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Better way to write "matching balanced parenthesis" program in Ruby
(8 answers)
Closed 8 years ago.
I just went through those interview coding quizzes for the first time and I'm somewhere between submerging myself in a tub of dran-o and investing in No Tears bubble bath products along with a bunch of toasters.
The problem was as follows:
If you're given a string like "zx(c)abcde[z{x]}", write a function that returns true if the syntax is correct and false if the syntax is incorrect: for example, in that string the brackets and braces are messed up. In other words "{hello}mot[o]" will pass but "{hello}mo{[t}" would not.
My throught process went like: keep a list of opening and closing bracket/brace/parens positions, then see if there is overlap. But that wasn't an optimal solution so I bombed it.
I'd like some help understanding how to solve this problem.
Thanks in advance.
[Edit: I've incorporated both of #sawa's excellent suggestions.]
One way you can do this is with a stack.
MATCH = { '['=>']', '('=>')', '{'=>'}' }
OPENING = MATCH.keys
CLOSING = MATCH.values
def check_for_match(str)
str.chars.each_with_object([]) do |c, arr|
case c
when *OPENING
arr << c
when *CLOSING
return false unless c.eql?(MATCH[arr.pop])
end
end.empty?
end
check_for_match("zx(c)abcde[z{x]}") #=> false
check_for_match("zx(c)abcde[z{x}]") #=> true
[Edit: I thought this question seemed familiar. I and several others answered it a while ago.]
Another way to do this is to first strip out the irrelevant characters, then sequentially remove adjacent matching pairs until either the string is empty (return true) or the string is not empty and there are no more matching adjacent pairs (return false).
def check_for_match(str)
str = str.gsub(/[^\(\)\[\]\{\}]/, '')
while str.gsub!(/\(\)|\[\]|\{\}/, ''); end
str.empty?
end
check_for_match("zx(c)abcde[z{x]}") #=> false
check_for_match("zx(c)abcde[z{x}]") #=> true
Reader challenge: provide a proof that the syntax is incorrect when false is returned.
I would replace each bracket with an XML tag, and just run it through an XML validator. It'll pick out weird stuff like this:
<bracket>stuff<curly>morestuff</bracket></curly>
This will fail XML validation, so you can just return that.
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The program reads in a file of tracks and produces a Track array (tracks). When the user searches for a specific track (search_string) the program returns that it cannot be found, despite there being a track with that name in the array.
class Track
attr_accessor :name, :location
def initialize (name, location)
#name = name
#location = location
end
end
# search for track by name.
# Returns the index of the track or -1 if not found
def search_for_track_name(tracks, search_string)
search_string = gets.chomp
index = 0
while (index < tracks.length)
tracks.include?(search_string)
index = index + 1
end
if tracks.include?(search_string)
return index
else
index = -1
end
return index
end
The condition if tracks.include?(search_string) will never be true because tracks is an array of instances of Track. And therefore the array doesn't include a specific string.
It might include a track which has a matching name. To find the index of a track that has a matching name Array#index might help:
tracks.index { |track| track.name == search_string }
Because index returns nil if the value is not found you can simplify your whole if...else block and probably the whole search_for_track_name method to just this line:
tracks.index { |track| track.name == search_string } || -1
I notice your searching loop does nothing. It checks N times if the array includes the search string, ignores the result of that check, and adds one to the index. So the index is always going to be N (3 in your case). Then you check again whether the array includes the search string, and return -1 if you don't. So your method will always return either N or -1.
Fortunately, Ruby is a language which has already done a lot of the heavy lifting for you. Quite frankly I think you want:
def search_for_track_name(tracks, search_string)
tracks.index(search_string) || -1
end
#index would return nil if the search string is not found; this is more standard for Ruby, but since your requirement is to return -1, the || -1 will put it in for you. (nil and false are the only falsy values in Ruby, so || is a convenient way to shortcut to a default object if the first part is nil.)
I've also just noticed something else, and I think it might be the problem. Do you by any chance have to press Enter twice? Because the first input from read_string will get overwritten by search_string = gets.chomp. You already set search_string before you passed it to the method...
