Checking Strings ================ It is often helpful to examine a character and test whether it is uppercase, lowercase, or whether it is a character or a digit. We can use bracket notation and string methods inside a :ref:`boolean expression ` to check if a string meets a certain condition. Checking Type ------------- The string methods only work on string objects. Let's say ``my_var = 'Python'``. ``my_var.upper()`` works just fine. However, if we were to reassign a number to the variable, like ``my_var = 3.14159``, then ``my_var.upper()`` throws an error. Let's see how we can use the ``type()`` function to check for a string. .. admonition:: Example .. raw:: html Run the program with each of the following values for ``my_var``: #. ``'1234'`` vs. ``1234`` #. ``True`` or ``False`` vs. ``"True"`` or ``"False"`` #. ``0.333`` vs. ``'0.333'`` **Follow Up**: #. Add an ``elif`` statement and code block to check for the ``int`` data type. #. Repeat this process for the ``float`` and ``bool`` data types. #. Rerun the program with the different ``my_var`` values. We can easily modify line 3 to check if ``my_var`` is an ``int``, ``float``, or ``bool`` data type. We could also use an ``if/elif/else`` block to check for the different data types. (*TRY IT*!) .. admonition:: Note Of course, we could avoid the conditional block completely with: .. sourcecode:: Python print("The value {0} is a {1} data type.".format(my_var, type(my_var)) Comparing Strings ----------------- We can use the comparison operators ``==, !=, <, >, <=, >=`` on strings. .. admonition:: Example .. sourcecode:: Python :linenos: my_pet = 'dog' your_pet = 'cat' other_pet = 'crow' print(my_pet == your_pet) print(your_pet[0] == other_pet[0]) print(my_pet < your_pet) **Console Output** :: False True False #. Line 5 evaluates if the strings ``'dog'`` and ``'cat'`` are identical and then prints the boolean result (``False``). #. Line 6 evaluates if the first character in ``'cat'`` and ``'crow'`` are the same (``True``). - Note that the first character comparison for ``'cat'`` and ``'Crow'`` would return ``False`` due to the different cases. - To make the expression *case-insensitive*, we need to convert each character to the same case (e.g. ``your_pet[0].lower() == other_pet[0].lower()``). #. Line 7 evaluates if ``'dog'`` is less than ``'cat'``. This comparison is a bit more complicated. What is really being compared is each character's Unicode value. All characters, even letters, have `Unicode numbers `__. The Unicode value of a letter corresponds with its alphabetical order. So a string that comes earlier in the alphabet is considered *less than* a string that comes later. Since ``'dog'`` follows ``'cat'`` alphabetically, the expression ``'dog' < 'cat'`` evaluates to ``False``. .. admonition:: Note Case matters when comparing characters! CAPITAL letters have smaller Unicode values than lowercase letters. ``'Zebra' < 'zebra'`` is ``True``, ``'Zebra' < 'apple'`` is ``True``, and ``'zebra' < 'apple'`` is ``False``. Checking with ``in`` and ``not in`` ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ If we want to find out if a certain character is in a string, we could iterate through the string and compare each character to the one we want. .. sourcecode:: Python :linenos: title = 'The Hunger Games' search_character = 'e' for char in title: if char == search_character: print("'{0}' is in '{1}'.".format(search_character, title)) However, this is inefficient, since the loop continues even after we find ``search_character``. As coded, the program prints the output once each time ``search_character`` is found. A better approach is to use the ``in`` operator (or its opposite, ``not in``) to return the same information. The ``in`` operator tests if one string is a substring of another. .. sourcecode:: Python :linenos: title = 'The Hunger Games' search_character = 'e' if search_character in title: print("'{0}' is in '{1}'.".format(search_character, title)) .. admonition:: Try It! This example uses the ``in`` operator to decide when to increase the value of ``vowel_count``. .. raw:: html #. The program does not quite work yet. There are 9 vowels in ``'Armadillos or anteaters'``, but the code does not count the capital ``A``. #. Fix the code to be *case-insensitive*. Both capital and lowercase vowels should increase ``vowel_count``. #. Refactor the code to report the number of consonants (non-vowels) in the string. (*Hint*: Use the ``not in`` operator). Checking Case ------------- Let's explore how we can check the case for a character, slice, or an entire string. Fortunately, Python provides methods that check the case of a string, and they deal with non-letter characters properly. .. admonition:: Example .. sourcecode:: Python :linenos: character = 'a' word = "yep!" non_letters = '$10.75' print(character.isupper()) print(word.islower()) print(non_letters.isupper()) **Console Output** :: False True False The ``isupper()`` method returns ``True`` if all the letters in a string are uppercase. If the string contains a single lowercase letter, or no letters at all, the method returns ``False``. The ``islower()`` method behaves in a similar way, but it checks for lowercase letters. Check Your Understanding ------------------------ Evaluate whether the following expressions are ``True`` or ``False``: .. admonition:: Question .. sourcecode:: Python "dog" < "doghouse" .. raw:: html

True
False

.. Answer = True .. admonition:: Question .. sourcecode:: Python "dog" < "Dog" .. raw:: html

True
False

.. Answer = False .. admonition:: Question .. sourcecode:: Python "dog" < "Doghouse" .. raw:: html

True
False

.. Answer = False .. admonition:: Question .. sourcecode:: Python "app" in "Happy" .. raw:: html

True
False

.. Answer = True .. admonition:: Question For which of the following would ``text.upper() == text`` return ``True``? .. raw:: html

text = 'Stop Yelling!'
text = 'STOP YELLING!'
text = 'stop yelling!'
text = 'STOP YELLINg!'
All return True
None return True

.. Answer = b