Recursion Examples
from typing import List, Optional, Sequence
# A recursive function is a function that calls itself. They're an alternative option to iteration, like using a while
# or for loop. Sometimes, they can make code easier to read and write. The examples below are overly simplified and
# have obviously better alternatives.
def multiply_string(s: str, n: int) -> str:
# There are 3 main parts to a recursive function.
# 1. A way to go to either the base case (no recursion) or recursive case (function calls itself).
if n == 0: # The switch between base and recursive is often an if statement.
# 2. The base case. This is the case that doesn't require recursion.
return '' # Any string times 0 is empty string. That's easy.
# 3. The recursive case. This same function gets called again, but slightly differently.
# This needs to eventually get to the base case.
return s + multiply_string(s, n - 1) # Add the string to result of multiplying the string times n -1.
# Eventually, n will be 0 and we'll be at the base case.
# This is also a good order to first write your recursive functions, even if you rewrite in a different order later.
# 1. What input is the base case?
# 2. What calculation needs to be done for the base case (if any)?
# 3. What calculation needs to be done for the recursive case?
# For the base case, there are some common patterns. It's often when an integer input is 0 or 1, the collection is
# totally empty, or you're at the thing you're looking for. A good question is: "What's the easiest possible case?"
# There can also be more than one base case! You've searched everywhere and haven't found the thing could be one
# base case, and you've found the thing could be another.
# For the recursive case, it's often easiest to think of being one step away from a base case. If your base case is
# n=0, think of how to get from n=0 to n=1. This is usually easier than how to get from n=9 to n=10.
def triangle_number(n: int) -> int:
"""Calculate the sum of 1, 2, n. This is like a triangle with n items on the bottom."""
if n == 0: # Easy case. n == 1 is just as easy, but this way our function also supports 0.
return 0
return n + triangle_number(n - 1) # This will sum the numbers from highest to lowest. The calculation is like:
# (3 + (2 + (1 + (0))))
# Recursive functions often have default arguments. These aren't used when you call the function from the outside,
# but they are used when you call the function recursively. They often hold data about the path to get where you are,
# or what you've already looked at.
def contains(item: object, sequence: Sequence, index: int = 0) -> bool:
# When this function gets called by a user, they won't provide an index and we'll start at 0.
if index == len(sequence): # This means we've gone past the last index in the sequence.
return False
if sequence[index] == item: # Another base case. We've found the item.
return True
# For the recursion, the item stays the same, the sequence stays the same, but let's check what's at the next index.
# Any outside user of this function won't provide the index, but we can provide it now and check the next item.
return contains(item, sequence, index + 1)
# The default arguments also might be mutated. This can help runtime. Rather than create a new thing every time,
# just mutate what was provided. Maybe you return it at the end.
def path_to_zero(n: int, path: Optional[List[int]] = None) -> List[int]:
"""Returns a list of how to get from n to 0 by subtracting 1."""
if path is None: # Classic Python mutable default stuff.
path = []
if n == 0: # If we're already at 0 (the end)
path.append(0) # Our base case, no recursion required.
return path
else:
path.append(n) # Put the current number on the path.
path_to_zero(n - 1, path) # The path will get mutated in the recursive calls, adding the remaining numbers.
return path
# It can be helpful to write recursive functions in a verbose (if: base case, else: recursive) way, then refactor
# them to be simpler. Here's the function above, refactored.
def path_to_zero_refactored(n: int, path: Optional[List[int]] = None) -> List[int]:
if path is None:
path = []
path.append(n) # Add the number we're at.
if n: # If we're not at 0
path_to_zero_refactored(n - 1, path) # Add the next thing until we are.
return path
# A good exercise is translating recursive functions to iterative, or vice versa.
def path_to_zero_iterative(n: int) -> List[int]:
# This isn't the best code, but it's the closest to the recursive version above.
path = []
while n != 0: # Recursive to iterative translations will often have "while not base case" loops.
path.append(n)
n -= 1 # This is the same transformation to n that happens in the recursive call above.
path.append(0) # Our while loop skips the base case, so we do it here.
return path
# The above examples are contrived, but this is an example where the recursive version makes for code that's just
# as good or better than the iterative version.
def collatz(n: int, steps: int = 0) -> int:
"""Calculates how many steps to get from n to 1 following the Collatz rules.
The Collatz rules are:
If the number is even, get the next number by dividing it by 2.
If the number is odd, get the next number by multiplying by 3 and adding 1.
"""
if n == 1: # We're at the end.
return steps
elif n % 2 == 0: # If n is even
return collatz(n // 2, steps + 1)
else: # n is odd
return collatz(n * 3 + 1, steps + 1) # Woah, a second recursive case!
# A downside to recursion in Python is that there is a limit to the call stack. This means there's a limit to how
# deep your recursion can go. The default is 1,000.
def deep_recursion():
"""Requires over 1,000 recursive calls, raising a RecursionError on most Python implementations."""
collatz(9780657630)
# Those are some recursion in Python basics. Go find some other recursive problems and enjoy!from typing import List, Optional, Sequence
# A recursive function is a function that calls itself. They're an alternative option to iteration, like using a while
# or for loop. Sometimes, they can make code easier to read and write. The examples below are overly simplified and
# have obviously better alternatives.
def multiply_string(s: str, n: int) -> str:
# There are 3 main parts to a recursive function.
# 1. A way to go to either the base case (no recursion) or recursive case (function calls itself).
if n == 0: # The switch between base and recursive is often an if statement.
