Taking on the CodeWar Challenge. Struggling to fix my code so that if the length of the string input is 0, it would show 0 instead of nil.
Code is below: (suggestions?)
def longest_palindrome(string)
i = 0
a = []
while !string[i..-1].empty?
j = -1
while !string[i..j].empty?
s = string[i..j]
if s.reverse == s
a << s.length
if s.length == nil
a.max = 0
end
end
j -= 1
end
i += 1
end
a.max
end
First, I'd like to point out a couple of issues with the code you posted.
The body of the innermost if statement is never executed because
0 != nil
This means that even if s.length evaluates to zero
s.length == nil
will still be false.
Another issue I'd like to point out is that
a.max = 0
will throw an error that looks like this:
undefined method max=' for []:Array (repl):17:inlongest_palindrome'
(repl):1:in `initialize'
You can't set the max value directly. The reason you never run into this error with the code you posted is because of the first issue I outlined above.
Now to answer your question. There are a lot of ways to do what you are asking. For example, you could just check whether the input string is empty
at the beginning of the code and immediately return 0 if it is therefore
never executing the while loops at all. Maybe with something like
return 0 if string.empty?
at the beginning of the code.
But from your question, I think what you are looking is something more like the following:
def longest_palindrome(string)
i = 0
a = []
while !string[i..-1].empty?
j = -1
while !string[i..j].empty?
s = string[i..j]
if s.reverse == s
a << s.length
end
j -= 1
end
i += 1
end
a.max.to_i
end
Of interest here is the second last line which makes sure a.max is converted
to an integer using the to_i method. Calling this method on nil converts it to 0.
Also, please note I have changed the code to fix the issues I had highlighted earlier.
Related
# Character Counter
class String
def count_char
#lcase_count ,#upcase_count, #num_count, #spl_char_count = [0, 0 ,0 ,0]
each_char { |char|
if ('a'..'z').cover?(char)
#lcase_count += 1
elsif ('A'..'Z').cover?(char)
#upcase_count += 1
elsif ('0'..'9').cover?(char)
#num_count += 1
else
#spl_char_count += 1
end
}
return #lcase_count,#upcase_count,#num_count,#spl_char_count
end
end
input = ARGV[0]
if ARGV.empty?
puts 'Please provide an input'
exit
end
puts 'Lowercase characters = %d' % [input.count_char[0]]
puts 'Uppercase characters = %d' % [input.count_char[1]]
puts 'Numeric characters = %d' % [input.count_char[2]]
puts 'Special characters = %d' % [input.count_char[3]]
Traceback (most recent call last):
1: from new.rb:25:in <main>'
new.rb:3:incount_char': can't modify frozen String (FrozenError)
I think as far, i didnt modify string not sure why getting FrozenError
You are monkeypatching the String class and at the same time introduce new instance variables to String, which already is a terrible design decision, because - unless you are the author of the String class -, you don't know whether or not these variables exist already. Then, in your code, you modify the variables by incrementing them. Since ARGV is an array of frozen strings, you get the error.
Using instance variables here is absolutely unnecessary. Just use normal local variables.
It’s impossible to tell what exactly is wrong with your code, it looks like one of the instance variables you use is initialized as string or likewise. Introducing instance variables in foreign classes is not a good practice in general, also you do abuse each for reducing. Here is an idiomatic ruby code for your task:
class String
def count_char
each_char.with_object(
{lcase_count: 0, upcase_count: 0, num_count: 0, spl_char_count: 0}
) do |char, acc|
case char
when 'a'..'z' then acc[:lcase_count] += 1
when 'A'..'Z' then acc[:upcase_count] += 1
when '0'..'9' then acc[:num_count] += 1
else acc[:spl_char_count] += 1
end
end
end
end
Please note, that this code deals with a simple latin alphabet only. Better approach would be to match regular expressions, like:
lcase_count = scan(/\P{Lower}/).count
upcase_count = scan(/\P{Upper}/).count
...
You can try following,
class String
def count_char
chars = { lcase_count: 0 ,upcase_count: 0, num_count: 0, spl_char_count: 0 }
each_char do |char|
if ('a'..'z').cover?(char)
chars[:lcase_count] += 1
elsif ('A'..'Z').cover?(char)
chars[:upcase_count] += 1
elsif ('0'..'9').cover?(char)
chars[:num_count] += 1
else
chars[:spl_char_count] += 1
end
end
return chars
end
end
str = 'Asdssd'
# => "Asdssd"
str.count_char
# => {:lcase_count=>5, :upcase_count=>1, :num_count=>0, :spl_char_count=>0}
str.count_char[:upcase_count]
# => 1
I couldn't find a document regarding ARGV being a frozen string.
But it seems to be that is the case.
