7.1 String Slicing

String slicing basics

• Strings are a sequence type, having characters ordered by index from left to right
• An index is an integer matching to a specific position in a string's sequence of characters
• An individual character is read using an index surrounded by brackets
  • Ex: my_str[5] reads the character at index 5 of the string my_str. Indices start at 0, so index 5 is a reference to the 6th character in the string.
• Multiple consecutive characters can be read using slice notation.
  • Slice notation has the form my_str[start:end]
  • If my_str is 'Boggle', then my_str[0:3] yields string 'Bog'
• Other sequence types like lists and tuples also support slice notation.

• The last character of the slice is one location before the specified end.
• Negative numbers can be used to specify an index relative to the end of the string.
  • Ex: If the variable my_str is 'Jane Doe!?', then my_str[0:-2] yields 'Jane Doe' because the -2 refers to the second-to-last character '!'

Slicing and slicing operations

• The Python interpreter creates a new string object for the slice.
• Thus, creating a slice of the string variable my_str, and then changing the value of my_str, does not also change the value of the slice.

• A programmer often wants to read all characters that occur before or after some index in the string
• Omitting a start index, such as in my_str[:end] yields the characters from indices 0 to end-1
  • Ex: my_str[:5] reads indices 0-4.
• Similarly, omitting the end index yields the characters from the start index to the end of the string
  • Ex: my_str[5:] yields all characters at and after index 5.

The slice stride

• The stride determines how much to increment the index after reading each element
  • For example, my_str[0:10:2] reads every other element between 0 and 10
• The stride defaults to 1 if not specified.