# 2. The base case. This is the case that doesn't require recursion.
return '' # Any string times 0 is empty string. That's easy.
# 3. The recursive case. This same function gets called again, but slightly differently.
# This needs to eventually get to the base case.
return s + multiply_string(s, n - 1) # Add the string to result of multiplying the string times n -1.
# Eventually, n will be 0 and we'll be at the base case.
# This is also a good order to first write your recursive functions, even if you rewrite in a different order later.
# 1. What input is the base case?
# 2. What calculation needs to be done for the base case (if any)?
# 3. What calculation needs to be done for the recursive case?
# For the base case, there are some common patterns. It's often when an integer input is 0 or 1, the collection is
# totally empty, or you're at the thing you're looking for. A good question is: "What's the easiest possible case?"
# There can also be more than one base case! You've searched everywhere and haven't found the thing could be one
# base case, and you've found the thing could be another.
# For the recursive case, it's often easiest to think of being one step away from a base case. If your base case is
# n=0, think of how to get from n=0 to n=1. This is usually easier than how to get from n=9 to n=10.
def triangle_number(n: int) -> int:
"""Calculate the sum of 1, 2, n. This is like a triangle with n items on the bottom."""
if n == 0: # Easy case. n == 1 is just as easy, but this way our function also supports 0.
return 0
return n + triangle_number(n - 1) # This will sum the numbers from highest to lowest. The calculation is like:
# (3 + (2 + (1 + (0))))
# Recursive functions often have default arguments. These aren't used when you call the function from the outside,
# but they are used when you call the function recursively. They often hold data about the path to get where you are,
# or what you've already looked at.
def contains(item: object, sequence: Sequence, index: int = 0) -> bool:
# When this function gets called by a user, they won't provide an index and we'll start at 0.
if index == len(sequence): # This means we've gone past the last index in the sequence.
return False
if sequence[index] == item: # Another base case. We've found the item.
return True
# For the recursion, the item stays the same, the sequence stays the same, but let's check what's at the next index.
# Any outside user of this function won't provide the index, but we can provide it now and check the next item.
return contains(item, sequence, index + 1)
# The default arguments also might be mutated. This can help runtime. Rather than create a new thing every time,
# just mutate what was provided. Maybe you return it at the end.
def path_to_zero(n: int, path: Optional[List[int]] = None) -> List[int]:
"""Returns a list of how to get from n to 0 by subtracting 1."""
if path is None: # Classic Python mutable default stuff.
path = []
if n == 0: # If we're already at 0 (the end)
path.append(0) # Our base case, no recursion required.
return path
else:
path.append(n) # Put the current number on the path.
path_to_zero(n - 1, path) # The path will get mutated in the recursive calls, adding the remaining numbers.
return path
# It can be helpful to write recursive functions in a verbose (if: base case, else: recursive) way, then refactor
# them to be simpler. Here's the function above, refactored.
def path_to_zero_refactored(n: int, path: Optional[List[int]] = None) -> List[int]:
if path is None:
path = []
path.append(n) # Add the number we're at.
if n: # If we're not at 0
path_to_zero_refactored(n - 1, path) # Add the next thing until we are.
return path
# A good exercise is translating recursive functions to iterative, or vice versa.
def path_to_zero_iterative(n: int) -> List[int]:
# This isn't the best code, but it's the closest to the recursive version above.
path = []
while n != 0: # Recursive to iterative translations will often have "while not base case" loops.
path.append(n)
n -= 1 # This is the same transformation to n that happens in the recursive call above.
path.append(0) # Our while loop skips the base case, so we do it here.
return path
# The above examples are contrived, but this is an example where the recursive version makes for code that's just
# as good or better than the iterative version.
def collatz(n: int, steps: int = 0) -> int:
"""Calculates how many steps to get from n to 1 following the Collatz rules.
The Collatz rules are:
If the number is even, get the next number by dividing it by 2.
If the number is odd, get the next number by multiplying by 3 and adding 1.
"""
if n == 1: # We're at the end.
return steps
elif n % 2 == 0: # If n is even
return collatz(n // 2, steps + 1)
else: # n is odd
return collatz(n * 3 + 1, steps + 1) # Woah, a second recursive case!
# A downside to recursion in Python is that there is a limit to the call stack. This means there's a limit to how
# deep your recursion can go. The default is 1,000.
def deep_recursion():
"""Requires over 1,000 recursive calls, raising a RecursionError on most Python implementations."""
collatz(9780657630)
# Those are some recursion in Python basics. Go find some other recursive problems and enjoy!#recursions example-2
def factorial(n):
print("Factoriaal called with " + str(n))
if n < 2:
print("Returning 1")
return 1
result = n * factorial(n - 1)
print("Returning " + str(result) + " for factorial of " + str(n))
return result
factorial(4)
#output
#Factoriaal called with 4
#Factoriaal called with 3
#Factoriaal called with 2
#Factoriaal called with 1
#Returning 1
#Returning 2 for factorial of 2
#Returning 6 for factorial of 3
#Returning 24 for factorial of 4from collections.abc import MutableSequence
from src.typehints import T
def selection_sort_recur(seq: MutableSequence[T], i=0) -> None:
"""Use selection sort recursively on a list in-place."""
if i >= len(seq) - 1:
return
min_val = min(seq[i:])
min_val_i = seq.index(min_val, i)
seq[min_val_i] = seq[i]
seq[i] = min_val
selection_sort_recur(seq, i + 1)Last updated