My code is supposed to print integers in an array.
odds_n_ends = [:weezard, 42, "Trady Blix", 3, true, 19, 12.345]
ints = odds_n_ends.select { |x| if x.is_a?(Integer) then return x end }
puts ints
It gives me an error in the 2nd line - in 'block in <main>': unexpected return (LocalJumpError)
When I remove the return, the code works exactly as desired.
To find the mistake in my understanding of blocks, I read related posts post1 and post2. But, I am not able to figure out how exactly are methods and blocks being called and why my approach is incorrect.
Is there some call stack diagram explanation for this ? Any simple explanation ?
I am confused because I have only programmed in Java before.
You generally don't need to worry exactly what blocks are to use them.
In this situation, return will return from the outside scope, e.g. if these lines were in a method, then from that method. It's the same as if you put a return statement inside a loop in Java.
Additional tips:
select is used to create a copied array where only the elements satisfying the condition inside the block are selected:
only_ints = odds_n_ends.select { |x| x.is_a?(Integer) }
You're using it as a loop to "pass back" variables that are integers, in which case you'd do:
only_ints = []
odds_n_ends.each { |x| if x.is_a?(Integer) then only_ints << x end }
If you try to wrap your code in a method then it won't give you an error:
def some_method
odds_n_ends = [:weezard, 42, "Trady Blix", 3, true, 19, 12.345]
ints = odds_n_ends.select { |x| if x.is_a?(Integer) then return true end }
puts ints
end
puts some_method
This code output is true. But wait, where's puts ints??? Ruby didn't reach that. When you put return inside a Proc, then you're returning in the scope of the entire method. In your example, you didn't have any method in which you put your code, so after it encountered 'return', it didn't know where to 'jump to', where to continue to.
Array#select basically works this way: For each element of the array (represented with |x| in your code), it evaluates the block you've just put in and if the block evaluates to true, then that element will be included in the new array. Try removing 'return' from the second line and your code will work:
ints = odds_n_ends.select { |x| if x.is_a?(Integer) then true end }
However, this isn't the most Ruby-ish way, you don't have to tell Ruby to explicitly return true. Blocks (the code between the {} ) are just like methods, with the last expression being the return value of the method. So this will work just as well:
ints = odds_n_ends.select { |x| if x.is_a?(Integer) } # imagine the code between {} is
#a method, just without name like 'def is_a_integer?' with the value of the last expression
#being returned.
Btw, there's a more elegant way to solve your problem:
odds_n_ends = [:weezard, 42, "Trady Blix", 3, true, 19, 12.345]
ints = odds_n_ends.grep(Integer)
puts ints
See this link. It basically states:
Returns an array of every element in enum for which Pattern ===
element.
To understand Pattern === element, simply imagine that Pattern is a set (let's say a set of Integers). Element might or might not be an element of that set (an integer). How to find out? Use ===. If you type in Ruby:
puts Integer === 34
it will evalute to true. If you put:
puts Integer === 'hey'
it will evalute to false.
Hope this helped!
In ruby a method always returns it's last statement, so in generall you do not need to return unless you want to return prematurely.
In your case you do not need to return anything, as select will create a new array with just the elements that return true for the given block. As ruby automatically returns it's last statement using
{ |x| x.is_a?(Integer) }
would be sufficient. (Additionally you would want to return true and not x if you think about "return what select expects", but as ruby treats not nil as true it also works...)
Another thing that is important is to understand a key difference of procs (& blocks) and lambdas which is causing your problem:
Using return in a Proc will return the method the proc is used in.
Using return in a Lambdas will return it's value like a method.
Think of procs as code pieces you inject in a method and of lambdas as anonymous methods.
Good and easy to comprehend read: Understanding Ruby Blocks, Procs and Lambdas
When passing blocks to methods you should simply put the value you want to be returned as the last statement, which can also be in an if-else clause and ruby will use the last actually reached statement.
I have a string which looks like a hash:
"{ :key_a => { :key_1a => 'value_1a', :key_2a => 'value_2a' }, :key_b => { :key_1b => 'value_1b' } }"
How do I get a Hash out of it? like:
{ :key_a => { :key_1a => 'value_1a', :key_2a => 'value_2a' }, :key_b => { :key_1b => 'value_1b' } }
The string can have any depth of nesting. It has all the properties how a valid Hash is typed in Ruby.
For different string, you can do it without using dangerous eval method:
hash_as_string = "{\"0\"=>{\"answer\"=>\"1\", \"value\"=>\"No\"}, \"1\"=>{\"answer\"=>\"2\", \"value\"=>\"Yes\"}, \"2\"=>{\"answer\"=>\"3\", \"value\"=>\"No\"}, \"3\"=>{\"answer\"=>\"4\", \"value\"=>\"1\"}, \"4\"=>{\"value\"=>\"2\"}, \"5\"=>{\"value\"=>\"3\"}, \"6\"=>{\"value\"=>\"4\"}}"
JSON.parse hash_as_string.gsub('=>', ':')
Quick and dirty method would be
eval("{ :key_a => { :key_1a => 'value_1a', :key_2a => 'value_2a' }, :key_b => { :key_1b => 'value_1b' } }")
But it has severe security implications.
