Algorithms Flashcards
Reverse and Integer
x = 12345
x = 12345
y = 0
while x > 0 do
y = y*10
y = y + (x%10)
x = x/10
end
puts yMake a nested array
[‘Dave’, 7, ‘Miranda’, 3, ‘Jason’, 11]
p [‘Dave’, 7, ‘Miranda’, 3, ‘Jason’, 11].each_slice(2).to_a
[[“Dave”, 7], [“Miranda”, 3], [“Jason”, 11]]
array = [“Jason”, “Jason”, “Teresa”, “Judah”, “Michelle”, “Judah”, “Judah”, “Allison”]
=> {“Jason”=>2, “Teresa”=>1, “Judah”=>3, “Michelle”=>1, “Allison”=>1}
count = Hash.new(0)
array.each {|v| count[v] += 1 }
or
array.inject(Hash.new(0)) { |total, e| total[e] += 1 ;total}
p count => {“Jason”=>2, “Teresa”=>1, “Judah”=>3, “Michelle”=>1, “Allison”=>1}
dividing an even number with biggest equal odd numbers as;
36 => [9,9,9,9]
54 => [27, 27]
56 => [7,7,7,7,7,7,7,7]
def fun(num) odd = num odd /= 2 while odd.even? [odd] * (num / odd) end
ruby prime number or not
- prime? => false
- prime? => true
class Fixnum
def prime?
('1' * self) !~ /^1?$|^(11+?)\1+$/
end
end19283945 => (5,4,9,3,8,2,9,1)
n.to_s.chars.reverse!.map(&:to_i)
n.to_s.split(“”).reverse!.map(&:to_i)
19283945 => (5,4,9,3,8,2,9,1)
filter_list([1,’a’,’b’,0,15]) => [1,0,15]
l. reject {|i| i.is_a? String }
l. delete_if {|i| i.is_a? String }
l. select {|i| i.is_a? Integer }
l. delete_if { |x| x.class == String }
l. select{ |w| w.is_a? Numeric }
filter_list([1,’a’,’b’,0,15]) => [1,0,15]
add_binary(a,b)
(51,12) => 111111
(1,1) => 10
def add_binary(a,b) (a+b).to_s(2) end (51,12) => 111111 (1,1) => 10
arr(0,1) => 0+1 = 1
arr(-3,2) => -3 + -2 + -1 + 0 + 1 + 2 = -3
if b > a (a..b).to_a.inject(0, &:+) else (b..a).to_a.inject(0, &:+) end
or
b > a ? (a..b).reduce(:+) : (b..a).reduce(:+)
arr(0,1) => 0+1 = 1
arr(-3,2) => -3 + -2 + -1 + 0 + 1 + 2 = -3
find the smallest word in an array
s.split.map(&:size).min
l = s.split(" ").min_by {|w| w.size }
return l.length
# l: length of the shortest wordIncrement by set, then sum all numbers
sequence_sum(2, 6, 2), 12)
sequence_sum(1, 5, 1), 15)
sequence_sum(1, 5, 3), 5)
def sequence_sum(begin_number, end_number, step)
return 0 if begin_number > end_number
sum = 0
(begin_number..end_number).step(step) {|v| sum += v}
sum
end
or
(begin_number..end_number).step(step).reduce(0, :+)
sum of the numbers in a range (enumerable method)
(5. .10).reduce(:+)
(5. .10).inject(:+)
(5. .10).inject {|sum, n| sum + n }
Find the binaries 225 76 10011001 101001011
225 = 128 + 64 + 32 + 0 + 0 + 0 + 0 + 1 –> 11100001
76 = 64 + 0 + 0 + 8 + 4 + 0 + 0 –> 1001100
10011001 –> 128 + 0 + 0 + 16 + 8 + 0 + 0 + 1 = 143
101001011 –> 256 + 0 + 128 + 0 + 0 + 16 + 0 + 2 + 1 = 403
Binary to Hex conversion
1 0 1 0 0 1 1 0
----1 0 1 0 0 1 1 0 = 166 8 4 2 1 - 8 4 2 1 1 0 1 0 - 0 1 1 0 0xA 0x6 ----> 0xA6
Hex to Binary conversion
0xF2
---0xF2 0xF 0x2 8 4 2 1 - 8 4 2 1 1 1 1 1 - 0 0 1 0 ------> 11110010 = 242
Decimal to Hex conversion
137
137 / 16 –> 8 whole(128) + 0x9 (remainder)
8 / 16 –> 0 + 8 (remainder)
——> 0x89
Decimal to Hex conversion
243
243 / 16 –> 15 whole(240) + 0x3 (remainder)
15 –> F
——> 0xF3
Hex to Decimal conversion
0x9F
—-0x9F
9 x 16 = 144
F –> 15
—–> 144 + 15 = 159
Hex to Decimal conversion
0xA59C
----0xA59C 10 x (16x16x16) = 40960 5 x (16x16) = 1280 9 x 16 = 144 C --> 12 --------> 42.396
Regex expressions for “a b”
/s
/S
[^a]
“a b”
/s –> whitespace between a and b
/S –> match only a and b
[^a] –> match whitespace and b (not a)
Regex expressions \d \D \h \H \w \W
\d – Any decimal digit (0-9)
\D – Any character but a decimal digit (not decimal)
\w – matches “word characters”,
\W – matches “non-word characters”.
\h – Any hexadecimal digit (0-9, A-F, a-f) (ruby only)
\H – Any character but a hexadecimal digit (ruby only)
Write a regex that matches any sequence of 3 characters delimited by whitespace characters.
