String Indexing and Slicing

String Indexing

• Examining parts of a string is a common operation
• Each character is a sting is referred to by an be index
  • 0 refers to the first character
  • length of the string minus 1 refers to the last character

Index operator

• consists of square brakets

Syntax

    string[index]

• return the item in string at index

s = “UC Berkeley”

Index 0 1 2 3 4 5 6 7 8 9 10 Character U C B e r k e l e y

Negative Indexes

• Indexes can also be negative
  • -1 (last character)
  • -len(s) (first character)
  • -len(s) + n (nth positive index) A generatlized formula to convert positive indexes

Negative Indexes Example

s = “UC Berkeley”

Index -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 Character U C B e r k e k e y

String slicing

• A range or substring of characters
• a Slice has two indexes:
  • start index - beginning of a slice
  • stop index - the end of a slice (exclusive)
    • character under the stop index itself is excluded

s = “UC Berkeley”

Index 0 1 2 3 4 5 6 7 8 9 10 Character U C B e r k e k e y

s[3:11]
#Output
Berkeley

Index operator (Slice)

string[start:stop]

• return the slice of string starting at index start and ending at index stop(exclusive)

Default Slice Values Examples

• stop index

s[:stop]
# equivalent
s[0:stop]

• Default Slice Example

s = "University of California"
print(s[0:10])
print(s[:10])
# Output
University
University

Default Slice Values

s[start:]
# equivalent
s[start:len(s)]
# stop index of the length of s

• Slice Default Example

s = "University of California"
print(s[11:len(s)])
print(s[11:])
# Output
of California
of California