It executes whatever it is passed, you must be 110% sure (as in, at least no user input anywhere along the way) it would contain only properly formed hashes or unexpected bugs/horrible creatures from outer space might start popping up.
The string created by calling Hash#inspect can be turned back into a hash by calling eval on it. However, this requires the same to be true of all of the objects in the hash.
If I start with the hash {:a => Object.new}, then its string representation is "{:a=>#<Object:0x7f66b65cf4d0>}", and I can't use eval to turn it back into a hash because #<Object:0x7f66b65cf4d0> isn't valid Ruby syntax.
However, if all that's in the hash is strings, symbols, numbers, and arrays, it should work, because those have string representations that are valid Ruby syntax.
I had the same problem. I was storing a hash in Redis. When retrieving that hash, it was a string. I didn't want to call eval(str) because of security concerns. My solution was to save the hash as a json string instead of a ruby hash string. If you have the option, using json is easier.
redis.set(key, ruby_hash.to_json)
JSON.parse(redis.get(key))
TL;DR: use to_json and JSON.parse
Maybe YAML.load ?
The solutions so far cover some cases but miss some (see below). Here's my attempt at a more thorough (safe) conversion. I know of one corner case which this solution doesn't handle which is single character symbols made up of odd, but allowed characters. For example {:> => :<} is a valid ruby hash.
I put this code up on github as well. This code starts with a test string to exercise all the conversions
require 'json'
# Example ruby hash string which exercises all of the permutations of position and type
# See http://json.org/
ruby_hash_text='{"alpha"=>{"first second > third"=>"first second > third", "after comma > foo"=>:symbolvalue, "another after comma > foo"=>10}, "bravo"=>{:symbol=>:symbolvalue, :aftercomma=>10, :anotheraftercomma=>"first second > third"}, "charlie"=>{1=>10, 2=>"first second > third", 3=>:symbolvalue}, "delta"=>["first second > third", "after comma > foo"], "echo"=>[:symbol, :aftercomma], "foxtrot"=>[1, 2]}'
puts ruby_hash_text
# Transform object string symbols to quoted strings
ruby_hash_text.gsub!(/([{,]\s*):([^>\s]+)\s*=>/, '\1"\2"=>')
# Transform object string numbers to quoted strings
ruby_hash_text.gsub!(/([{,]\s*)([0-9]+\.?[0-9]*)\s*=>/, '\1"\2"=>')
# Transform object value symbols to quotes strings
ruby_hash_text.gsub!(/([{,]\s*)(".+?"|[0-9]+\.?[0-9]*)\s*=>\s*:([^,}\s]+\s*)/, '\1\2=>"\3"')
# Transform array value symbols to quotes strings
ruby_hash_text.gsub!(/([\[,]\s*):([^,\]\s]+)/, '\1"\2"')
# Transform object string object value delimiter to colon delimiter
ruby_hash_text.gsub!(/([{,]\s*)(".+?"|[0-9]+\.?[0-9]*)\s*=>/, '\1\2:')
puts ruby_hash_text
puts JSON.parse(ruby_hash_text)
Here are some notes on the other solutions here
#Ken Bloom and #Toms Mikoss's solutions use eval which is too scary for me (as Toms rightly points out).
#zolter's solution works if your hash has no symbols or numeric keys.
#jackquack's solution works if there are no quoted strings mixed in with the symbols.
#Eugene's solution works if your symbols don't use all the allowed characters (symbol literals have a broader set of allowed characters).
#Pablo's solution works as long as you don't have a mix of symbols and quoted strings.
This short little snippet will do it, but I can't see it working with a nested hash. I think it's pretty cute though
STRING.gsub(/[{}:]/,'').split(', ').map{|h| h1,h2 = h.split('=>'); {h1 => h2}}.reduce(:merge)
Steps
1. I eliminate the '{','}' and the ':'
2. I split upon the string wherever it finds a ','
3. I split each of the substrings that were created with the split, whenever it finds a '=>'. Then, I create a hash with the two sides of the hash I just split apart.
4. I am left with an array of hashes which I then merge together.
EXAMPLE INPUT: "{:user_id=>11, :blog_id=>2, :comment_id=>1}"
RESULT OUTPUT: {"user_id"=>"11", "blog_id"=>"2", "comment_id"=>"1"}
I prefer to abuse ActiveSupport::JSON. Their approach is to convert the hash to yaml and then load it. Unfortunately the conversion to yaml isn't simple and you'd probably want to borrow it from AS if you don't have AS in your project already.
We also have to convert any symbols into regular string-keys as symbols aren't appropriate in JSON.
However, its unable to handle hashes that have a date string in them (our date strings end up not being surrounded by strings, which is where the big issue comes in):
string = '{'last_request_at' : 2011-12-28 23:00:00 UTC }'
ActiveSupport::JSON.decode(string.gsub(/:([a-zA-z])/,'\\1').gsub('=>', ' : '))
Would result in an invalid JSON string error when it tries to parse the date value.
Would love any suggestions on how to handle this case
works in rails 4.1 and support symbols without quotes {:a => 'b'}
just add this to initializers folder:
class String
def to_hash_object
JSON.parse(self.gsub(/:([a-zA-z]+)/,'"\\1"').gsub('=>', ': ')).symbolize_keys
end
end
Please consider this solution. Library+spec:
File: lib/ext/hash/from_string.rb:
require "json"
module Ext
module Hash
module ClassMethods
# Build a new object from string representation.
#
# from_string('{"name"=>"Joe"}')
#
# #param s [String]
# #return [Hash]
def from_string(s)
s.gsub!(/(?<!\\)"=>nil/, '":null')
s.gsub!(/(?<!\\)"=>/, '":')
JSON.parse(s)
end
end
end
end
class Hash #:nodoc:
extend Ext::Hash::ClassMethods
end
File: spec/lib/ext/hash/from_string_spec.rb:
require "ext/hash/from_string"
describe "Hash.from_string" do
it "generally works" do
[
# Basic cases.
['{"x"=>"y"}', {"x" => "y"}],
['{"is"=>true}', {"is" => true}],
['{"is"=>false}', {"is" => false}],
['{"is"=>nil}', {"is" => nil}],
['{"a"=>{"b"=>"c","ar":[1,2]}}', {"a" => {"b" => "c", "ar" => [1, 2]}}],
['{"id"=>34030, "users"=>[14105]}', {"id" => 34030, "users" => [14105]}],
# Tricky cases.
['{"data"=>"{\"x\"=>\"y\"}"}', {"data" => "{\"x\"=>\"y\"}"}], # Value is a `Hash#inspect` string which must be preserved.
].each do |input, expected|
output = Hash.from_string(input)
expect([input, output]).to eq [input, expected]
end
end # it
end
Here is a method using whitequark/parser which is safer than both gsub and eval methods.
It makes the following assumptions about the data:
Hash keys are assumed to be a string, symbol, or integer.
Hash values are assumed to be a string, symbol, integer, boolean, nil, array, or a hash.
# frozen_string_literal: true
require 'parser/current'
class HashParser
# Type error is used to handle unexpected types when parsing stringified hashes.
class TypeError < ::StandardError
attr_reader :message, :type
def initialize(message, type)
#message = message
#type = type
end
end
def hash_from_s(str_hash)
ast = Parser::CurrentRuby.parse(str_hash)
unless ast.type == :hash
puts "expected data to be a hash but got #{ast.type}"
return
end
parse_hash(ast)
rescue Parser::SyntaxError => e
puts "error parsing hash: #{e.message}"
rescue TypeError => e
puts "unexpected type (#{e.type}) encountered while parsing: #{e.message}"
end
private
def parse_hash(hash)
out = {}
hash.children.each do |node|
unless node.type == :pair
raise TypeError.new("expected child of hash to be a `pair`", node.type)
end
key, value = node.children
key = parse_key(key)
value = parse_value(value)
out[key] = value
end
out
end
def parse_key(key)
case key.type
when :sym, :str, :int
key.children.first
else
raise TypeError.new("expected key to be either symbol, string, or integer", key.type)
end
end
def parse_value(value)
case value.type
when :sym, :str, :int
value.children.first
when :true
true
when :false
false
when :nil
nil
when :array
value.children.map { |c| parse_value(c) }
when :hash
parse_hash(value)
else
raise TypeError.new("value of a pair was an unexpected type", value.type)
end
end
end
and here are some rspec tests verifying that it works as expected:
# frozen_string_literal: true
require 'spec_helper'
RSpec.describe HashParser do
describe '#hash_from_s' do
subject { described_class.new.hash_from_s(input) }
context 'when input contains forbidden types' do
where(:input) do
[
'def foo; "bar"; end',
'`cat somefile`',
'exec("cat /etc/passwd")',
'{:key=>Env.fetch("SOME_VAR")}',
'{:key=>{:another_key=>Env.fetch("SOME_VAR")}}',
'{"key"=>"value: #{send}"}'
]
end
with_them do
it 'returns nil' do
expect(subject).to be_nil
end
end
end
context 'when input cannot be parsed' do
let(:input) { "{" }
it 'returns nil' do
expect(subject).to be_nil
end
end
context 'with valid input' do
using RSpec::Parameterized::TableSyntax
where(:input, :expected) do
'{}' | {}
'{"bool"=>true}' | { 'bool' => true }
'{"bool"=>false}' | { 'bool' => false }
'{"nil"=>nil}' | { 'nil' => nil }
'{"array"=>[1, "foo", nil]}' | { 'array' => [1, "foo", nil] }
'{foo: :bar}' | { foo: :bar }
'{foo: {bar: "bin"}}' | { foo: { bar: "bin" } }
end
with_them do
specify { expect(subject).to eq(expected) }
end
end
end
end
I built a gem hash_parser that first checks if a hash is safe or not using ruby_parser gem. Only then, it applies the eval.