“reds and blues
the lazy cat sleeps”
“reds and blues
the lazy cat sleeps”
/\s…\s/ – matches \whitespace(and/the/cat)whitespace\
%w(1 2 3 4)
=> [“1”, “2”, “3”, “4”]
check if the number is integer?
def integer?(num)
num.to_i.to_s == num
end
How to load a file like .yml?
require ‘yaml’
MESSAGES = YAML.load_file(‘calculator_messages.yml’)
what does .yml file return?
welcome: ‘welcome to programming’
Hash
=> def messages(message)
MESSAGES[message]
end
print float m with only 2 digits
{format(‘%02.2f’, m)}
process => Another game! Y/N
answer = gets.chomp
unless answer.downcase.start_with?(‘y’)
if answer !~ /y/i
famous_words = ‘seven years ago..’
add ‘Four score and’ in front of the famous words string
“Four score and “ + famous_words
famous_words.prepend(“Four score and “)
“Four score and “ «_space;famous_words
flintstones = { “Fred” => 0, “Wilma” => 1, “Barney” => 2, “Betty” => 3}
get the array [“Barney”, 2] ?
flintstones.assoc(“Barney”)
# check that "Spot" is present
ages = { "Herman" => 32, "Lily" => 30, "Grandpa" => 402 }ages. has_key?(“Spot”)
ages. include?(“Spot”)
ages. member?(“Spot”)
ages. key?(“Lily”)
monsters = “The Monsters are creepy in a good way.”
=> “tHE mUNSTERS ARE CREEPY IN A GOOD WAY.”
monsters.swapcase
ages = { "Herman" => 32, "Lily" => 30 }
additional_ages = { "Marilyn" => 22, "Spot" => 237 }ages.merge!(additional_ages)
flintstones = %w(Fred Barney Wilma)
add “Dino” and “Hoppy” to array
flintstones.concat(%w(Dino Hoppy))
statement = “The Flintstones Rock!”
count number of “t” in string
=> statement.scan(‘t’).count
=> statement.chars.count{|char| char =~ /t/}
clear screen in command line?
def clear_screen
system(“clear”) || system(“cls”)
end
produce = {
'apple' => 'Fruit',
'carrot' => 'Vegetable',
'pear' => 'Fruit',
'broccoli' => 'Vegetable'
}
select_fruit(produce) # => {“apple”=>”Fruit”, “pear”=>”Fruit”}
def select_fruit(str)
h = str.delete_if {|key,value| str[key] != 'Fruit' }
end
select_fruit(produce) # => {“apple”=>”Fruit”, “pear”=>”Fruit”}
{ a: “ant”, b: “bear”, c: “cat” }.each_with_index {|pair, index| puts “The index of #{pair} is #{index}.” }
[1, 2, 3].each_with_object([]) {|num, array|
array «_space;num if num.odd? }
{ a: “ant”, b: “bear”, c: “cat” }.first(2)
each_with_index => # The index of [:a, “ant”] is 0.
each_with_object => # [1, 3] # array is initialized in method!
firtst(2) => # [[:a, “ant”], [:b, “bear”]]
odd, even = [1, 2, 3].partition do |num|
num.odd?
end
long, short = { a: “ant”, b: “bear”, c: “cat” }.partition do |key, value|
value.size > 3
end
odd # => [1, 3]
even # => [2]
partition # => [[[:b, “bear”]], [[:a, “ant”], [:c, “cat”]]]
long. to_h # => { :b => “bear” }
short. to_h # => { :a => “ant”, :c => “cat” }
[1, 2, 3].reject do |num|
num > 3
end
reject => [1, 2, 3] (return values if false)
hash = { a: ‘ant’, b: ‘bear’ }
hash.shift
=> [:a, “ant”]
arr = [1, 2, 3, 4, 5]
arr. take(2)
arr. fetch(2)
arr. slice(2)
arr = [1, 2, 3, 4, 5]
arr. take(2) => # [1, 2]
arr. fetch(2) => # 3
arr. slice(2) => # 3
ages = { “Herman” => 32, “Lily” => 30, “Grandpa” => 5843}
sum all values
total_ages = 0
ages.each { |key, value| total_ages += value }
total_ages
ages.values.inject(:+)
ages = { “Herman” => 32, “Lily” => 30, “Grandpa” => 402 }
remove if the age is over 100
ages. keep_if { |_, age| age < 100 }
ages. select {|k,v| v < 100 }
ages. delete_if {|k,v| v > 100 }
flintstones = %w(Fred Barney Wilma Betty BamBam Pebbles)
get index of element starting with ‘Be’
flintstones. index {|v| v.start_with?(‘Be’) }
flintstones. index { |name| name[0, 2] == “Be” }
flintstones. each_with_index {|v, index| puts index if v.chars.first(2).join == ‘Be’}
flintstones = %w(Fred Barney Wilma Betty BamBam Pebbles)
=> [“Fre”, “Bar”, “Wil”, “Bet”, “Bam”, “Peb”]
flintstones. map {|v| v.split(‘’).take(3).join }
flintstones. map! { |name| name[0, 3] }
{ “F”=>1, “R”=>1, “T”=>1, “c”=>1, “e”=>2, … }
statement = “The Flintstones Rock”
{ “F”=>1, “R”=>1, “T”=>1, “c”=>1, “e”=>2, … }
statement.chars.each_with_object({}) {|letter, hash| hash[letter] = statement.scan(letter).count unless letter == ‘ ‘ }
munsters = {
“Lily” => { “age” => 30, “gender” => “female” },
“Grandpa” => { “age” => 402, “gender” => “male” },
“Eddie” => { “age” => 10, “gender” => “male” }, }
add an age_group acc. to the age
{ “Lily” => {“age” => 30, “gender” => “female”, “age_group” => “adult” },
“Grandpa” => { “age” => 402, “gender” => “male”, “age_group” => “senior” },
“Eddie” => { “age” => 10, “gender” => “male”, “age_group” => “kid” } }
munsters.values.each do |hash|
case hash['age']
when (0..18)
hash['age_group'] = 'kid'
when (18...65)
hash['age_group'] = 'adult'
else
hash['age_group'] = 'senior'
end
end2 <=> 1 1 <=> 2 2 <=> 2 'b' <=> 'a' 'a' <=> 'b' 'b' <=> 'b' 1 <=> 'a'
2 <=> 1 # => 1 1 <=> 2 # => -1 2 <=> 2 # => 0 'b' <=> 'a' # => 1 'a' <=> 'b' # => -1 'b' <=> 'b' # => 0 1 <=> 'a' # => nil
’!’ <=> ‘A’
‘b’ <=> ‘}’
‘A’ <=> ‘a’
How do you determine a string’s ASCII position?
