Python program to get all permutations of size r of a string

Python's itertools.permutations() function generates all permutations of a specified length from a string. A permutation is an arrangement of elements where order matters.

Syntax

itertools.permutations(iterable, r)

Parameters:

• iterable − The input string or sequence
• r − Length of each permutation (optional, defaults to full length)

Example

Let's generate all permutations of size 3 from the string "HELLO" ?

from itertools import permutations

def solve(s, r):
    vals = list(permutations(s, r))
    result = []
    for x in vals:
        result.append(''.join(x))
    return result

s = "HELLO"
r = 3
print(solve(s, r))

['HEL', 'HEL', 'HEO', 'HLE', 'HLL', 'HLO', 'HLE', 'HLL', 'HLO', 'HOE', 'HOL', 'HOL', 'EHL', 'EHL', 'EHO', 'ELH', 'ELL', 'ELO', 'ELH', 'ELL', 'ELO', 'EOH', 'EOL', 'EOL', 'LHE', 'LHL', 'LHO', 'LEH', 'LEL', 'LEO', 'LLH', 'LLE', 'LLO', 'LOH', 'LOE', 'LOL', 'LHE', 'LHL', 'LHO', 'LEH', 'LEL', 'LEO', 'LLH', 'LLE', 'LLO', 'LOH', 'LOE', 'LOL', 'OHE', 'OHL', 'OHL', 'OEH', 'OEL', 'OEL', 'OLH', 'OLE', 'OLL', 'OLH', 'OLE', 'OLL']

How It Works

The algorithm follows these steps:

1. Use permutations(s, r) to generate all r-length permutations as tuples
2. Convert each tuple to a string using ''.join()
3. Collect all permutation strings in a list

Shorter Approach Using List Comprehension

We can simplify the solution using list comprehension ?

from itertools import permutations

def get_permutations(s, r):
    return [''.join(p) for p in permutations(s, r)]

s = "ABC"
r = 2
print(get_permutations(s, r))

['AB', 'AC', 'BA', 'BC', 'CA', 'CB']

Key Points

• Permutations consider order: 'AB' and 'BA' are different
• Duplicate characters in input create duplicate permutations
• Total permutations = n!/(n-r)! where n is string length
• Returns tuples by default, use join() to convert to strings

Conclusion

Use itertools.permutations() to generate all arrangements of r characters from a string. Convert tuples to strings using join() for readable output.