You can use it as
require 'hash_parser'
# this executes successfully
a = "{ :key_a => { :key_1a => 'value_1a', :key_2a => 'value_2a' },
:key_b => { :key_1b => 'value_1b' } }"
p HashParser.new.safe_load(a)
# this throws a HashParser::BadHash exception
a = "{ :key_a => system('ls') }"
p HashParser.new.safe_load(a)
The tests in https://github.com/bibstha/ruby_hash_parser/blob/master/test/test_hash_parser.rb give you more examples of the things I've tested to make sure eval is safe.
This method works for one level deep hash
def convert_to_hash(str)
return unless str.is_a?(String)
hash_arg = str.gsub(/[^'"\w\d]/, ' ').squish.split.map { |x| x.gsub(/['"]/, '') }
Hash[*hash_arg]
end
example
> convert_to_hash("{ :key_a => 'value_a', :key_b => 'value_b', :key_c => '' }")
=> {"key_a"=>"value_a", "key_b"=>"value_b", "key_c"=>""}
I came to this question after writing a one-liner for this purpose, so I share my code in case it helps somebody. Works for a string with only one level depth and possible empty values (but not nil), like:
"{ :key_a => 'value_a', :key_b => 'value_b', :key_c => '' }"
The code is:
the_string = '...'
the_hash = Hash.new
the_string[1..-2].split(/, /).each {|entry| entryMap=entry.split(/=>/); value_str = entryMap[1]; the_hash[entryMap[0].strip[1..-1].to_sym] = value_str.nil? ? "" : value_str.strip[1..-2]}
Ran across a similar issue that needed to use the eval().
My situation, I was pulling some data from an API and writing it to a file locally. Then being able to pull the data from the file and use the Hash.
I used IO.read() to read the contents of the file into a variable. In this case IO.read() creates it as a String.
Then used eval() to convert the string into a Hash.
read_handler = IO.read("Path/To/File.json")
puts read_handler.kind_of?(String) # Returns TRUE
a = eval(read_handler)
puts a.kind_of?(Hash) # Returns TRUE
puts a["Enter Hash Here"] # Returns Key => Values
puts a["Enter Hash Here"].length # Returns number of key value pairs
puts a["Enter Hash Here"]["Enter Key Here"] # Returns associated value
Also just to mention that IO is an ancestor of File. So you can also use File.read instead if you wanted.
I had a similar issue when trying to convert a string to a hash in Ruby.
The result from my computations was this:
{
"coord":{"lon":24.7535,"lat":59.437},
"weather":[{"id":803,"main":"Clouds","description":"broken clouds","icon":"04d"}],
"base":"stations",
"main":{"temp":283.34,"feels_like":281.8,"temp_min":282.33,"temp_max":283.34,"pressure":1021,"humidity":53},
"visibility":10000,
"wind":{"speed":3.09,"deg":310},
"clouds":{"all":75},
"dt":1652808506,
"sys":{"type":1,"id":1330,"country":"EE","sunrise":1652751796,"sunset":1652813502},
"timezone":10800,"id":588409,"name":"Tallinn","cod":200
}
I checked the type value and confirmed that it was of the String type using the command below:
result =
{
"coord":{"lon":24.7535,"lat":59.437},
"weather":[{"id":803,"main":"Clouds","description":"broken clouds","icon":"04d"}],
"base":"stations",
"main":{"temp":283.34,"feels_like":281.8,"temp_min":282.33,"temp_max":283.34,"pressure":1021,"humidity":53},
"visibility":10000,
"wind":{"speed":3.09,"deg":310},
"clouds":{"all":75},
"dt":1652808506,
"sys":{"type":1,"id":1330,"country":"EE","sunrise":1652751796,"sunset":1652813502},
"timezone":10800,"id":588409,"name":"Tallinn","cod":200
}
puts result.instance_of? String
puts result.instance_of? Hash
Here's how I solved it:
All I had to do was run the command below to convert it from a String to a Hash:
result_new = JSON.parse(result, symbolize_names: true)
And then checked the type value again using the commands below:
puts result_new.instance_of? String
puts result_new.instance_of? Hash
This time it returned true for the Hash