’!’ <=> ‘A’ # => -1
‘b’ <=> ‘}’ # => -1
‘A’ <=> ‘a’ # => -1
You can determine a string’s ASCII position by calling ord on the string.
‘b’.ord # => 98
‘}’.ord # => 125
[‘cot’, ‘bed’, ‘mat’]
sort acc. to second character of strings
people = { Kate: 27, john: 25, Mike: 18 }
sort acc. to age
['cot', 'bed', 'mat'].sort_by { |word| word[1] }
# => ["mat", "bed", "cot"]
people.sort_by { |name, age| age }
# => [[:Mike, 18], [:john, 25], [:Kate, 27]]
arr1 = [“a”, “b”, “c”]
arr2 = arr1.dup or arr2 = arr1.clone
arr2[1].upcase!
arr2 # => [“a”, “B”, “c”]
arr1 # => [“a”, “B”, “c”]
arr1 = [“a”, “b”, “c”].freeze
arr2 = arr1.clone
arr2 «_space;“d” => ?
=> RuntimeError: can’t modify frozen Array
arr1 = [“a”, “b”, “c”].freeze
arr2 = arr1.dup
arr2 «_space;“d” =>
arr2 # => [“a”, “b”, “c”, “d”]
arr1 # => [“a”, “b”, “c”]
str = “abc”.freeze
str «_space;“d”
str = >
=> RuntimeError: can’t modify frozen String
Only mutable objects can be frozen because immutable objects, like integers, are already frozen. We can check if an object is frozen with the frozen? method.
5.frozen? # => true
arr = [‘10’, ‘11’, ‘9’, ‘7’, ‘8’]
sort in descending order
# => [“11”, “10”, “9”, “8”, “7”]
arr = [‘10’, ‘11’, ‘9’, ‘7’, ‘8’]
arr. map! {|v| v.to_i }.sort.reverse
arr. sort {|a,b| b.to_i <=> a.to_i }
munsters = {
“Herman” => { “age” => 32, “gender” => “male” },
“Grandpa” => { “age” => 402, “gender” => “male” },
“Marilyn” => { “age” => 23, “gender” => “female”}
}
get the sum of the ages of males
sum = 0
#munsters.values.each {|spec| sum += spec['age'] if spec['gender'] == 'male'}
#munsters.each_value {|spec| sum += spec['age'] if spec['gender'] == 'male' }
sum => # 434hsh = {first: [‘the’, ‘quick’], second: [‘brown’, ‘fox’], third: [‘jumped’], fourth: [‘over’, ‘the’, ‘lazy’, ‘dog’]}
get the vowels as string
hsh = {first: [‘the’, ‘quick’], second: [‘brown’, ‘fox’], third: [‘jumped’], fourth: [‘over’, ‘the’, ‘lazy’, ‘dog’]}
hsh.values.each {|arr| arr.map! {|str| str.scan(/[aeoui]/) } }.flatten.join
=> “euiooueoeeao”
arr = [{a: [1, 2, 3]}, {b: [2, 4, 6], c: [3, 6], d: [4]}, {e: [8], f: [6, 10]}]
get the keys contain only the even values
=> [{:e=>[8], :f=>[6, 10]}]
arr = [{a: [1, 2, 3]}, {b: [2, 4, 6], c: [3, 6], d: [4]}, {e: [8], f: [6, 10]}]
arr.select {|hash| hash.all? {|k,v| v.all? {|val| val.even? } } }
get a random hexadecimal number
num = Random.rand(16).to_s(16)
!!nil => false
def winner?(brd)
!!detect_winner(brd)
end
while detect_winner method returns nothing yet (nil)
!!nil returns false if it returns a value (which is true) then returns true for the winner method to be used.
number = 12345
number, remainder = number.divmod(10)
number? remainder?
number => [1234, 5]
number => 1234
remainder => 5
arr = [1,2,3]
new_arr = arr[0], arr[1], arr[2]
new_arr => ?
arr = [1,2,3]
new_arr = *arr
new_arr => [1,2,3]
vehicles = [‘car’, ‘car’, ‘truck’, ‘car’, ‘SUV’, ‘truck’, ‘motorcycle’, ‘motorcycle’, ‘car’, ‘truck’]
how to count the quantity of ‘car’
puts vehicles.count(‘car’) => 4
puts "==> Enter the first number:"
first = gets.chomp # 3
puts "==> Enter the second number:"
second = gets.chomp # 2
%w(+ - * / % **).each do |op|
equation = "#{first} #{op} #{second}"
puts "==> #{equation} = #{eval(equation)}"
end
eval => Evaluates the Ruby expression(s) in string.
==> Enter the first number: 3 ==> Enter the second number: 2 ==> 3 + 2 = 5 ==> 3 - 2 = 1 ==> 3 * 2 = 6 ==> 3 / 2 = 1 ==> 3 % 2 = 1 ==> 3 ** 2 = 9
array = [3, 2]
%w(+ - * / % **).
perform calculations for the array elements
array = [3, 2]
%w(+ - * / % **).each { |op| array.inject(op) }
==> 3 + 2 = 5
==> 3 - 2 = 1
==> 3 * 2 = 6
==> 3 / 2 = 1
==> 3 % 2 = 1
==> 3 ** 2 = 9oddities([2, 3, 4, 5, 6]) == [2, 4, 6]
oddities([‘abc’, ‘def’]) == [‘abc’]
oddities([123]) == [123]
oddities([]) == []
oddities([2, 3, 4, 5, 6]) == [2, 4, 6]
def oddities(arr)
arr.select { |val| arr.index(val).even? }
end
group([3, 2, 6, 2, 2, 2]) => [[3], [2, 2, 2, 2], [6]]
how to group an array by occurence of its values?
def group(arr)
arr.group_by {|v| v}.values
end
arr.group_by {|v| v} => returns a hash like {3=>[3], 2=>[2, 2, 2, 2], 6=>[6]}
delete_nth([20,37,20,21], 1) # [20,37,21], “From array [20,37,20,21],1 you get”)
delete_nth([1,1,3,3,7,2,2,2,2], 3) # [1, 1, 3, 3, 7, 2, 2, 2], “From array [1,1,3,3,7,2,2,2,2],3 you get”)
delete_nth([20,37,20,21], 1) # [20,37,21], “From array [20,37,20,21],1 you get”)
def delete_nth(array, max_e)
hash = Hash.new(0)
arr.reject {|val| (hash[val] += 1) > max_e }
end
make a password generator which is:
- 6 - 20 characters long
- contains at least one lowercase letter
- contains at least one uppercase letter
- contains at least one number
- contains only alphanumeric characters (no special characters)
ALPHA_NUM = [*(‘a’..’z’), *(‘A’..’Z’), *(‘0’..’9’)]
def password_gen
Array.new(20) { ALPHA_NUM.sample }.take(rand(6..20)).join
end
reverse a string without using String.reverse method
def reversed(str)
reversed_str = ''
str.chars.each {|val| reversed_str.prepend(val) }
reversed_str
end
reversed(‘hello’) => ‘olleh’
create a hash from the hexadecimal strings to integer values
numbers = ‘0123456789ABCDEF’.split(‘’).zip(0..16).to_h
convert an Integer to a String
integer_to_string(4321) == ‘4321’
integer_to_string(0) == ‘0’
integer_to_string(5000) == ‘5000’
integer_to_string(4321) == ‘4321’
def integer_to_string(num)
return [num].join if num == 0
arr = []
while num > 0
arr.unshift(num%10)
num /= 10
end
arr.join
end
convert an Integer to a String (Alternative way)
integer_to_string(4321) == ‘4321’
integer_to_string(0) == ‘0’
integer_to_string(5000) == ‘5000’
integer_to_string(4321) == ‘4321’
def integer_to_string(num)
arr = [*'0'..'9']
str = ''
loop do
num, rem = num.divmod(10)
str.prepend(arr[rem])
break if num == 0
end
str
end
ascii_value(‘Four score’) #== 984
ascii_value(‘Launch School’) #== 1251
ascii_value(‘a’) # == 97
ascii_value(‘’) #== 0
ascii_value(‘Four score’) #== 984
def ascii_value(str)
str.inject(0) {|sum, v| sum + v.ord }
end
or str.sum => return direclty sum of ascii numbers
alternatively => str.map(&:ord).reduce(0, :+)
char.ord.chr == char => .chr is to convert number to ascii character
time_of_day(0) == "00:00" time_of_day(-3) == "23:57" time_of_day(35) == "00:35" time_of_day(-1437) == "00:03" time_of_day(3000) == "02:00"
time_of_day(35) == “00:35”
def time_of_day(minutes)
hours, mins = minutes.divmod(60)
hours = hours % 24
format('%02d:%02d', hours, mins)
end
time_of_week(3000) == “Tuesday 02:00”
time_of_week(800) == “Sunday 13:20”
time_of_week(-4231) == “Thursday 01:29”
time_of_week(-4231) == “Thursday 01:29”
def time_of_week(mins)
t = Time.new(2017, 1, 1) # start from a sunday 00:00
(t + (mins * 60)).strftime(‘%A %H:%M’) # convert minutes to seconds before adding
end
after_midnight(‘00:00’) == 0
after_midnight(‘12:34’) == 754
after_midnight(‘12:34’) == 754
def after_midnight(str)
hour, min = str.split(‘:’).map(&:to_i)
(hour * 60 + min) % (24 * 60)
end
hour, min = str.scan(/\d\d/).map(&:to_i)
Write the method to find primes number between a range.
find_primes(3, 10) => 3, 5, 7
find_primes(1, 2) => 1, 2
find_primes(3, 10) => 3, 5, 7
def is_prime?(number) (2..(number-1)).each do |div| return false if number % div == 0 end end
def find_primes(num1, num2)
(num1..num2).select do |num|
is_prime?(num)
end
end
swap the first and last character of each word of string.
p swap('Oh what a wonderful day it is') == 'hO thaw a londerfuw yad ti si'
p swap('Abcde') == 'ebcdA'
p swap('a') == 'a'
p swap(‘Oh what a wonderful day it is’) == ‘hO thaw a londerfuw yad ti si’
def swap(str)
str.split.each {|word| word[0], word[-1] = word[-1], word[0] }.join(', ')
end
alternative way => str.gsub(/(\w)(\w*)(\w)/, ‘\3\2\1’)
Exchange the values of a and b
a, b = b, a
cleanup(“—what’s my +*& line?”) == ‘ what s my line ‘
cleanup(“—what’s my +*& line?”) == ‘ what s my line ‘
def cleanup(str)
str.gsub(/[^a-z]+/i, ‘ ‘)
end
str. tr_s(‘^a-zA-Z’, ‘ ‘)
str. gsub(/[^a-z]/i, ‘ ‘).squeeze(‘ ‘)
alphabetic_number_sort((0..19).to_a) == [ 8, 18, 11, 15, 5, 4, 14, 9, 19, 1, 7, 17, 6, 16, 10, 13, 3, 12, 2, 0]
NUMBERS = %w(zero one two three fou, five six seven eight nine ten eleven twelve thirteen fourteen fifteen sixteen seventeen eighteen nineteen)
def alphabetic_number_sort(numbers)
numbers.sort_by {|number| NUMBERS[number] }
end
inter_leave([1,2,3], [‘a’, ‘b’, ‘c’]) => [1, “a”, 2, “b”, 3, “c”]
inter_leave([1,2,3], [‘a’, ‘b’, ‘c’]) => [1, “a”, 2, “b”, 3, “c”]
def inter_leave(arr1, arr2) # [arr1, arr2].transpose.flatten # arr1.zip(arr2).flatten end
crunch('ddaaiillyy ddoouubbllee') == 'daily double'
crunch('4444abcabccba') == '4abcabcba'
crunch('ggggggggggggggg') == 'g'
crunch('a') == 'a'
crunch('') == ''crunch(‘ddaaiillyy ddoouubbllee’) == ‘daily double’
def crunch(str)
arr = str.chars
arr.map.with_index {|v, idx| v unless v == arr[idx+1] }.compact.join
end
def crunch(str)
new_arr = []
arr = str.chars
arr.each_with_index {|v, idx| new_arr << v unless v == arr[idx+1] }
new_arr.join
end
str. squeeze
str. scan(/(.)(?!\1+)/).join
str. tr_s(‘ -z’, ‘ -z’)
str. chars.select.with_index(1) { |char, idx| char != str[idx] }.join
remove the fist character of a string
str = ‘hello’ => ‘ello’
str = ‘hello’ => ‘ello’
str[0] = ‘’
str.slice!(0)
str[1..-1] => ‘ello’ # no mutation
Write a method to calculate n elements of a fibonacci serie.
