Simple even and odd number loop in Ruby

ruby-talk
1

Hello guys, I am trying to develop a simple loop in Ruby. You need to
type in a number. When this number is an "even" number, it will divide
it by 2. If it is an odd number, it will multiply it by 3. This goes in
a loop... so for instance if you type in number "20", Ruby would write
an answer like this: 20, 10, 5, 15, 45,... infinitelly...always either
dividing or multiplying the number that it gets depending on whether it
is an odd or even number.

I tried so many things these days, but none of them were working.

···

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2

I hope this is not homework. For example this would work:

def f n # this function calculates next number in your sequence
  if n % 2 == 0 # n is even
    return n / 2
  else
    return n * 3
  end
end

num = gets.to_i # read in a number
while true # infinitely...
  num = f(num) # find the next number
  puts num # print it
  sleep 1 # wait one second
end

Is there anything you don't understand about this code?

-- Matma Rex

3

Please show the code so we can give you concrete advice (vs. writing
the program for you).

Kind regards

robert

···

On Sun, Feb 5, 2012 at 3:34 PM, Viera Tarcova <faithfromslovakia@gmail.com> wrote:

I tried so many things these days, but none of them were working.

--
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/

4

How would you make this loop stop when it reaches number one?

···

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5

please is there any ideas on how to do this for a range of number say 2
to 5 and them count the number of counts before it ( that is the numbers
2,3,4,5 )gets to 1. after that be able to get the number with the
largest count?

for instance 3 gives count as follows

3,10,5,16,8,4,2,1

and for instance 4 gives count as follows

4,2,1

therefore between 2 to 5
the largest count is 8 which is produced by number 3

···

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6

Viera Tarcova wrote in post #1044163:

Hello guys, I am trying to develop a simple loop in Ruby. You need to
type in a number. When this number is an "even" number, it will divide
it by 2. If it is an odd number, it will multiply it by 3. This goes in
a loop... so for instance if you type in number "20", Ruby would write
an answer like this: 20, 10, 5, 15, 45,... infinitelly...always either
dividing or multiplying the number that it gets depending on whether it
is an odd or even number.

I tried so many things these days, but none of them were working.

This should provide some clarity with the following two examples:

If you wanted to print out only even integers from 0 - 20

i = 20
loop do
i -= 1
next if i % 2 != 0
print "#{i}"
break if i <= 0
end

(note technically it will only return even values up to 18, but you can
adjust the i = value as necessary)

If you wanted to print out only odd integers from 0 -20

i = 20
loop do
  i -= 1
  next if i % 2 == 0
  print "#{i}"
  break if i <= 0
end

(note technically it will only return odd values from 1 - 19)

···

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Posted via http://www.ruby-forum.com/\.

7

This should provide some clarity with the following two examples:

If you wanted to print out only even integers from 0 - 20

i = 20
loop do
i -= 1
next if i % 2 != 0
print "#{i}"
break if i <= 0
end

(note technically it will only return even values up to 18, but you can
adjust the i = value as necessary)

If you wanted to print out only odd integers from 0 -20

i = 20
loop do
  i -= 1
  next if i % 2 == 0
  print "#{i}"
  break if i <= 0
end

(note technically it will only return odd values from 1 - 19)

···

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Posted via http://www.ruby-forum.com/.

8

There's also Integer#even? and #odd?.
e.g.
if n.even?
  return n / 2
else
  return n * 3
end

···

2012/2/5 Bartosz Dziewoński <matma.rex@gmail.com>:

I hope this is not homework. For example this would work:

def f n # this function calculates next number in your sequence
if n % 2 == 0 # n is even
return n / 2
else
return n * 3
end
end

num = gets.to_i # read in a number
while true # infinitely...
num = f(num) # find the next number
puts num # print it
sleep 1 # wait one second
end

9

Thank you Bartosz, this is exactly what I was looking for. My loop was
not good at all. I am still learning. If you are interested in doing
online mentoring, please let me know. I would love to have such a good
mentor like you.

Bartosz Dziewoński wrote in post #1044167:

···

I hope this is not homework. For example this would work:

def f n # this function calculates next number in your sequence
  if n % 2 == 0 # n is even
    return n / 2
  else
    return n * 3
  end
end

num = gets.to_i # read in a number
while true # infinitely...
  num = f(num) # find the next number
  puts num # print it
  sleep 1 # wait one second
end

Is there anything you don't understand about this code?

-- Matma Rex

--
Posted via http://www.ruby-forum.com/\.

10

Instead of looping forever (while true), you should only loop until num is 1.

Jesus.

···

On Mon, Feb 6, 2012 at 7:58 PM, Viera Tarcova <faithfromslovakia@gmail.com> wrote:

How would you make this loop stop when it reaches number one?

11

Remember that Ruby provides useful predicates for testing the even/odd -ness of numbers, so the original poster's problem could be solved like this which seems to use terms close to those in the statement of the problem:

#!/usr/bin/env ruby

print "Enter a starting number: "
num = Integer(gets)

loop do
  puts num

  if num.even?
    num /= 2
  else
    num *= 3
  end
end

And it seems to produce the expected results for as long as I ran it, but I don't expect it to run infinitely because eventually my computer will run out of memory to store a big Bignum!

Hope this helps,

Mike

···

On Jan 31, 2014, at 7:45 PM, Il Knowledge <lists@ruby-forum.com> wrote:

Viera Tarcova wrote in post #1044163:

Hello guys, I am trying to develop a simple loop in Ruby. You need to
type in a number. When this number is an "even" number, it will divide
it by 2. If it is an odd number, it will multiply it by 3. This goes in
a loop... so for instance if you type in number "20", Ruby would write
an answer like this: 20, 10, 5, 15, 45,... infinitelly...always either
dividing or multiplying the number that it gets depending on whether it
is an odd or even number.

I tried so many things these days, but none of them were working.

This should provide some clarity with the following two examples:

If you wanted to print out only even integers from 0 - 20

i = 20
loop do
i -= 1
next if i % 2 != 0
print "#{i}"
break if i <= 0
end

(note technically it will only return even values up to 18, but you can
adjust the i = value as necessary)

If you wanted to print out only odd integers from 0 -20

i = 20
loop do
i -= 1
next if i % 2 == 0
print "#{i}"
break if i <= 0
end

(note technically it will only return odd values from 1 - 19)

--

Mike Stok <mike@stok.ca>
http://www.stok.ca/~mike/

The "`Stok' disclaimers" apply.

12

Remember that Ruby provides useful predicates for testing the even/odd -ness of numbers, so the original poster's problem could be solved like this which seems to use terms close to those in the statement of the problem:

We can exploit more knowledge: the program will output in two phases:
1. even numbers are printed; they are divided by two all the time
until reaching an odd number
2. odd numbers are printed until the end of time; each one is three
times the number before

That also makes it clear that there is just one point where we can
reach 1 which was required as termination criterion.

print "Enter a starting number: "
num = Integer(gets)

while num.even?
  puts num
  num /= 2
end

puts num

loop do
  puts num *= 3
end if num > 1

There are various variants how we can write the last part

# variant
num == 1 or loop do
  puts num *= 3
end

# variant
if num > 1
  loop do
    puts num *= 3
  end
end

# variant
while num > 1
  puts num *= 3
end

The last condition is a bit silly though as it tests too often unnecessarily.

And it seems to produce the expected results for as long as I ran it, but I don't expect it to run infinitely because eventually my computer will run out of memory to store a big Bignum!

:slight_smile:

Cheers

robert

···

On Sat, Feb 1, 2014 at 5:23 PM, Mike Stok <mike@stok.ca> wrote:

--
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/

13

The original problem (if I understand it) seems like an ideal
opportunity to use an Enumerator. Here's my go:

#!/usr/bin/env ruby

class TwoOrThree

  attr_accessor :value

  def initialize(value)
    self.value = value
  end

  def each
    return enum_for(:each) unless block_given?

    loop do
      yield self.value

      if self.value.odd?
        self.value = self.value * 3
      else
        self.value = self.value / 2
      end

    end

  end

end

if ARGV.count < 2
  puts "Usage: #{$0} START COUNT"
  exit(-1)
end

start_value = Integer(ARGV.shift)
take_value = Integer(ARGV.shift)

e = TwoOrThree.new(start_value).each

p e.take(take_value)

···

----
$ bin/two_or_three 19 7
[19, 57, 171, 513, 1539, 4617, 13851]

$ bin/two_or_three 1 1
[1]

$ bin/two_or_three 0 3
[0, 0, 0]

$ bin/two_or_three -1 4
[-1, -3, -9, -27]

$ bin/two_or_three 20 20
[20, 10, 5, 15, 45, 135, 405, 1215, 3645, 10935, 32805, 98415, 295245,
885735, 2657205, 7971615, 23914845, 71744535, 215233605, 645700815]

14

tamouse m. wrote in post #1135295:

  def each
    return enum_for(:each) unless block_given?

    loop do
      yield self.value

      if self.value.odd?
        self.value = self.value * 3
      else
        self.value = self.value / 2
      end

    end

  end

end

I could be wrong, but aren't there 4 superfluous "self." in that code?
As far as I know, you'd only need to use self when you're specifying the
attr_writer rather than a local variable.

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15

Yes.

I use them to keep myself clear. They are entirely superfluous for the code.

···

On Tue, Feb 4, 2014 at 11:34 AM, Joel Pearson <lists@ruby-forum.com> wrote:

tamouse m. wrote in post #1135295:

  def each
    return enum_for(:each) unless block_given?

    loop do
      yield self.value

      if self.value.odd?
        self.value = self.value * 3
      else
        self.value = self.value / 2
      end

    end

  end

end

I could be wrong, but aren't there 4 superfluous "self." in that code?
As far as I know, you'd only need to use self when you're specifying the
attr_writer rather than a local variable.

--
Posted via http://www.ruby-forum.com/\.