Ruby：NoMethodError：nil：NilClass的未定义方法“ +”

Question

Would anybody have an idea, why following code returns error:

stock = {"M9788375085969"=>5, "M9788392289760"=>5, "M9788389371461"=>1, "M9788389371447"=>3, "M9788392289761"=>2}
add = {"M9788375085969"=>1, "M9788392289760"=>2, "NEW9788392289753"=>1 }

add.each do |key, value|
  stock[key] += value
end

NoMethodError: undefined method `+' for nil:NilClass

while similar thing works fine:

key = "M9788375085969"
value = 1
stock[key] += value
=> 6

Answer 1

There is one key in your add hash that is missing in your stock hash : "NEW9788392289753".

When executing stock["NEW9788392289753"] , nil is returned, as the key is not mapped.

Answer 2

The key "NEW9788392289753" is not present in the Hash stock ,but present in add hash. See below :

stock = {"M9788375085969"=>5, "M9788392289760"=>5, "M9788389371461"=>1, "M9788389371447"=>3, "M9788392289761"=>2}
stock['NEW9788392289753'] # => nil
nil.respond_to?(:+) # => false # means NilClass don't has method called :+

Thus nil.+(value) throwing a valid error. Do as below :

stock = {"M9788375085969"=>5, "M9788392289760"=>5, "M9788389371461"=>1, "M9788389371447"=>3, "M9788392289761"=>2}
add = {"M9788375085969"=>1, "M9788392289760"=>2, "NEW9788392289753"=>1 }

add.each do |key, value|
  p stock[key] += value if stock.has_key?(key) # it will take care of the error.
end

output

6
7

As per OP's comment I would do as :

add.each do |key, value|
  if stock.has_key?(key) 
     stock[key] += value 
  else
     stock[key] = value
  end
end

Answer 3

因为add的密钥NEW9788392289753不包含在stock 。

Answer 4

Another way of treating non-existent keys is providing a default of zero:

stock = {"M9788375085969"=>5, "M9788392289760"=>5, "M9788389371461"=>1, "M9788389371447"=>3, "M9788392289761"=>2}
add = {"M9788375085969"=>1, "M9788392289760"=>2, "NEW9788392289753"=>1 }

stock.default = 0 

add.each do |key, value|
  stock[key] += value
end

p stock #=> {"M9788375085969"=>6, "M9788392289760"=>7, "M9788389371461"=>1, "M9788389371447"=>3, "M9788392289761"=>2, "NEW9788392289753"=>1}