Third thing I've noticed - your track names are being populated with newlines (\n). You'll need to chomp them as well, or follow other users' advice and look for partial matches instead of exact ones.
This question already has an answer here:
How do I count the frequencies of the letters in user input?
(1 answer)
Closed 5 years ago.
Those of you who are marking this as a duplicate - seriously, try to be more responsible. The question that you have marked as the same is quite different to the question I've asked. I've already got a great answer that was not available on any of the questions that were asked about similar topics.
My Original Question:
I see that many have asked how to find the most common letter in a string. My code for that is below. But I'd like to know how to get multiple answers to return. That is to say, if there are several letters that have the same count as most common, how do I get my method to return those only and all of them?
def ltr(string)
results = string.scan(/\w/).reduce(Hash.new(0)) {|h,c| h[c] += 1; h}
sorted = results.sort_by{|key,value| value}
sorted[-1]
end
For example, if the string I input to this method is ("oh how i hate to get up in the morning")...the there are 4 each of the letters 'h', 'o', and 't'. My current method only returns the 't' with the count of '4'. How do I get the others to be returned as well?
Note: Please read questions carefully before you decide to mark them as duplicates. The question that was suggested as a possible duplicate simply shows how to count the frequencies of characters, not how to have it return only those which are the most common. Matt's answer is perfect.
You had a good answer. Just keep going thru the resulting array keeping only the elements where the count is as high as the count of the first item:
def ltr(string)
results = string.scan(/\w/).reduce(Hash.new(0)) {|h,c| h[c] += 1; h}
sorted = results.sort_by{|key,value| value}.reverse
top = sorted[0][1]
sorted.take_while{|key,value| value == top}
end
I have been trying to teach myself to code and have gravitated towards Ruby.
Upon working on if functions I have come across this problem where even if the user input is == to the answer variable it still comes back as false in the if function. It will not come back as true
I can get it to work if it is an Integer, but for some reason it always returns false when having a string. Tried to convert is as well but can not figure it out.
Thank you for any help.
puts("For each question select A, B, or C")
puts("What is the capital of Kentucky?")
puts()
puts("A. Louisville")
puts("B. Frankfort")
puts("C. Bardstown")
puts()
answer = String("B")
text = gets()
puts()
if text == answer
puts("correct")
else
puts("incorrect")
puts("The correct answer was " + answer + ".")
end
There is an additional method you can call when declaring the "text" variable that will solve this.
The method you used preserves a line break at the end when you press enter to submit an answer so it will never actually correspond to the answer. To remove the line break use the "chomp" method.
text = gets.chomp
Hope this helped. Good luck.
I'm trying to use the two following methods to recursively traverse arrays of arrays until the bottom and then come back up with the match results.
You know how in a tennis tournament they start with 32 matches and pair by pair the winner moves ahead, and at the end there's only one winner? That's what I want to replicate in Ruby.
I created a match_winner that always returns the first array for the sake of simplicity. Then, I send the whole tournament array into winner that calls itself recursively until it finds a simple array corresponding to a single match.
def match_winner(array_of_arrays)
return array_of_arrays[0]
end
def winner(tournament)
if tournament[0][0].is_a?(String)
return match_winner(tournament)
else
tournament.each{|e|
winner(e)
}
end
end
tournament = [
[["one", "two"],["three", "four"]],
[["five", "six"],["seven", "eight"]]
]
puts winner(tournament).inspect
Which outputs:
[[["one", "two"], ["three", "four"]], [["five", "six"], ["seven", "eight"]]]
I tried different permutations and variations on this algorithm but I couldn't make it work correctly and return only the final winner.
Does anyone see anything obviously wrong here?
Now I'm calling winner.
I know that the question looks like it's answered, but I just did the same problem and I have to say that simply changing each to map didn't work for me, because, as the code posted, the result is an array of the first-round winners. What worked for me is:
def winner(tournament)
if tournament[0][0].is_a?(String)
return match_winner(tournament)
else
tournament.map!{|e| #use map!, because we need to apply winner() to new values
e=winner(e) #assign new value somewhere, so recursion can climb back
}
end
end
Maybe more experienced developers can explain why that is. Without these two tips it won't work.
