Substring. In Python we use slice syntax to get parts of existing strings—we can extract words from strings. We specify starts and ends.
With indexes and slices, we have one, two or three parts. We use a start index and an end index. The final value is a step—an amount to skip between.
First example. This code slices strings in many ways. The loop iterates through a range of the string, and it then tests several syntax forms.
Part 1 We start with the simplest possible example. We get a substring starting at index 1, and ending at index 3.
Part 2 We get more substrings in a for-loop. We try the start, end, and skip values.
Info The item access, s[n], requires only one number. This is an index access. It can be thought of as a 1-element slice.
# Part 1: the simplest possible example.
animal = "bird"
# Part 2: more complex syntax for substrings.
s = "abcdefghijklmnopqrs"# ... Loop over some indexes.for n in range(2, 4):
# ... Print slices.
print(n, s[n:n + 2])
print(n, s[n:n + 3])
print(n, s[n:n + 4:2])
print(n, s[n:n + 6:2])ir
One character. Consider this program. It uses index accesses to get one-character substrings. We can test these substrings in an if-statement.
value = "web"# Loop over every index in string.for index in range(0, len(value)):
# Get one-character substring.
char = value[index]
# Display substring.
# Test substring.
if char == "w":
Lengths. We do not specify a length to get a substring with slice syntax. But when the start is equal to zero, the second argument will be the length.
Tip The length of the slice is the difference between the two arguments (subtract the first from the second).
string = "abcdefgh"# Two-character substring.
two = string[0:2]
# Three-character substring.
three = string[0:3]
# Four-character substring.
four = string[0:4]
# Five-character substring.
five = string[0:5]
Offsets. This example shows the relation between the start and the end index. If you want a 3-character slice, add three to the start index (the offset).
string = "abcdefgh"# Three characters starting at index 2.
offset = 2
length = 3
sliced = string[offset:offset+length]
Omit start. Sometimes there is no start index in a slice. This simply means the slice starts at zero. So the zero is implicit, assumed, and we can omit it.
value = "rainy day"# Omitting the start means start at 0.
test = value[:2]
print("OMIT START:", test)
test = value[0:2]
print("START AT 0:", test)OMIT START: ra
START AT 0: ra
Negative values. A negative start begins the slice based on the last index (the end) not the first. Here we extract the last two characters of a string.
value = "apple"# Get last two characters.
end = value[-2:]
Item assignment error. A string cannot be mutated. So we cannot use slice syntax to assign into an existing string. Instead, we create a new string—we can concatenate slices.
Detail This error is used for many problems in the Python language. It indicates invalid type usage.
value = "dotperls"
value = " net " # Does not work.
print(value)Traceback (most recent call last):
File "...\file.py", line 6, in <module>
value[3:] = " net "
TypeError: 'str' object does not support item assignment
First words. Here we use a string slice to get the first words in a string. The first_words method counts spaces. When it reaches the requested number, it returns a slice of the string.
Result When we use first_words, we extract the first words separated by spaces. A slice is used to take the substring.
def first_words(input, words):
for i in range(0, len(input)):
# Count spaces in the string.
if input[i] == ' ':
words -= 1
if words == 0:
# Return the slice up to this point.
# Test our method.
test = "Stately, plump Buck Mulligan came from the stairhead."
result = first_words(test, 4)
print(result)Stately, plump Buck Mulligan
All possible substrings. Suppose we want to get all possible substrings in a string. This can be used for some search algorithms—it lets us instantly test whether the substring is found.
Tip We can place all possible substrings in a dictionary, and then do constant-time lookups to search a string.
Detail We use the for-range loop in Python to iterate through all lengths and start indexes in the source string.
test = "abcdefghi"# Loop over possible lengths.for length in range(1, len(test)):
# Loop over possible start indexes.for start in range(0, len(test) - length + 1):
# Take substring and print it.
part = test[start:length + start]
Right. The rightmost few characters of a string can be extracted. We use slice syntax for this too. But we begin with a negative index.