand}.last
end
end

Cheers
Chris

ChrisH wrote:

I can’t seem to get around the `entries = entries’ part – I don’t want
to convert the enumerable to an array more than once. Any ideas?

Enumerable supports sort_by, so could just use:

module Enumerable
def rand
self.sort_by{Kernel.rand}.last
end
end

I think sorting the entire enumerable may be slight overkill, although
it of course is prettier

Daniel

Daniel S. [email protected] writes:

to convert the enumerable to an array more than once. Any ideas?
There’s also this, which is possibly more memory-efficient (if the
Enumerable in question chews large amounts of memory when being
converted to an array, but not with each), but is almost certainly
much slower, since it calls rand once per element:

module Enumerable
def rand
ret, i = nil, 0
each {|v| ret = v if 0 == Kernel.rand(i += 1)}
ret
end
end

Test code for irb:

h=Hash.new(0); 10000.times {h[%w{a b c d e}.rand] += 1}; h

irb(main):215:0> (-10…10).rand
=> 2
irb(main):216:0> (-10…10).rand
=> 6
irb(main):217:0> (-10…10).rand
=> 4
irb(main):218:0> (-10…10).rand
=> -1

Sniff God I love Ruby.

[…]

The Kernel#rand(-5…15) version wouldn’t suffer from this problem. It
would be a good idea to add this to ruby core, because this
functionality is something, that is needed very often.

yes, this was exactly my point
if we already had Kernel#rand with Range object, people wouldn’t
implement their own methods, making scripts shoter and slightly more
portable (regarding copy&paste other code)

Regards, Daniel

Daniel M. wrote:

h=Hash.new(0); 10000.times {h[%w{a b c d e}.rand] += 1}; h

I think you shouldn’t name this method rand. It shadows Kernel.rand in
all classes, that already call rand and include Enumerable. The same is
true for Range#rand, if people inherit from Range and already call rand.
Ruby makes those things possible, but it’s also very easy to shoot
yourself or others into the foot, if you aren’t careful.

The Kernel#rand(-5…15) version wouldn’t suffer from this problem. It
would be a good idea to add this to ruby core, because this
functionality is something, that is needed very often.