Substrings

Python does not have pre-defined functions for creating substrings. Strings can be treated as an array of characters. Python starts counting from zero.

Index values

Positive index

Positive index values, with 0 being the first element.

Index: 0  1  2  3  4  5  6  7  8  9  10
Value: H  e  l  l  o     W  o  r  l  d

Negative index

Negative index values, with -1 being the last element.

Index: -11  -10  -9  -8  -7  -6  -5  -4  -3  -2  -1
Value:   H    e   l   l   o       W   o   r   l   d

Individual character

# Positive index
print("Hello World"[6])

# Negative index
print("Hello World"[-5])

myString = "Hello World"
myCharacter = myString[1]
print(myCharacter)

Multiple characters

To extract more than a single character a second parameter is used.

string[start : stop]

Notes: * The stop value is always one more than the last character to be included. * The number of characters selected can be calculated using stop-start

print("Hello World"[0:4])

myString = "Hello World"
mySubstring = myString[6:11]
print(mySubstring)

Left substring

If the first character is included in a substring then the start parameter can be omitted.

myString = "Hello World"
mySubstring = myString[ :4]
print(mySubstring)

Right substring

If the last character is included in a substring then the stop parameter can be omitted.

myString = "Hello World"
mySubstring = myString[6: ]
print(mySubstring)

Using negative index values can be easier.

myString = "Hello World"
mySubstring = myString[-5: ]
print(mySubstring)

Mid substring

myString = "Hello World"
mySubstring = myString[3:8]
print(mySubstring)