Python hacks for competitive programming

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Here are some Python hacks for competitive programming that can help you write efficient and concise code.

Loops


for i in range(2,n) # 2 to n-1
for i in range(0,n) # 0 to n-1
for i in range(0,n,2) # 0,2,4,6 ...
for i in range(n) # 0 to n-1
for i in range(n,0,-1) # n to 1
for i in range(n,-1,-1) # n to -1
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Array initialize

Python does not have built-in support for arrays like other languages (C, Java), but lists can be used as dynamic arrays.

arr = [0] * 10  # Creates an array of size 10 with all elements as 0
arr = [1] * 10  # Creates an array of size 10 with all elements as 1
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Negative index in List 

lst = [1,2,3,4,5,6,7,8,9]
print(lst[-1]) #9
print(lst[-2]) #8
print(lst[-3]) #7
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Use defaultdict for Auto Dictionary Handling

Instead of manually checking if a key exists in a dictionary, use defaultdict.

from collections import defaultdict
d = defaultdict(int)  # Default value is 0
d['a'] += 1
print(d['a'])  # 1
print(d['b'])  # 0 (instead of KeyError)
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For lists:

d = defaultdict(list)
d['a'].append(10)
print(d['a'])  # [10]
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Use itertools for Combinations & Permutations

from itertools import permutations, combinations
print(list(permutations([1, 2, 3], 2)))  # [(1,2), (1,3), (2,1), ...]
print(list(combinations([1, 2, 3], 2)))  # [(1,2), (1,3), (2,3)]
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Use enumerate to Avoid Extra Index Handling

arr = [10, 20, 30]
for i, val in enumerate(arr, start=1):
    print(i, val)
Python

Use divmod() for Simultaneous Quotient & Remainder

q, r = divmod(10, 3)
print(q, r)  # 3 1
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Swap Two Variables

a, b = 5, 10
a, b = b, a
print(a, b)  # 10, 5
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    Use enumerate() for Cleaner Loops

    Instead of:

    arr = [10, 20, 30]
for i in range(len(arr)):
    print(i, arr[i])
    Python

    Use:

    for i, val in enumerate(arr, start=1):
    print(i, val)
    Python

    Reverse a String or List in One Line

    s = "hello"
print(s[::-1])  # "olleh"

arr = [1, 2, 3, 4]
print(arr[::-1])  # [4, 3, 2, 1]
    Python

    For in-place list reversal (better performance):

    arr.reverse()
    Python

    One-Liner If-Else (Ternary Operator)

    Instead of:

    x = 10
if x > 5:
    y = "big"
else:
    y = "small"
    Python

    Use:

    y = "big" if x > 5 else "small"
    Python

    Find Unique Elements in a List Faster

    arr = [1, 2, 2, 3, 3, 4]

unique_arr = list(set(arr))
print(unique_arr)
    Python

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