How to detect lowercase letters in Python?

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I need to know if there is a function that detects the lowercase letters in a string. Say I started writing this program:

s = input('Type a word')

Would there be a function that lets me detect a lowercase letter within the string s? Possibly ending up with assigning those letters to a different variable, or just printing the lowercase letters or number of lowercase letters.

While those would be what I would like to do with it I’m most interested in how to detect the presence of lowercase letters. The simplest methods would be welcome.

To check if a character is lower case, use the islower method of str. This simple imperative program prints all the lowercase letters in your string:

for c in s:
    if c.islower():
         print c

Note that in Python 3 you should use print(c) instead of print c.


Possibly ending up with assigning those letters to a different variable.

To do this I would suggest using a list comprehension, though you may not have covered this yet in your course:

>>> s="abCd"
>>> lowercase_letters = [c for c in s if c.islower()]
>>> print lowercase_letters
['a', 'b', 'd']

Or to get a string you can use ''.join with a generator:

>>> lowercase_letters="".join(c for c in s if c.islower())
>>> print lowercase_letters
'abd'

There are 2 different ways you can look for lowercase characters:

  1. Use str.islower() to find lowercase characters. Combined with a list comprehension, you can gather all lowercase letters:

    lowercase = [c for c in s if c.islower()]
    
  2. You could use a regular expression:

    import re
    
    lc = re.compile('[a-z]+')
    lowercase = lc.findall(s)
    

The first method returns a list of individual characters, the second returns a list of character groups:

>>> import re
>>> lc = re.compile('[a-z]+')
>>> lc.findall('AbcDeif')
['bc', 'eif']

There are many methods to this, here are some of them:

  1. Using the predefined str method islower():

    >>> c="a"
    >>> c.islower()
    True
    
  2. Using the ord() function to check whether the ASCII code of the letter is in the range of the ASCII codes of the lowercase characters:

    >>> c="a"
    >>> ord(c) in range(97, 123)
    True
    
  3. Checking if the letter is equal to it’s lowercase form:

    >>> c="a"
    >>> c.lower() == c
    True
    
  4. Checking if the letter is in the list ascii_lowercase of the string module:

    >>> from string import ascii_lowercase
    >>> c="a"
    >>> c in ascii_lowercase
    True
    

But that may not be all, you can find your own ways if you don’t like these ones: D.

Finally, let’s start detecting:

d = str(input('enter a string : '))
lowers = [c for c in d if c.islower()]

# here i used islower() because it's the shortest and most-reliable
# one (being a predefined function), using this list comprehension
# is (probably) the most efficient way of doing this

You should use raw_input to take a string input. then use islower method of str object.

s = raw_input('Type a word')
l = []
for c in s.strip():
    if c.islower():
        print c
        l.append(c)
print 'Total number of lowercase letters: %d'%(len(l) + 1)

Just do –

dir(s)

and you will find islower and other attributes of str

import re
s = raw_input('Type a word: ')
slower="".join(re.findall(r'[a-z]',s))
supper="".join(re.findall(r'[A-Z]',s))
print slower, supper

Prints:

Type a word: A Title of a Book
itleofaook ATB

Or you can use a list comprehension / generator expression:

slower="".join(c for c in s if c.islower())
supper="".join(c for c in s if c.isupper())
print slower, supper

Prints:

Type a word: A Title of a Book
itleofaook ATB

If you don’t want to use the libraries and want simple answer then the code is given below:

  def swap_alpha(test_string):
      new_string = ""
      for i in test_string:
          if i.upper() in test_string:
              new_string += i.lower()

          elif i.lower():
                new_string += i.upper()

          else:
              return "invalid "

      return new_string


user_string = input("enter the string:")
updated = swap_alpha(user_string)
print(updated)


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