You can use dup to fix your error.
input = ARGV[0].dup
This function is supposed to take a string and return the characters in reverse order.
def reverse(string)
reversedString = "";
i = string.length - 1
while i >= 0
reversedString = reversedString + string[i]
i -= 1
end
puts reversedString
end
however all the tests return false:
puts(
'reverse("abc") == "cba": ' + (reverse("abc") == "cba").to_s
)
puts(
'reverse("a") == "a": ' + (reverse("a") == "a").to_s
)
puts(
'reverse("") == "": ' + (reverse("") == "").to_s
)
Does anyone see what the problem is?
Try to use the default String class reverse method like this:
"Hello World".reverse
"Hello World".reverse!
Check Ruby's String class API at https://ruby-doc.org/core-2.4.0/String.html
If you want to make your custom method, you could use a map like this:
string = String.new
"Hello World".chars.each { | c | string.prepend c }
The problem is your function isn't returning its result, it's printing it. It needs to return reversedString.
As a rule of thumb, functions should return their result. Another function should format and print it.
def reverse(string)
reversedString = "";
i = string.length - 1
while i >= 0
reversedString = reversedString + string[i]
i -= 1
end
return reversedString
end
Note: This was probably an exercise, but Ruby already has String#reverse.
It's good that you're writing tests, but the way you're writing them it's hard to tell what went wrong. Look into a Ruby testing framework like MiniTest.
require "minitest/autorun"
class TestReverse < Minitest::Test
def test_reverse
assert_equal "cba", reverse("abc")
assert_equal "a", reverse("a")
assert_equal "", reverse("")
end
end
That would have told you that your function is returning nil.
1) Failure:
TestReverse#test_reverse [test.rb:16]:
Expected: "cba"
Actual: nil
To make this more Ruby-like yet avoid using the built-in String#reverse method you'd do this:
def reverse(string)
string.chars.reverse.join('')
end
Remember that in Ruby the result of the last operation is automatically the return value of the method. In your case the last operation is puts which always returns nil, eating your value. You want to pass it through.
Try to design methods with a simple mandate, that is, this function should focus on doing one job and one job only: reversing a string. Displaying it is beyond that mandate, so that's a job for another method, like perhaps the caller.
To avoid calling any sort of reverse method at all:
def reverse(string)
result = ''
length = string.length
length.times do |i|
result << string[length - 1 - i]
end
result
end
You can often avoid for almost completely and while frequently if you use things like times or ranges (0..n) to iterate over.
puts prints and returns nil, so the whole method returns nil. If, for debugging reasons , you want to inspect what your method is returning, use p which returns it's argument (reversedString in this case).
def reverse(string)
reversedString = ""
i = string.length - 1
while i >= 0
reversedString = reversedString + string[i]
i -= 1
end
p reversedString # !!!
end
And all 3 tests return true
If I was going to do this, I'd probably take advantage of an array:
ary = 'foo bar baz'.chars
reversed_ary = []
ary.size.times do
reversed_ary << ary.pop
end
reversed_ary.join # => "zab rab oof"
pop removes the last character from the array and returns it, so basically it's walking backwards through ary, nibbling at the end and pushing each character onto the end of reversed_ary, effectively reversing the array.
Alternately it could be done using a string:
ary = 'foo bar baz'.chars
reversed_str = ''
ary.size.times do
reversed_str << ary.pop
end
reversed_str # => "zab rab oof"
or:
reversed_str += ary.pop
I just saw that #tadman did a similar thing with the string. His would run more quickly but this is more readable, at least to my eyes.
I've come across this error twice now in exercises when I'm trying to iterate over indexes in a string with a conditional. When I break it all out with individual cases the logic seems to work, but Ruby doesn't like something about the way it's expressed.
def NumberAddition(str)
def is_num(num)
(0..9).include? num.to_i
end
i = 0
sum = 0
while i < str.length
if is_num(str[i])
if !is_num(str[i+1])
sum += str[i].to_i
else
mult = str[i]
n = 1
while is_num(str[i+n])
mult << str[i+n]
n += 1
end
sum += mult.to_i
end
end
i += 1
end
sum
end
NumberAddition("75Number9")
throws this error:
no implicit conversion of nil into String
(repl):18:in `NumberAddition'
for the line:
mult << str[i+n]
so obviously instead of returning, say, the string "5" for str[i+n],
where i=0 and n=1, it finds nil. Is there a way to express this with my methodology or do I need to retool the whole loop?
Your is_num function doesn't take into account that nil.to_i is 0. That's why you're getting error, because you're trying to append nil to a string. You need to use something like this:
def is_num(num)
# convert string to integer and back to string should be equal to string itself
num.to_i.to_s == num
end
Or, if you want to make sure that you concatenating strings, just convert the argument to a string
mult << str[i+n].to_s # nil gets converted to an empty string
I am wondering why my code works in one instance but doesn't in another. Does it have something to do with local and global variables?
