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I'm trying to figure out how to place a value into one of three arrays and then shuffle those arrays and have the program output the index location of the value.
Here is what I have so far:
# The purpose of this program is to randomly place the name Zac
# in one of three arrays and return the array number and position of
# Zac
A1 = ["John","Steve","Frank","Charles"]
A2 = ["Sam","Clint","Stuart","James"]
A3 = ["Vic","Jim","Bill","David"]
n = [A1,A2,A3]
name = "Zac"
def placename(title, namelist)
mix = rand(2)
namelist[mix] << title
namelist.shuffle
return namelist
end
allnames = [] << placename(name, n)
def findname(allnames, key)
allnames.each do |i|
until allnames[i].include?(key) == true
i+=1
end
location = allnames[i].find_index(key)
puts "The location and value of #{key} is #{location}"
end
end
findname(allnames, name)
At the moment I'm getting a "undefined method for Nil Class" error (no method error)
Can someone please clarify what I'm doing wrong with this or if there is a more effective way of going about this? Thanks in advance!!
Your approach assumes that in the block starting...
allnames.each do |i|
... that i will contain the index of the allnames element. This isn't true. i will contain the VALUE (contents) of the element.
What you could try as an alternative is...
allnames.each_with_index do |_value, i|
or, you can do...
allnames.each do |value|
and then replace all references to allnames[i] with value
another problem is that...
allnames = [] << placename(name, n)
puts the returned array of arrays inside ANOTHER array. I think what you want to do is..
allnames = placename(name, n)
I modified the last fewlines. I hope this is what you wanted
allnames = placename(name, n)
def findname allnames, key
r = allnames.map.with_index{|x,i|x.include?(key) ? i : p}-[p]
puts "The location of value #{key} is array number #{r[0]} and item number #{allnames[r[0]].index(key)}"
end
findname(allnames, name)
Edit: Randomization
To get randomized array number and item number you have to do the following
def placename(title, namelist)
mix = rand(3) # Since the number of arrays (nested within) is 3 we can use 3 instead of 2
namelist[mix] << title
namelist.map!{|x|x.shuffle}.shuffle! # Shuffling each item and the whole array in place.
return namelist
end
Assuming you want to modify the array in place, I'd do it like this:
# insert name into random subarray
def insert_name name
subarray_idx = rand #name_arrays.size
subarray = #name_arrays[subarray_idx]
insertion_idx = rand subarray.size
#name_arrays[subarray_idx].insert insertion_idx, name
sprintf '"%s" inserted at #name_arrays[%d][%d]',
name, subarray_idx, insertion_idx
end
# define starting array, then print & return the
# message for further parsing if needed
#name_arrays = [
%w[John Steve Frank Charles],
%w[Sam Clint Stuart James],
%w[Vic Jim Bill David],
]
p(insert_name 'Zac')
This has a few benefits:
You can inspect #name_arrays to validate that things look the way you expect.
The message can be parsed with String#scan if desired.
You can modify #insert_name to return your indexes, rather than having to search for the name directly.
If you don't capture the insertion index as a return value, or don't want to parse it from your message String, you can search for it by leveraging Enumerable#each_with_index and Array#index. For example:
# for demonstration only, set this so you can get the same
# results since the insertion index was randomized
#name_arrays =
[["John", "Steve", "Frank", "Charles"],
["Sam", "Clint", "Stuart", "James"],
["Vic", "Jim", "Zac", "Bill", "David"]]
# return indices of nested match
def find_name_idx name
#name_arrays.each_with_index
.map { [_2, _1.index(name)] }
.reject { _1.any? nil }
.pop
end
# use Array#dig to retrieve item at nested index
#name_arrays.dig *find_name_idx('Zac')
I am trying to search an array for a substring and if that substring exists, shovel it into a new array. The problem I am having is that it keeps coming back with this error message:
`block in substrings': undefined method `<<' for nil:NilClass
I have verified that the index in the method is not nil by printing it. I have also done index == nil to double check.
What am I missing here?
Thanks in advance for your help!
new_array = []
def substrings(word, array)
new_array = array.each do |index|
if index.include? (word)
p index
p index == nil
new_array << index
end
end
end
dictionary = ["below", "down", "go", "going", "horn", "how", "howdy", "it", "i", "low", "own", "part", "partner", "sit"]
substrings("i", dictionary)
You basically combine two different ways of solving this problem. The first is to assign the new_array the result of looping though the array, but in that case, the new_array variable is not available to use inside the block.