fibonacci(7) => [1, 1, 2, 3, 5, 8, 13]
fibonacci(7) => [1, 1, 2, 3, 5, 8, 13]
def fibonacci(quantity)
fibonacci = [1, 1]
(quantity-2).times { fibonacci << fibonacci.last(2).inject(:+) }
fibonacci
end
reverse an array without using reverse method. (with mutation the array)
list = [1,2,4,3]
reverse!(list) # => [3,4,2,1]
list = [1,2,4,3]
reverse!(list) # => [3,4,2,1]
def reverse!(arr)
result = arr.clone
n = arr.size
count = 0
while n > 0
arr[count] = result[n-1]
n -= 1
count += 1
end
arr
end
list.sort_by!.with_index { |_, idx| -idx }
list.each_index { |idx| list.insert(idx, list.pop) }
reverse an array (not mutating)
list = [1,2,4,3]
reverse(list) # => [3,4,2,1]
list = [1,2,4,3]
reverse(list) # => [3,4,2,1]
def reverse(array)
result_array = []
array.reverse_each { |element| result_array << element }
result_array
end
def reverse(arr)
arr.map.with_index {|_, idx| arr[-idx-1] }
end
def reverse(arr)
list = []
arr.each_index {|idx| list << arr[-idx-1] }
list
end
merge([1, 3, 5], [3, 6, 9]) # == [1, 3, 5, 6, 9]
merge([1, 3, 5], [3, 6, 9]) # == [1, 3, 5, 6, 9]
def merge(arr1, arr2) #arr1.concat(arr2).uniq # arr1 | arr2 # (arr1 + arr2).uniq end
halvsies([1, 2, 3, 4]) == [[1, 2], [3, 4]]
halvsies([1, 5, 2, 4, 3]) == [[1, 5, 2], [4, 3]]
halvsies([5]) == [[5], []]
halvsies([]) == [[], []]
halvsies([1, 2, 3, 4]) == [[1, 2], [3, 4]]
halvsies([1, 5, 2, 4, 3]) == [[1, 5, 2], [4, 3]]
def halvsies(arr)
first, second = arr.partition.with_index do |_, idx|
arr.size.even? ? idx < (arr.size/2) : idx < (arr.size/2)+1
end
end
def halvsies(array)
first_half = array.slice(0, (array.size / 2.0).ceil)
second_half = array.slice(first_half.size, array.size - first_half.size)
[first_half, second_half]
end
find the repeating number
find_dup([1, 5, 3, 1]) #== 1
find_dup([1, 5, 3, 1]) #== 1
def find_dup(arr)
arr.find {|val| arr.count(val) == 2 }
end
def find_dup(arr)
arr.group_by {|v| v}.find {|key, value| value.size > 1 }.first
end
def find_dup(arr)
arr.group_by(&:itself).each {|key, value| return key if value.size > 1 }
end
Write an include? method
include?([1,2,3,4,5], 3) == true include?([1,2,3,4,5], 6) == false include?([], 3) == false include?([nil], nil) == true include?([], nil) == false
include?([1,2,3,4,5], 3) == true
def include?(arr, val)
arr.any? {|v| v == val }
end
arr.member?(val)
!!arr.find_index(val)
triangle(5, 4)
# * # ** # *** # **** #*****
triangle(5, 4)
def triangle(number, coord=1)
case coord
when 1 #top-left
number.downto(1) do |count|
puts ('*' * count).ljust(number)
end
when 2 #top-right
number.downto(1) do |count|
puts ('*' * count).rjust(number)
end
when 3 #bottom-left
1.upto(number) do |count|
puts ('*' * count).ljust(number)
end
when 4 #bottom-right
1.upto(number) do |count|
puts ('*' * count).rjust(number)
end
end
end
letter_case_count(‘abCdef 123’) == { lowercase: 5, uppercase: 1, neither: 4 }
letter_case_count(‘abCdef 123’) == { lowercase: 5, uppercase: 1, neither: 4 }
def letter_case_count(str)
hash = {}
hash[:lowercase] = str.chars.count {|char| char.match(/[a-z]/) }
hash[:uppercase] = str.chars.count {|char| char.match(/[A-Z]/) }
hash[:neither] = str.chars.count {|char| char.match(/[^a-z]/i) }
hash
end
def letter_case_count(str)
counts = {}
counts[:lowercase] = str.scan(/[a-z]/).length
counts[:uppercase] = str.scan(/[A-Z]/).length
counts[:neither] = str.scan(/[^A-Za-z]/).length
counts
end
word_cap(‘four score and seven’) == ‘Four Score And Seven’
word_cap(‘four score and seven’) == ‘Four Score And Seven’
def word_cap(str)
str.split.each {|char| char[0] = char.chr.upcase }.join(' ')
end
def word_cap(str)
str.gsub(/\S+/, &:capitalize)
end
# without using String#capitalize: def word_cap(str) str.downcase.gsub(/(?<=\A|\s)\S/, &:upcase) end
def word_cap(str)
str.downcase.gsub(/(\A\w|\s\S)/) { |match| match.upcase }
end
regexp only the block with quotes.