This works:
def factorial num
result = 1
while (num > 1)
result = result * num
num -= 1
end
puts result
end
This doesn't work:
result = 1
def factorial num
while (num > 1)
result = result.to_i * num
num -= 1
end
puts result
end
Everything inside of a method definition cannot see local variables from other places. That sounds weird, but here's two ways to fix it:
result = 1
number = 10
def factorial(num,result_value)
while (num > 1)
result_value = result_value.to_i * num
num -= 1
end
puts result_value
end
factorial(number, result)
That passes result as an argument. That's a great way of handling the method because it doesn't allow you to change the value of result from within the method. That might not seem like a big deal but "pure methods" like this become very valuable as the size the code increases.
This is the "dirty" or un-pure way of doing the same thing:
#result = 1
def factorial(num)
while (num > 1)
#result = #result.to_i * num
num -= 1
end
puts #result
end
Putting an # in front of a variable name allows its scope to expand to methods defined outside of its scope. This becomes a problem as the complexity of your code increases.
Random personal opinion: even though Ruby doesn't require you to put the parentheses next to a method definition, you always should. It makes the code a lot more explicit and easier to read. Follow your heart though ;)
You could experiment by prepending all results with a $ sign, making it global. Prepending with a # results in an instance variable, also interesting. Sidenote: puts prints and returns nil, so your method returns nil.
result = 1 # line 1
def factorial num
while (num > 1)
result = result.to_i * num
num -= 1
end
puts result
end
In this code, factorial doesn't know about result variable from the line 1.
When Ruby find result = result.to_i * num in your method it will first assign nil to the result. Then Ruby will try to run result.to_i * num. Since result is already nil, result.to_i is equal 0.
Here is another example:
def foo
a = a
puts "#{a.class}"
end
foo #NilClass
In the Doesn't Work version the result variable you've assigned to 1 isn't visible inside the factorial method.
Now there is a possibly unexpected behaviour in Ruby that if you try to assign a variable and you refer to the same variable on the right hand side of the assignment, if that variable doesn't have a value yet then it is treated as nil rather than raising an error. So the first time round the loop when you perform
result = result.to_i * num
it's equivalent to result = nil.to_i * num and nil.to_i is equal to 0 so this then sets up result to be 0 for subsequent iterations of the loop and as you're just multiplying the value of result stays on 0.
i tried writing my own alphabet search for Chris Pine tutorial chapter 7, and i waned to implement a binary method. there is no validity for string input, so i don't know what will happen with integers mixed with strings, but the idea was to do it for a list of strings only.
#get list of strings
puts "type words to make a list. type 'exit' to leave program."
x = ()
list = []
while x.to_s.upcase != 'EXIT'
x = gets.chomp
list.push(x)
end
list.pop
#binary method
nano = list.length
half= list.each_slice(nano/2).to_a
left = half[0]
right = half[1]
nanol=left.length
nanor=right.length
#initialize results array
A = []
for i in 0..nano-1
smallest_left = left.min
smallest_right = right.min
#no difference with this commented out or not
#if nanol==0
# A<<smallest_right
#end
#if nanor==0
# A<<smallest_left
#end
#error message points to the line below (rb:44)
if smallest_left<smallest_right
A << smallest_left
print A
left.pop[i]
elsif smallest_left>smallest_right
A << smallest_right
print A
right.pop[i]
else
print A
end
end
for input = ['z','b','r','a'] i can see the list being sorted in the error:
["a"]["a", "b"]["a", "b", "r"] rb:44:in `<': comparison of String with nil failed (ArgumentError)
please help me see my error :) Thanks in advance!
The exception is occurring because you are trying to compare nil. You get a different exception when nil is on the left.
'1' < nil
#=> scratch.rb:1:in `<': comparison of String with nil failed (ArgumentError)
nil > '1'
scratch.rb:1:in `<main>': undefined method `>' for nil:NilClass (NoMethodError)
Your code gets into this situation when the left or right array is empty (ie all of its elements have been added to A already). Presumably, this is why you had originally added the if-statements for nanol == 0 and nanor == 0 (ie to handle when one of the arrays is empty).
Your if-statements have a couple of issues:
You do need the nanol == 0 and nanor == 0 statements
The three if-statements are always run, even though only one would apply in an iteration
nanol and nanor are never re-calculated (ie they will never get to zero)
When the left and right values are equal, you don't actually add anything to the A array
The inside of your iteration should be:
smallest_left = left.min
smallest_right = right.min
nanol=left.length
nanor=right.length
if nanol == 0 #Handles left no longer having values
A << right.delete_at(right.index(smallest_right) || right.length)
elsif nanor == 0 #Handles right no longer having values
A << left.delete_at(left.index(smallest_left) || left.length)
elsif smallest_left < smallest_right
A << left.delete_at(left.index(smallest_left) || left.length)
elsif smallest_left > smallest_right
A << right.delete_at(right.index(smallest_right) || right.length)
else #They are equal so take one
A << left.delete_at(left.index(smallest_left) || left.length)
end
You will still have an issue (no error, but unexpected results) when your list has an odd number of elements. But hopefully that answers your question.