.. _strings-things-extracting-characters-from-a-string:

Extracting characters from a string
-----------------------------------
.. index::
   pair: string; string extraction

Strings are called "strings" because they are made up of a sequence, or
string, of characters.
Imagine a string of beads, where each character is a bead in the string.


.. graphviz::
   :name: fig_string_characters
   :caption: Characters in a string
   :alt: a sequence of character cells representing a string
   :align: center


   graph char_array {
     fontname = "Bitstream Vera Sans"
     node [
        fontname = "Bitstream Vera Sans"
        fontsize = 14
        shape = "circle"
        style=filled
        fillcolor=lightblue
     ]
     
     a -- b -- c -- d -- e -- f -- h -- i -- j -- k [ constraint=false]
   }

The first operation we are going to perform on a
string is to extract one of the characters. C++ uses square brackets
(``[`` and ``]``) for this operation.

Take a look at the active code below. We extract the character
at index 1 from string ``fruit`` using ``[`` and ``]``.

.. tb-code:: cpp
   :name: extracting_characters_AC_1
   :caption: Accessing a string character

   #include <iostream>
   #include <string>

   int main() {
       std::string fruit = "orange";
       char letter = fruit[1];
       std::cout << letter;
   }

The expression ``fruit[1]`` indicates that I want character number 1
from the string named ``fruit``. The result is stored in a ``char``
named ``letter``.
When I output the value of ``letter``, I get a surprise:

::

   r

``r`` is not the first letter of ``"orange"``.
Unless you are a computer scientist.
Programmers (and most programming languages) always start
counting from zero.
The 0th letter (“zeroeth”) of ``"orange"`` is ``o``.
The 1th letter (“oneth”) is ``r`` and the 2th (“twoeth”) letter is
``a``.

.. note::
   In C++, indexing begins at 0!

If you want the zereoth letter of a string, 
then you have to put zero in the square brackets.

The active code below accesses the first character in string ``fruit``.

.. tb-code:: cpp
   :name: extracting_characters_AC_2
   :caption: Accessing a string character

   #include <iostream>
   #include <string>

   int main() {
       string fruit = "orange";
       char letter = fruit[0];
       cout << letter << '\n';
   }

.. tb-group::
   :name: self_check

   .. tb-tab:: Q1

      .. tb-choice::
         :name: extracting_characters_1

         What would replace the "?" in order to access the letter "b" in the string below?

         .. code-block:: cpp
            :linenos:

            #include <iostream>
            using namespace std;

            int main () {
              string bake = "bake a cake!";
              char letter = bake[?];
            }

         - [ ] 1

           Don't forget that computer scientists do not start counting at 1!
         - [x] 0

           Yes, this would access the letter "b".
         - [ ] 2

           This would access the letter "k".

   .. tb-tab:: Q2

      .. tb-choice::
         :name: extracting_characters_2

         What is printed when the code below is run?

         .. code-block:: cpp
            :linenos:

            #include <iostream>
            using namespace std;

            int main () {
              string lunch = "hello";
              string person = "deejay";
              lunch[0] = lunch[3];
              cout << lunch;
            }

         - [ ] lunch

           When we <code>cout</code> a <code>string</code> we print its content not its name.
         - [ ] jello

           Carefully check which string(s) we are indexing into.
         - [x] lello

           Correct! We copy the 'l' from position 3 of "hello" to position 0.
         - [ ] heljo

           Consider which string(s) we are indexing into.

   .. tb-tab:: Q3

      .. tb-click::
         :name: extracting_characters_3

         Click on each spot where a character in a string is accessed.

         .. code-block:: cpp

            int main() {
                string fruit = "apple";
                char letter = fruit[2];
                cout << fruit << '\n';
                cout <<  fruit[4]  << '\n';
            }



         .. tb-miss:: text:int main() {

            Remember, square brackets [] are used to access a character in a string.

         .. tb-miss:: text:string fruit = "apple";

            Remember, square brackets [] are used to access a character in a string.

         .. tb-hit:: text:fruit[2];

            Correct.

         .. tb-miss:: text:cout << fruit << '\n';

            Remember, square brackets [] are used to access a character in a string.

         .. tb-hit:: text:fruit[4]

            Correct.

   .. tb-tab:: Q4

      .. tb-parsons::
         :name: extracting_characters_4

         Construct a block of code that correctly prints the letter "a".

         .. code-block:: cpp

            {{group}}
            string x;
            {{endgroup}}
            {{group}}
            x = "It is warm outside!";
            {{endgroup}}
            {{distractor}}
            {{group}}
            x = "It is warm outside"
            {{endgroup}}
            {{group}}
            cout << x[7] << '\n';
            {{endgroup}}
            {{distractor}}
            {{group}}
            cout << x[8] << '\n'; #distractor
            {{endgroup}}