“this is a string in quotes”. but this one is not.
“this is a string in quotes”. but this one is not.
/([’”]).+?\1/
“this is a string in quotes” will be selected
text = %(We read “War of the Worlds”.)
==> We read “The Time Machine”.
text = %(We read “War of the Worlds”.)
puts text.sub(/([’”]).+\1/, ‘\1The Time Machine\1’)
prints: We read “The Time Machine”.
p fields("Pete,201,Student")
# -> ['Pete', '201', 'Student']
p fields("Pete \t 201 , TA")
# -> ['Pete', '201', 'TA']
p fields("Pete \t 201")
# -> ['Pete', '201']
p fields("Pete \t 201 , TA")
# -> ['Pete', '201', 'TA']
def fields(str)
str.split(/[ \t,]+/)
end
change the FIRST operator to a ?
mystery_math('4 + 3 - 5 = 2')
# -> '4 ? 3 - 5 = 2'
mystery_math('(4 * 3 + 2) / 7 - 1 = 1')
# -> '(4 ? 3 + 2) / 7 - 1 = 1'
def mystery_math(str)
str.sub(/[+-*\/]/, ‘?’)
end
sub => replace the FIRST occurence
gsub => replace ALL occurences
tr => just writing string not regex
tr_s => removes all found duplicates while replacing
format_date(‘2016-06-17’) # -> ‘17.06.2016’
hint => think about capture groupes defined by (…)
format_date(‘2016-06-17’) # -> ‘17.06.2016’
def format_date(str)
#str.split('-').reverse.join('.')
str.sub(/\A(\d\d\d\d)-(\d\d)-(\d\d)\z/, ‘\3.\2.\1’)
end
([a-c])x\1x\1 matches?
([a-c])x\1x\1 matches axaxa, bxbxb and cxcxc.
q(?!u)
q(?=u)
(?
q(?!u) => match a q NOT followed by u
q(?=u) => match a q followed by u
(? match b NOT preceded by an a
(?<=a)b => match b preceded by a
swapcase the every second character of the words
staggered_case(‘I Love Launch School!’) #== ‘I LoVe lAuNcH ScHoOl!’
staggered_case(‘I Love Launch School!’) #== ‘I LoVe lAuNcH ScHoOl!’
def staggered_case(str)
str.gsub(/..?/, &:capitalize)
end
def staggered_case(str)
str.chars.each_slice(2).map { |a, b| [a.upcase, b.to_s.downcase] }.join
end
str.scan(/..?/).map(&:capitalize).join
multiply each element of first arry with second arry and sort the numbers.
multiply_all_pairs([2, 4], [4, 3, 1, 2]) == [2, 4, 4, 6, 8, 8, 12, 16]
multiply_all_pairs([2, 4], [4, 3, 1, 2]) == [2, 4, 4, 6, 8, 8, 12, 16]
def multiply_all_pairs(arr1, arr2)
arr1.product(arr2).map {|a, b| a * b }.sort
end
enultimate(‘last word’) == ‘last’
penultimate(‘Launch School is great!’) == ‘is’
enultimate(‘last word’) == ‘last’
penultimate(‘Launch School is great!’) == ‘is’
def enultimate(str)
str[/\S+(?=\s\S+$)/]
end
str.split[-2]
sum_of_sums([3, 5, 2]) == (3) + (3 + 5) + (3 + 5 + 2) # -> (21)
sum_of_sums([1, 5, 7, 3]) == (1) + (1 + 5) + (1 + 5 + 7) + (1 + 5 + 7 + 3) # -> (36)
sum_of_sums([4]) == 4
sum_of_sums([1, 2, 3, 4, 5]) == 35
sum_of_sums([3, 5, 2]) == (3) + (3 + 5) + (3 + 5 + 2) # -> (21)
def sum_of_sums(arr)
sum = 0
arr.each.with_index(1) {|_, idx| sum += arr.first (dx).inject(:+) }
sum
end
def sum_of_sums(arr)
arr.size.downto(1).reduce(0) { |sum, n| sum + arr[-n] * n }
end
write a madlib game
“Do you walk your blue dog quickly? That’s hilarious!”