So you could either choose to create the variable first, like this
new_array = []
array.each do |index|
if index.include?(word)
new_array << index
end
end
Alternatively you could use a method called reduce which takes a more functional programming approach. That could look like this
new_array = array.reduce([]) do |arr, index|
if index.include?(word)
arr << index
else
arr
end
end
What reduce does is that the block argument arr is always set to the return value of the previous block execution. that can make the syntax a little longer than it has to be, so Ruby also has an alternate approach to reduce, called each_with_object, that does the same, but by mutating the same variable, instead of requiring a return value. I actually prefer this way and would solve it like this.
new_array = array.each_with_object([]) do |index, arr|
arr << index if index.include?(word)
end
I would like to extend a bit the (correct) answer given by #DanneManne: While it is correct that you can access local variables from outer blocks from within an inner block, you can't do it within a def, and it is unnecessary in your example, because you can initialize new_array inside the body of your method and return it as result. But in case you ever really need this kind of construct, there indeed is a workaround:
So, this does NOT work:
a=5
def f
puts a; # WRONG. a is not known here
end
and this works different than you seem to expect:
a=5
def f
a=6
end
puts a # prints 5
f
puts a # prints 5 again
But if you define your method like this, it works:
a=5
define_method(:f) do
puts a; # OK, refers to outer variable a
end
By using a block, you create a closure with this, so if you do now a
f;
a=6;
f
5 and 6 is printed, in this order.
I have verified that the index in the method is not nil by printing it. I have also done index == nil to double check.
What am I missing here?
Not index is nil, but new_array. The error message says:
undefined method `<<' for nil:NilClass
and refers to the line new_array << index. Here, << is the method and new_array is the receiver. And for some reason, new_array is nil.
You probably expected new_array to be [] because you explicitly said new_array = []. But methods have their own local variable scope. If you define a local variable outside of a method, it won't be available inside, or vice-versa.
Typically when referring to an undefined variable you'd get:
undefined local variable or method `new_array'
but here, the 2nd assignment conceals the actual problem:
new_array = array.each do |index|
^^^^^^^^^^^
When Ruby encounters this line, it immediately creates a local variable new_array with an initial value of nil. (local variables are created when the line is parsed, not when the assignment occurs)
To get the expected result, you have to move new_array = [] into the method, get rid of the new_array = array.each { ...} assignment and return new_array at the end:
def substrings(word, array)
new_array = []
array.each do |index|
if index.include?(word)
new_array << index
end
end
new_array
end
The variable names are a bit arbitrary, maybe even misleading. Having index.include?(word) looks like you're comparing a numerical index to a string. I'd use something like this:
def substrings(substring, words)
result = []
words.each do |word|
if word.include?(substring)
result << word
end
end
result
end
Code-wise, you can incorporate the array into the loop via each_with_object which will also return the array:
def substrings(substring, words)
words.each_with_object([]) do |word, result|
if word.include?(substring)
result << word
end
end
end
However, selecting elements based on a condition is such a common task that Ruby provides a dedicated method select – you merely have to return true or false from the block to indicate whether the element should be selected:
def substrings(substring, words)
words.select do |word|
word.include?(substring)
end
end
out_file = File.open "out_file.txt" , 'w' do |f|
matrix = [
[1,2,3],
[4,5,6],
[7,8,9]
]
f.puts "matrix test"
f.puts " int at [0,2]: #{matrix[0][2]}"
f.puts " int at [2,0]: #{matrix[2][0]}"
f.puts " int at {1,1]: #{matrix[1][1]}"
above code produces this:
"matrix test
int at [0,2]: 3
int at [2,0]: 7
int at {1,1]: 5"
but this code using the same matrix variable declaration ..
rows = Array(0..3)
cols = Array(0..3)
rows.each do |r|
cols.each do |c|
f.puts "row:#{r} col:#{c} = #{matrix[r][c]},"
end
end
produces an error:
undefined method `[]' for nil:NilClass (NoMethodError)
Can anybody please tell me what's going on?
The problem is your Array(0..3), it is generating an array [0,1,2,3] instead of what you want: [0,1,2].
You actually want to use ... : Array(0...3) => [0,1,2].
Or you could just change the range inside to 0..2 : Array(0..2) => [0,1,2]
Check out the documentation for Range for more information.
As Tony suggests, using rows=Array(0..2) or rows=Array(0...3) will work for you.
You can also use the range directly and forgo the array creation, like this:
rows = 0...3
cols = 0...3
...
There are 2 types of ranges, the inclusive .. and the exclusive ... which doesn't include the right most digit.
A range such as 0..5 will have every number including the 5. (ie. 0,1,2,3,4,5)
A range such as '0...5' will have every number excluding the 5. (ie. 0,1,2,3,4).
So if you notice your error message,
undefined method `[]' for nil:NilClass (NoMethodError)
You need to begin to wonder what could be running a method on nil. Well, you have this matrix declaration of:
matrix = [
[1,2,3],
[4,5,6],
[7,8,9]
]
So that when this range pops up that is expressed as:
rows = Array(0..3)
It will go through 0,1,2, and also 3. Well, there is no 3 index in that array since your array begins counting at 0 and ends at 2. So when the 3 index hits, the value of it is beyond anything you've declared - it's nil. When you try to run that method on it (to call the spot in the array you want), the error message tells you that you can't run a method (which the [] is actually) on nil.
Paying close attention to your error messages, as well as understanding the 2 types of ranges should help you catch these sorts of errors in the future as well. Leave a comment if this doesn't make total sense.
The previous answers are right, but I thought I would raise the issue of the approach...
Why are you creating ranged arrays instead of using the actual length of the matrix arrays in question...?
