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any() and all()

Introduction

The any() and all() functions take an iterable and evaluate the truthiness of its elements. They are incredibly useful for writing concise conditional checks in interviews.

any()

Returns True if at least one element in the iterable evaluates to True. If the iterable is empty, it returns False.

bools = [False, False, True, False]
print(any(bools)) # True

Short-Circuit Evaluation

any() stops evaluating as soon as it finds the first True value.

def check_positive(n):
    print(f"Checking {n}")
    return n > 0

nums = [-1, -2, 5, -4]
# Stops printing after "Checking 5"
result = any(check_positive(x) for x in nums)

all()

Returns True if every element in the iterable evaluates to True. If the iterable is empty, it returns True.

bools = [True, True, True]
print(all(bools)) # True

Short-Circuit Evaluation

all() stops evaluating as soon as it finds the first False value.

nums = [1, 2, -5, 4]
# Stops evaluating when it hits -5
result = all(x > 0 for x in nums) 
print(result) # False

Combining with Generator Expressions

The true power of any() and all() in interviews comes from combining them with generator expressions. This allows you to replace multi-line for loops with a single, readable line of code.

Example: Checking a Valid Sudoku Row

Instead of: 

def is_valid(row):
    for val in row:
        if val < 1 or val > 9:
            return False
    return True

Write: 

def is_valid(row):
    return all(1 <= val <= 9 for val in row)

Example: Finding an Overlap

Instead of: 

def has_overlap(list1, list2):
    set2 = set(list2)
    for x in list1:
        if x in set2:
            return True
    return False

Write: 

def has_overlap(list1, list2):
    set2 = set(list2)
    return any(x in set2 for x in list1)

Time and Space Complexity

  • Time Complexity: \(O(N)\) in the worst case (it must check every element). \(O(1)\) in the best case due to short-circuiting.
  • Space Complexity: \(O(1)\) when used with a generator expression. (Warning: Using a list comprehension [x > 0 for x in nums] forces \(O(N)\) space before any() even starts. Always use generators (...)).

Summary

  • Use any() to check if at least one condition is met.
  • Use all() to check if all conditions are met.
  • Both functions short-circuit to save time.
  • Always use them with generator expressions (condition for item in iterable) to save space.