.. _string-iteration:
Loop Through a String
=====================
The ``range`` command assigns the loop variable a new integer value each time
the loop repeats. What if we want to assign the variable something *other* than
whole numbers?
Characters Instead of Integers
------------------------------
Recall that a string is a series of characters enclosed in quotes, like
``'Hello, World!'`` or ``"Python"``. Just like ``range`` assigns a new integer
to the loop variable, we can set up a ``for`` loop to assign different
characters from a string.
Run the following program to see how this works:
.. raw:: html
Some points to notice:
#. In the ``for`` statement, a string (or a variable containing a string)
replaces ``range``.
#. The first time the loop runs, the loop variable gets assigned the first
character in the string (``character = 'H'`` or ``char = 'T'``).
#. Each time the loop repeats, the variable holds the next character in the
string.
#. Unlike ``range(n)``, which does NOT assign the value of ``n`` to the loop
variable, the last character in the string IS used.
#. Once Python reaches the end of the string, the loop ends.
Characters Another Way
----------------------
Take a look at the following code and its output:
.. admonition:: Example
.. sourcecode:: Python
:linenos:
word = 'Python'
print(word)
print(word[0])
print(word[1])
print(word[5])
**Console Output**
::
Python
P
y
n
Notice how lines 4 - 6 each print a single character from the string.
``print(word[0])`` displays only the first letter (``P``). ``word[1]`` prints
the second character, and ``word[5]`` prints the last.
.. index:: index
We will explore strings in more detail in a later chapter. For now, we just
need to recognize that every character in a string has an *index*. An index
is a number that tells us the location of a character in the string.
Like ``range``, the index values of a string start counting at ``0``.
.. figure:: figures/string-index.png
:alt: Diagram showing the index values of a string.
The index values of a string.
Indexes provide a different way to loop through a string:
.. admonition:: Example
.. raw:: html
Some items to note:
#. By using ``range(6)``, the ``index`` variable takes values from 0 - 5.
#. ``word[index]`` refers to the character in the string at position ``index``.
#. As written, the loop only displays characters up to index 5.
- Try replacing the string with a longer one.
#. If we replace the string with a shorter one, we get an error! For
``word[5]`` to work, there must be a character at index 5.
#. To prevent "index out of range" errors in this loop, we should NOT use
a specific number inside ``range``.
#. Replace ``range(6)`` with ``range(len(word))``. ``len(word)`` always returns
the length of the current string, so changing that string will not break the
loop.
Check Your Understanding
------------------------
.. admonition:: Question
If ``phrase = 'Code for fun'``, then ``phrase[2]`` evaluates to:
.. raw:: html
"o"
"d"
"for"
"fun"
.. Answer = b
.. admonition:: Question
What will be printed in the THIRD iteration of the loop:
.. sourcecode:: Python
:linenos:
for char in 'ABCDEFGHIJ':
print(char)
.. raw:: html