def madlib
noun, verb, adj, adv = %w[noun verb adjective adverb].map do |word_type|
puts “Please enter a#{‘n’ if word_type[/\Aa/]} #{word_type}:”
gets.chomp
end
puts format(“Do you %s your %s %s %s? That’s hilarious!”, verb, adj, noun, adv)
end
substrings_at_start(‘abc’) #== [‘a’, ‘ab’, ‘abc’]
substrings_at_start(‘a’) #== [‘a’]
substrings_at_start(‘xyzzy’) #== [‘x’, ‘xy’, ‘xyz’, ‘xyzz’, ‘xyzzy’]
substrings_at_start(‘abc’) #== [‘a’, ‘ab’, ‘abc’]
def substrings_at_start(str, arr=[])
(0..str.size-1).each_with_object([]) {|idx, result| result << str[0..idx] }
end
# recursive def substrings_at_start(str, arr=[]) return arr if arr.size == str.size arr << str[0..arr.size] substrings_at_start(str, arr) end
remove first and last characters
str = ‘hello’
arr = [1, 2, 3, 4, 5]
str = ‘hello’
unmutated methods
str.chr => ‘h’
str.chop => ‘o’
mutated methods
str.prepend(a) => prepends a
str[0] = ‘’
str[-1] =’’
arr = [1, 2, 3, 4, 5]
unmutated methods
arr.first => 1
arr.last => 5
mutated methods
arr. shift => 1
arr. pop => 5
arr. unshift(a) => prepends a
Regex cheat sheet
/a/ character ‘a’
/\// character ‘/’ (/\?*+{[.|()^$ need to be escaped with )
/./ any character (including newline for /…/m)
/a?/ 0..1 'a'
/a*/ 0..n 'a'
/a+/ 1..n 'a'
/a{2,7}/ 2..7 'a'
/a{2,}/ 2..n 'a'
/a{,7}/ 0..7 'a'
/a?bc?/ ‘b’ or ‘ab’ or ‘bc’ or ‘abc’
/a|bc/ ‘a’ or ‘bc’
/(a|b)c/ ‘ac’ or ‘bc’
/[abc]/ a or b or c
/[^abc]/ any character except a or b or c
/[a-cF-H]/ a or b or c or F or G or H
/\d/ any digit [0-9]
/\w/ any letters, numbers or underscores [a-zA-Z0-9_]
/\s/ any whitespace character (including newline for /…/m)
/\D/ any character except digits
/\W/ any character except letters, numbers or underscores
/\S/ any character except whitespace
/^abc/ abc after line start
/abc$/ abc before line end
Regex cheat sheet
/a/ character ‘a’
/\// character ‘/’ (/\?*+{[.|()^$ need to be escaped with )
/./ any character (including newline for /…/m)
/a?/ 0..1 'a'
/a*/ 0..n 'a'
/a+/ 1..n 'a'
/a{2,7}/ 2..7 'a'
/a{2,}/ 2..n 'a'
/a{,7}/ 0..7 'a'
/a?bc?/ ‘b’ or ‘ab’ or ‘bc’ or ‘abc’
/a|bc/ ‘a’ or ‘bc’
/(a|b)c/ ‘ac’ or ‘bc’
/[abc]/ a or b or c
/[^abc]/ any character except a or b or c
/[a-cF-H]/ a or b or c or F or G or H
/\d/ any digit [0-9]
/\w/ any letters, numbers or underscores [a-zA-Z0-9_]
/\s/ any whitespace character (including newline for /…/m)
/\D/ any character except digits
/\W/ any character except letters, numbers or underscores
/\S/ any character except whitespace
/^abc/ abc after line start
/abc$/ abc before line end
substrings('abcde') # == ['a', 'ab', 'abc', 'abcd', 'abcde', 'b', 'bc', 'bcd', #'bcde',
# 'c', 'cd', 'cde',
# 'd', 'de',
# 'e' ]p substrings('abcde') # == ['a', 'ab', 'abc', 'abcd', 'abcde', 'b', 'bc', 'bcd', #'bcde',
# 'c', 'cd', 'cde',
# 'd', 'de',
# 'e' ]
def substrings(str, arr = [])
return arr if str.empty?
str.size.times {|idx| arr << str[0..idx] }
str[0] = ''
substrings(str, arr)
end
twice(37) == 74 twice(44) == 44 twice(334433) == 668866 twice(444) == 888 twice(107) == 214 twice(103103) == 103103 twice(3333) == 3333 twice(7676) == 7676 twice(123_456_789_123_456_789) == 123_456_789_123_456_789 twice(5) == 10
twice(103103) == 103103
twice(3333) == 3333
def twice(num)
num.to_s[/\A(\d+)\1\z/] ? num : num * 2
end
alternative way
def twice(number)
return number * 2 if number.to_s.size.odd?
arr = number.to_s.chars
arr.first(arr.size/2) == arr.last(arr.size/2) ? number : number * 2
end
uppercase?('t') == false
uppercase?('T') == true
uppercase?('Four Score') == false
uppercase?('FOUR SCORE') == true
uppercase?('4SCORE!') == true
uppercase?('') == truedef uppercase?(str)
!str[/[a-z]/]
end
def uppercase?(str) str == str.upcase end
p sequence(5, 1) == [1, 2, 3, 4, 5] p sequence(4, -7) == [-7, -14, -21, -28] p sequence(3, 0) == [0, 0, 0] p sequence(0, 1000000) == []
p sequence(5, 1) == [1, 2, 3, 4, 5] p sequence(4, -7) == [-7, -14, -21, -28]
def sequence(count, start)
start.step(by: start).take(count)
end
def sequence(count, start)
(1..count).map { |idx| idx * start }
end
Grade
90 <= score <= 100 'A' 80 <= score < 90 'B' 70 <= score < 80 'C' 60 <= score < 70 'D' 0 <= score < 60 'F'
get_grade(95, 90, 93) == “A”
get_grade(50, 50, 95) == “D”
GRADES = { A: 90..1000, B: 80...90, C: 70...80, D: 60...70, F: 0...60 }
def get_grade(*scores)
GRADES.find { |k, v| v.cover?(scores.reduce(:+).fdiv(scores.size)) }.first.to_s
endbuy_fruit([[“apples”, 3], [“orange”, 1], [“bananas”, 2]])
def buy_fruit(arr)
arr.map {|a, b| b}
end
buy_fruit([[“apples”, 3], [“orange”, 1], [“bananas”, 2]])
def buy_fruit(arr)
arr.map {|a, b| b}
end
a => [“apples”, “orange”, “bananas”]
b => [3, 1, 2]
diamond(9)
*
***
*****
*******
*********
*******
*****
***
*
def diamond(n)
[*(1...n), *n.downto(1)].each { |i| puts ('*' * i).center(n) if i.odd? }
end
[*(1…n), *n.downto(1)] => [1, 2, 3, 4, 5, 6, 7, 8, 9, 8, 7, 6, 5, 4, 3, 2, 1]
word_to_digit(‘Please call me at five five five one two three four. Thanks.’) == ‘Please call me at 5 5 5 1 2 3 4. Thanks.
Tip for creating the hash:
DIGITS = %w(one two three four five six seven eight nine).zip([1,2,3,4,5,6,7,8,9]).to_h
word_to_digit(‘Please call me at five five five one two three four. Thanks.’) == ‘Please call me at 5 5 5 1 2 3 4. Thanks.