And yes, I know "bang" is a bad coding style, danger danger high voltage, but it's my second day with Ruby and I wanted to get this to work.
And, to understand recursion, you have to understand recursion.
Looks like you want map, not each, and, as a commenter above notes, you didn't call winner in the above code.
When you call:
tournament.each {...}
that method actually returns the tournament, which is thus what winner returns.
What you want is to replace it with
tournament.map {...}
which returns a new array consisting of calling "winner" on each element of tournament.
Assuming you have 2^n number of games always and match_winner works ok:
def winner(game)
if game[0][0][0] == game[0][0][0][0]
match_winner( [ game[0], game[1] ] )
else
match_winner( [winner(game[0]), winner(game[1])] )
end
end
I'm currently trying to making my obfuscated, short mandelbrot set code into a one-liner, but I'm having a lot of trouble in doing so. \
It was originally written in Python, but due to Python's limitations, I could not compress the code to one line. So now I'm going to try Ruby. I'm familiar with the language, but I'm not very skilled in using blocks - which is where I am having the issue.
Anyway, the code I want to "convert" is
for q in range(801):
if q%40==0:print s;s=''
i,k=0,0
while(abs(k)<2*(i<15)):k,i=k**2+complex(q%40*.075-2,q/40*-.1+1),i+1
s+=h[i/3]
Which I've attempted to rewrite in Ruby...
h,s='.;*&8#',''
0.upto(800).each {|q|
if !q%40
s=''
p s
end
i,k=0,0
while(k.abs<2*(i<15))
k,i=k**2+Complex(q%40*0.075-2,q/40*-0.1+1),i+1
end
s+=h[i/3]
}
Which throws the error
Line 2:in `upto': no block given (LocalJumpError)
from t.rb:2
After sorting this out, I'd like to shorten it further to one line. Which I've started here...
h,s='.;*&8#','';0.upto(800).each {|q| if !q%40 then s='';p s end;i,k=0,0;while(k.abs<2*(i<15))do k,i=k**2+Complex(q%40*0.075-2,q/40*-0.1+1),i+1 end}
But anyway, I'm just doing this for fun, and hoping to learn a little more Ruby in the process. So if anyone can explain to me what is throw these errors, that would be great.
require 'complex'
h,s='.;*&8#',''
0.upto(800).each {|q|
if q%40 == 0
p s
s=''
end
i,k=0,0
while(k.abs<2 && (i<15))
k,i=k**2+Complex(q%40*0.075-2,q/40*-0.1+1),i+1
end
s+=h[i/3, 1]
}
Issues I dealt with:
Ruby conditionals return boolean values, not 1 or 0, and ! has a high priority
You were clobbering s before printing it
To work on 1.8.x and 1.9.x you need to index strings with [x, 1]
And here it is rearranged a little as a better starting point for a one-liner:
require 'complex'
h,s='.;*&8#',''
800.times { |q|
(p s; s='') if q%40 == 0
i,k=0,0
k,i=k**2+Complex(q%40*0.075-2,q/40*-0.1+1),i+1 while k.abs<2 && i<15
s+=h[i/3, 1]
}
Fist, get rid of the each, the block should go with upto. Once you did that, you'll get another error: undefined method '%' for false:FalseClass. This is because of !q%40, since precedence will first do the logical negation of q (anything but nil and false are true) and then try to evaluate false%40. Also you seem to assume that a zero would evaluate to false, which it doesn't. Then the next problem will be in the condition of your while loop, since k.abs<2 as well as i<15 evaluate to boolen values (`*': true can't be coerced into Fixnum). This should get you started...
Here's a multi-line version; feel free to put it all on one line:
h,s='.;*&8#','';
0.upto(800).each { |q|
(puts s;s='') if q%40==0;
i,k=0,0;
k,i=k**2+Complex(q%40*0.075-2,q/40*-0.1+1),i+1 while k.abs<2*(i<15?1:0);
s+=h[i/3]
}