In Ruby How to Tell If a String Input Is in Uppercase or Lowercase

How can I check a word is already all uppercase?

You can compare the string with the same string but in uppercase:

'go234' == 'go234'.upcase  #=> false
'GO234' == 'GO234'.upcase  #=> true

How to convert a string to lower or upper case in Ruby

Ruby has a few methods for changing the case of strings. To convert to lowercase, use downcase:

"hello James!".downcase    #=> "hello james!"

Similarly, upcase capitalizes every letter and capitalize capitalizes the first letter of the string but lowercases the rest:

"hello James!".upcase      #=> "HELLO JAMES!"
"hello James!".capitalize  #=> "Hello james!"
"hello James!".titleize    #=> "Hello James!" (Rails/ActiveSupport only)

If you want to modify a string in place, you can add an exclamation point to any of those methods:

string = "hello James!"
string.downcase!
string   #=> "hello james!"

Refer to the documentation for String for more information.

How to convert lowercase string to uppercase and vice versa?

As another pointed out, swapcase already exists, but I suspect it's something you're meant to implement on your own

Here's a functional approach

class String
  def is_upper?
    self == self.upcase
  end

  def is_lower?
    self == self.downcase
  end

  def head
    self[0]
  end

  def tail
    self[1..-1]
  end

  def swapcase
    if empty?
      ""
    elsif head.is_lower?
      head.upcase + tail.swapcase
    elsif head.is_upper?
      head.downcase + tail.swapcase
    else
      head + tail.swapcase
    end
  end
end

puts "abcdE".swapcase
#=> ABCDe

The looping pattern in swapcase is pretty obnoxious and should be decomposed out into a generic function

class String
  def is_upper?
    self == self.upcase
  end

  def is_lower?
    self == self.downcase
  end

  def head
    self[0]
  end

  def tail
    self[1..-1]
  end

  def map &f
    if empty?
      ""
    else
      yield(head) + tail.map(&f)
    end
  end

  def swapcase
    map do |x|
      if x.is_lower?
        x.upcase
      elsif x.is_upper?
        x.downcase
      else
        x
      end
    end
  end
end

It works the same but swapcase is a lot nicer now

puts "abcdE".swapcase
#=> ABCDe

How to check if a string is all upper or lower case in Go?

You can of course compare the upper and lower cased strings in their entirety, or you can short-circuit the comparisons on the first failure, which would be more efficient when comparing long strings.

func IsUpper(s string) bool {
    for _, r := range s {
        if !unicode.IsUpper(r) && unicode.IsLetter(r) {
            return false
        }
    }
    return true
}

func IsLower(s string) bool {
    for _, r := range s {
        if !unicode.IsLower(r) && unicode.IsLetter(r) {
            return false
        }
    }
    return true
}

Is there an existing library method that checks if a String is all upper case or lower case in Java?

Guava's CharMatchers tend to offer very expressive and efficient solutions to this kind of problem.

CharMatcher.javaUpperCase().matchesAllOf("AAA"); // true
CharMatcher.javaUpperCase().matchesAllOf("A SENTENCE"); // false
CharMatcher.javaUpperCase().or(CharMatcher.whitespace()).matchesAllOf("A SENTENCE"); // true
CharMatcher.javaUpperCase().or(CharMatcher.javaLetter().negate()).matchesAllOf("A SENTENCE"); // true
CharMatcher.javaLowerCase().matchesNoneOf("A SENTENCE"); // true

A static import for com.google.common.base.CharMatcher.* can help make these more succinct.

javaLowerCase().matchesNoneOf("A SENTENCE"); // true

In Ruby, can I check if a string contains a letter without using regular expressions?

Try this

str.count("a-zA-Z") > 0

The count function accepts character sets as argument.

This might still fail with ArgumentError: invalid byte sequence in UTF-8 though. If your input is invalid there is likely no way around fixing the encoding.

NB, this scans the entire string, but so does downcase. For a performance benchmark see Eric's answer, performance varies a lot between worst case and best case scenario. As always though, readability comes before premature optimization.

How to check if there are 2 upper case letters, 3 lower case letters, and 1 number in JAVA

This looks like a homework question so I won't solve it directly. However here are some steps you could take:

• Create a method to validate the input (public static boolean isValid(String str))

• Convert the String to a character Array. (There's a method for that!)

• Iterate over the letters and keep track of how many upper and lower case letters there are and how many digits there are. (Using Character.isDigit(), Character.isUpperCase() and Character.isLowerCase())

• If all the requirements are met, return true. Return false otherwise.

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