Python String Indexing and Slicing: Essential Techniques for Composite Data Types

In Python, an index (also called a subscript) identifies the position of an element in a sequence, starting from 0; using an index you can retrieve the character at that position.

str1 = "welcome"
# print(str1[3])  # c

Slicing creates a new string by copying a specified range from the original string.

The slicing syntax is string[start:end:step] where:

start – the starting index (included in the result).

end – the ending index (excluded from the result).

step – the stride; the default value is 1.

Example string: str2 = "welcome to beijing" Typical slice operations and their outputs:

print(str2[0:3])      # wel (includes start, excludes end)
print(str2[1:])       # elcome to beijing (from start index to end)
print(str2[:4])       # welc (from beginning up to index 4)
print(str2[1:4:2])     # ec (step of 2)
# print(str2[1:4:0])  # step cannot be zero – would raise an error
print(str2[::])       # welcome to beijing (full copy)
print(str2[::-1])     # gnijieb ot emoclew (reversed string)
print(str2[-9:-3])    # o beij (both indices negative, counting from the right)

The corresponding output lines are:

wel
welcome to beijing
welc
ec
welcome to beijing
gnijieb ot emoclew
o beij