From: Damien Joly <do_joly@...>
Date: 2002-03-12T05:25:51+09:00
Subject: painfully newbie question

Last week I posted a question about using Ruby in
population ecology simulation, and I am very grateful
for the replies I got.  It seems Ruby is exactly what
I am looking for.

Of course, I have my first question/conundrum, and I
hope it isn't too painful for experienced Ruby/OO
programmers.

I am trying to simulate "death" in an array of objects
("Individuals") by tossing a "coin".  However, for
some reason, I get results I am not expecting.  Here
is my code:

#Start

class Individual
  attr_writer :alive, :mother
def initialize(alive = 1,mother = 0)
  @alive = alive
  @mother = mother
end

def to_s
"Alive: #{@alive} Pregnant: #{@mother} \n"
end

def survival
  if rand<0.5
    @alive = 0
  else
    @alive = 1
  end
end

end


population = Array.new(5,Individual.new(1,0))
puts "Before random death trials: \n"
puts population.to_s

puts "During random death trials: \n"

for i in 0..(population.length-1)
  population[i].survival
 puts population[i].to_s
end

puts "After random death trials: \n"

puts population.to_s   

# End

The output I get is:

Before random death trials:
Alive: 1 Pregnant: 0
Alive: 1 Pregnant: 0
Alive: 1 Pregnant: 0
Alive: 1 Pregnant: 0
Alive: 1 Pregnant: 0
During random death trials:
Alive: 1 Pregnant: 0
Alive: 1 Pregnant: 0
Alive: 1 Pregnant: 0
Alive: 0 Pregnant: 0
Alive: 0 Pregnant: 0
After random death trials:
Alive: 0 Pregnant: 0
Alive: 0 Pregnant: 0
Alive: 0 Pregnant: 0
Alive: 0 Pregnant: 0
Alive: 0 Pregnant: 0

When I would expect to get:

Before random death trials:
Alive: 1 Pregnant: 0
Alive: 1 Pregnant: 0
Alive: 1 Pregnant: 0
Alive: 1 Pregnant: 0
Alive: 1 Pregnant: 0
During random death trials:
Alive: 1 Pregnant: 0
Alive: 1 Pregnant: 0
Alive: 1 Pregnant: 0
Alive: 0 Pregnant: 0
Alive: 0 Pregnant: 0
After random death trials:
Alive: 1 Pregnant: 0
Alive: 1 Pregnant: 0
Alive: 1 Pregnant: 0
Alive: 0 Pregnant: 0
Alive: 0 Pregnant: 0

The value in "Alive" seem to be copied from the last
iteration into the array population once the for loop
is complete. 

I hope this question isn't too inane for the list, but
I am stumped and would appreciate any insight.

Thank you in advance,

Damien 


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