Add content on strings (#31946)

* Add content on strings

Added some content on Indexing, Slicing, Concatenation and Repetition.

* modify code and text formatting

guide/english/python/data-structures/strings/index.md

@@ -19,6 +19,15 @@ Python allows `str` objects, or _strings_, to be expressed in a few different wa
         Traceback (most recent call last):
                 File "<stdin>", line 1, in <module>
         TypeError: 'str' object does not support item assignment
+        
+        Instead, you can convert the string into a list, modify the list element (string character) you wish to change, and then join  the list elements back to a string, like so:
+        
+        >>> foo = "my string"
+        >>> foo_list_form = list(foo)
+        >>> foo_list_form[0] = "a"
+        >>> foo = ' '.join(foo_list_form)
+        >>> print(foo)
+        ay string # The required output
 *   Indexable: You can access any character of `str` object by specifying its index. And as it supports slicing like in `list` and `tuple` objects.
     
         >>> foo = "my string"
@@ -29,14 +38,64 @@ Python allows `str` objects, or _strings_, to be expressed in a few different wa
         >>> foo[::-1]
         'gnirts ym'
         
-    Instead, you can convert the string into a list, modify the list element (string character) you wish to change, and then join  the list elements back to a string, like so:
         
-        >>> foo = "my string"
-        >>> foo_list_form = list(foo)
-        >>> foo_list_form[0] = "a"
-        >>> foo = ' '.join(foo_list_form)
-        >>> print(foo)
-        ay string # The required output
+If we have a string `S`, we can access its length with the command `len(S)`.  
+```python
+>>> S = 'Hero'
+>>> len(S)
+4
+```
+  
+#### Accessing elements, Indexing and Slicing:
+Strings are zero-indexed. We can associate each element in a string with a number, the `1st` element indexed at `0`. So, a string of `N` characters would have `N` indices, from `0` to `N-1`.  
+Indices help us to access elements of the string.  
+```python
+>>> S = 'ABCDEFGH'
+>>> S[0]
+'A'
+>>> S[2]
+'C'
+>>> S[8]
+Traceback (most recent call last):
+  File "<pyshell#9>", line 1, in <module>
+    S[8]
+IndexError: string index out of range
+>>>
+>>> S[7]
+'H'
+```  
+As we see above, we type `S[0]` to access the 1st element of the string, i.e., `'A'`. Also note that since the indices start at `0`, the last element in the string `ABCDEFGH` has the index `7` and not `8`, even though the string itself has length `8`. Hence, we see that `IndexError` message when we try to access `S[8]`.  
+Another cool thing we can do with strings is slicing. Instead of accessing individual elements, we can access chunks of the string with slices. The syntax is `S[start:stop:step]` where `start` (default value `0`) refers to the index at which we want to begin slicing, `stop` (default value `len(S)`) is the index where we want to end slicing, and `step` (default value `1`) is the jump after each element. Let's look at this in action.  
+```python
+>>> S = 'ABCDEFGH'
+>>> S[0:5:1]
+'ABCDE'
+>>> S[2:8]
+'CDEFGH'
+>>> S[1::2]
+'BDFH'
+>>> S[::-1]
+'HGFEDCBA'
+```  
+Please note that the part of string that gets printed out goes up to `stop - 1` value and not `stop`. Hence, `S[0:5:1]` starts at `S[0]` and stops at `S[4]`.  
+When we don't specify a value for a parameter, Python automatically takes the default value as stated above.  
+See how making the `step` equal to `-1` just reverses the string. Cool, huh?  
+Feel free to experiment with different combinations to get comfortable with slicing. It's going to be very useful.  
+  
+#### Concatenation and Repetition:  
+```python
+>>> 2 + 3
+5
+>>> 'My' + ' ' + 'Hero' + ' ' + 'Academia'
+'My Hero Academia'
+>>>
+>>> 4 * 9
+36
+>>> 'Yo!' * 3
+'Yo!Yo!Yo!'
+```  
+The `+` and `*` operators are said to be `overloaded` because they behave differently for different types of objects.  
+Using the `+` operator on strings leads to `concatenation`, while the `*` operator results in `repetition`.  
 
 ## Reference: