Failed to pass the value of an instance variable to another from within the same class (Ruby)

Question

this is my code.

class Dog attr_accessor :name attr_reader :breed, :age def initialize(name, breed, age) @name = name @breed = breed @age = age @distance_in_kms = [] end def walk(distance_in_kms) @distance_in_kms << {distance: distance_in_kms} end def walked_distance @walked_distance = @distance_in_kms.inject(0) {|sum, hash| sum + hash[:distance]} end def display_walks puts "#{@name} has had #{@distance_in_kms.length} walks and walked #{@walked_distance} kms today:" @distance_in_kms.each do |each_walk| puts "#{each_walk[:distance]} km" end end #overriding to_s to print a meaningful representation of a class def to_s return "Dog: breed-#{@breed} name-#{@name}" end end doggo = Dog.new("Roy", "Labrador", 8) doggo.walk(3) doggo.walk(5) doggo.walk(1) puts doggo.name = "Dang" puts doggo.breed doggo.display_walks

The result I have is this

Dang Labrador Dang has had 3 walks and walked kms today: 3 km 5 km 1 km

The expected value before kms should be 9, the sum of each walk in a day. Obviously, I have done something wrong with the walked_distance method. I was trying to pass the value of array @distance_in_kms into @walked_distance, but it didn't work, the return value was nil.But what can I do to fix the problem and get expected results? Thank you!!

Change #{@walked_distance} string to #{walked_distance}. Instance variable from the method never was called in your code. — mikdiet
– mikdiet, Commented Sep 20, 2020 at 10:17

iGian · Accepted Answer · 2020-09-20 10:21:10Z

You must call the method to update the @walked_distance, for example:

def display_walks walked_distance # <------ This line puts "#{@name} has had #{@distance_in_kms.length} walks and walked #{@walked_distance} kms today:" @distance_in_kms.each do |each_walk| puts "#{each_walk[:distance]} km" end end

Or change the walked_distance method and call it directly from the string interpolation:

def walked_distance @distance_in_kms.inject(0) {|sum, hash| sum + hash[:distance]} end def display_walks puts "#{@name} has had #{@distance_in_kms.length} walks and walked #{walked_distance} kms today:" @distance_in_kms.each do |each_walk| puts "#{each_walk[:distance]} km" end end

Off topic, just a small change to evaluate:

@walked_distance = @distance_in_kms.values.sum

Sergio Tulentsev · Accepted Answer · 2020-09-20 10:15:05Z

puts "#{@name} has had #{@distance_in_kms.length} walks and walked #{@walked_distance} kms today:"

But you never call the code to calculate @walked_distance. Either update it when you add a new walk or calculate total distance here (by calling walked_distance, for example)

diaojiulishifei · Accepted Answer · 2020-10-08 11:09:43Z

I could've done this right at the beginning too:

class Dog attr_accessor :name attr_reader :breed, :age def initialize(name, breed, age) @name = name @breed = breed @age = age @distance_in_kms = [] @walked_distance = [] end def walk(distance_in_kms) @distance_in_kms << {distance: distance_in_kms} @walked_distance = @distance_in_kms.inject(0) {|sum, hash| sum + hash[:distance]} end def display_walks puts "#{@name} has had #{@distance_in_kms.length} walks and walked #{@walked_distance} kms today:" @distance_in_kms.each do |each_walk| puts "#{each_walk[:distance]} km" end end #overriding to_s to print a meaningful representation of a class def to_s return "Dog: breed-#{@breed} name-#{@name}" end end doggo = Dog.new("Roy", "Labrador", 8) doggo.walk(3) doggo.walk(5) doggo.walk(1) puts doggo.name = "Dang" puts doggo.breed doggo.display_walks

or use class methods to create a counter then try to link it to the instances(It's a very long way though)

Indeed, understanding variable scope is a real struggle for a beginner. Anyway, much appreciation for anyone who answered my question. Thank you!

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