DIGITS = %w(zero one two three four five six seven eight nine)
def word_to_digit(str)
str.gsub(/\w+/) { |word| DIGITS.index(word) || word }
end
DIGIT_HASH = {
‘zero’ => ‘0’, ‘one’ => ‘1’, ‘two’ => ‘2’, ‘three’ => ‘3’, ‘four’ => ‘4’,
‘five’ => ‘5’, ‘six’ => ‘6’, ‘seven’ => ‘7’, ‘eight’ => ‘8’, ‘nine’ => ‘9’
}.freeze
def word_to_digit(words)
DIGIT_HASH.keys.each do |word|
words.gsub!(/\b#{word}\b/, DIGIT_HASH[word])
end
words
end
def word_to_digit(str)
NUMERIC_WORDS.each do |number|
str = str.gsub(number, NUMERIC_WORDS.index(number).to_s)
end
str
end
p fibonacci(20) #== 6765 p fibonacci(100) #== 354224848179261915075 p fibonacci(100_001)
p fibonacci(20) #== 6765 p fibonacci(100) #== 354224848179261915075
def fibonacci(n)
(3..n).reduce([1, 1]) { |(a, b), _| [b, a + b] }.last
end
Count how many 13th friday in a year.
p friday_13th(2015) #== 3 p friday_13th(1986) #== 1 p friday_13th(2019) #== 2
friday_13th(2015) #== 3
friday_13th(1986) #== 1
friday_13th(2019) #== 2
require ‘date’
def friday_13th(year)
(1..12).count do |month|
Date.new(year, month, 13).friday?
end
end
p find_fibonacci_index_by_length(2) #== 7 p find_fibonacci_index_by_length(10) #== 45 p find_fibonacci_index_by_length(100) #== 476 p find_fibonacci_index_by_length(1000) #== 4782 p find_fibonacci_index_by_length(10000) #== 47847
def find_fibonacci_index_by_length(digit)
fib = [1, 1]
idx = 2
loop do
fib << fib.last(2).reduce(:+)
return idx+1 if fib[idx].to_s.size == digit
idx += 1
end
endThe method should determine if any two numbers in the array sum to the target number.
p sum([1, 2, 3, 4], 5) #== [1, 4] p sum([9, 3, 11, 5, 7], 18) #== [11, 7] p sum([1, 2], 15) #== "No match was found"
p sum([1, 2, 3, 4], 5) #== [1, 4] p sum([9, 3, 11, 5, 7], 18) #== [11, 7] p sum([1, 2], 15) #== "No match was found"
def sum(arr, sum)
arr.each_index do |idx|
count = 1
loop do
return [arr[idx], arr[count]] if arr[idx] + arr[count] == sum
count += 1
break if count == arr.size
end
end
'No match was found'
end
def sum(arr, sum)
result = arr.combination(2).to_a.select {|val| val.inject(:+) == sum }
result.empty? ? ‘No match was found’ : result.min
end
*** alternative method for the consecutive array elements => each_cons(param)
list = [‘x’,’cat’, ‘dog’, ‘x’, ‘dolphin’, ‘cougar’, ‘whale’]
slice the array where x is.
[[“cat”, “dog”], [“dolphin”, “cougar”, “whale”]]
list = [‘x’,’cat’, ‘dog’, ‘x’, ‘dolphin’, ‘cougar’, ‘whale’]
def solution(list)
list.slice_when {|prev, curr| curr == 'x' }.to_a.map {|v| v.drop(1) }
end
slice the array if there are at least 3 incremented elements
list = [-6, -3, -2, -1, 0, 1, 3, 4, 5, 7, 8, 9, 11, 12, 15]
==> “-6,-3-1,3-5,7-9,11,12,15”
def solution(list)
list.slice_when {|prev, curr| curr != prev.next }.to_a.map do |arr|
arr.size < 3 ? arr.first(2) : “#{arr[0]}-#{arr[-1]}”
end.join(‘,’)
end
Find the shortest way by eliminating inefficient directions.
a = ["NORTH", "SOUTH", "SOUTH", "EAST", "WEST", "NORTH", "WEST"] p dirReduc(a) #, ["WEST"])
u=["NORTH", "WEST", "SOUTH", "EAST"] p dirReduc(u) #, ["NORTH", "WEST", "SOUTH", "EAST"])
a = [“NORTH”, “SOUTH”, “SOUTH”, “EAST”, “WEST”, “NORTH”, “WEST”]
OPPOSITE = {
"NORTH" => "SOUTH",
"SOUTH" => "NORTH",
"EAST" => "WEST",
"WEST" => "EAST"
}
def dirReduc(arr)
stack = []
arr.each do |dir|
OPPOSITE[dir] == stack.last ? stack.pop : stack << dir
end
stack
end
transpose any matrix
p transpose([[1, 2, 3, 4]]) == [[1], [2], [3], [4]] p transpose([[1], [2], [3], [4]]) == [[1, 2, 3, 4]] p transpose([[1, 2, 3, 4, 5], [4, 3, 2, 1, 0], [3, 7, 8, 6, 2]]) == [[1, 4, 3], [2, 3, 7], [3, 2, 8], [4, 1, 6], [5, 0, 2]] p transpose([[1]]) == [[1]]
def transpose(matrix)
count = 0
result = []
while count < matrix[0].size
result << matrix.each_index.map do |idx|
matrix[idx][count]
end
count += 1
end
result
end
Alternative way:
def transpose(matrix)
matrix.first.zip(*matrix.drop(1))
end
[1, 2, 3].zip([4, 5, 6], [7, 8, 9]) # => [[1, 4, 7], [2, 5, 8], [3, 6, 9]]
matrix = [[1, 2, 3, 4, 5], [4, 3, 2, 1, 0], [3, 7, 8, 6, 2]]
matrix.drop(1) # => [[4, 3, 2, 1, 0], [3, 7, 8, 6, 2]]
matrix.first # => [1, 2, 3, 4, 5]
.zip(*matrix.drop(1)) # => [[1, 4, 3], [2, 3, 7], [3, 2, 8], [4, 1, 6], [5, 0, 2]]