Maybe something like this would remove the need to assume the matrix's composition:
out_file = File.open "out_file.txt" , 'w' do |f|
matrix = [
[1,2,3],
[4,5,6],
[7,8,9]
]
f.puts "matrix test"
matrix.length.times do |r|
matrix[r].length.times do |c|
f.puts "row:#{r} col:#{c} = #{matrix[r][c]},"
end
end
Trying to make a method skip_animals that takes an animals array and a skip integer and returns an array of all elements except first skip number of items.
input: skip_animals(['leopard', 'bear', 'fox', 'wolf'], 2)
expected output: ["2:fox", "3:wolf"]
def skip_animals(animals, skip)
arr = Array.new
animals.each_with_index{|animal, index| arr.push("#{animal}:#{index}") }
puts arr.drop(skip)
end
This instead puts each output on a separate line and doesn't add them to the array arr. I thought the arr.push would add them correctly. What do I have to do to get the elements added to the array?
I want to use these methods, not map or something more advanced. I need to tinker with this each_with_index line, not overhaul it.
(This is a challenge on Hackerrank, so it uses STDIN and STDOUT)
EDIT
Here is my updated code with p instead of puts. It's giving me a weird output of two different arrays, not sure why.
def skip_animals(animals, skip)
arr = Array.new
animals.each_with_index{|animal, index| arr.push("#{index}:#{animal}") }
p arr.drop(skip)
end
This gives me two lines of output:
["3:panda", "4:tiger", "5:deer"]
["0:leopard", "1:bear", "2:fox", "3:wolf", "4:dog", "5:cat"]
I'm assuming the top is the correct array, but I don't get why the second is printing also, or why it has a different set of animals.
Use p instead of puts.
irb(main):001:0> puts ['1', '2']
1
2
=> nil
irb(main):002:0> p ['1', '2']
["1", "2"]
According to the documentation, puts:
Writes the given objects to ios as with IO#print. Writes a record
separator (typically a newline) after any that do not already end with
a newline sequence. If called with an array argument, writes each
element on a new line. If called without arguments, outputs a single
record separator.
BTW, I would code like this (using Enumerable#map + returning result instead of printing inside the function):
def skip_animals(animals, skip)
animals.drop(skip).each_with_index.map { |animal, index|
("#{index + skip}:#{animal}")
}
end
p skip_animals(['leopard', 'bear', 'fox', 'wolf'], 2)
just remove puts remove form this line puts arr.drop(skip)
def skip_animals(animals, skip)
arr = Array.new
animals.each_with_index{|animal, index| arr.push("#{animal}:#{index}") }
arr.drop(skip)
end
I am trying to find the duplicate values in an array of strings between 1 to 1000000.
However, with the code I have, I get the output as all the entries that are doubled.
So for instance, if I have [1,2,3,4,3,4], it gives me the output of 3 4 3 4 instead of 3 4.
Here is my code:
array = [gets]
if array.uniq.length == array.length
puts "array does not contain duplicates"
else
puts "array does contain duplicates"
print array.select{ |x| array.count(x) > 1}
end
Also, every time I test my code, I have to define the array as array = [1,2,3,4,5,3,5]. The puts works but it does not print when I use array [gets].
Can someone help me how to fix these two problems?
How I wish we had a built-in method Array#difference:
class Array
def difference(other)
h = other.tally
reject { |e| h[e] > 0 && h[e] -= 1 }
end
end
though #user123's answer is more straightforward. (Array#difference is probably the more efficient of the two, as it avoids the repeated invocations of count.) See my answer here for a description of the method and links to its use.
In a nutshell, it differs from Array#- as illustrated in the following example:
a = [1,2,3,4,3,2,4,2]
b = [2,3,4,4,4]
a - b #=> [1]
a.difference b #=> [1, 3, 2, 2]
For the present problem, if:
arr = [1,2,3,4,3,4]
the duplicate elements are given by:
arr.difference(arr.uniq).uniq
#=> [3, 4]
For your first problem, you need to uniq function like
array.select{ |x| array.count(x) > 1}.uniq
For your second problem, when you receive a value using array = [gets] it would receive your entire sequence of array numbers as a single string, so everything would be stored in a[0] like ["1, 2 3 4\n"].
puts "Enter array"
array = gets.chomp.split(",").map(&:to_i)
if array.uniq.length == array.length
puts "array does not contain duplicates"
else
puts "array does contain duplicates"
print array.select{ |x| array.count(x) > 1}.uniq
end
copy this code in ruby file and try to run using
ruby file_name.rb
Coming to your 'gets' problem,
When you are doing a gets, your are basically getting a string as an input but not an array.
2.2.0 :001 > array = [gets]
1,2,1,4,1,2,3
=> ["1,2,1,4,1,2,3\n"]
See the above example, how the ruby interpreter took all your elements as a single string and put it in an array as a single array element. So you need to explicitly convert the input to an array with comma as a delimiter. The below will address both your questions.
array = gets.chomp
array = array.split(',').map(&:to_i)
if array.uniq.length == array.length
puts "array does not contain duplicates"
else
puts "array does contain duplicates"
print array.select{ |x| array.count(x) > 1}.uniq!
end