Convert list of ints to one number?

Question

I have a list of integers that I would like to convert to one number like:

numList = [1, 2, 3] num = magic(numList)  print num, type(num) >>> 123, <type 'int'> 

What is the best way to implement the magic function?

EDIT
I did find this, but it seems like there has to be a better way.

Answer 1

# Over-explaining a bit: def magic(numList):         # [1,2,3]     s = map(str, numList)   # ['1','2','3']     s = ''.join(s)          # '123'     s = int(s)              # 123     return s   # How I'd probably write it: def magic(numList):     s = ''.join(map(str, numList))     return int(s)   # As a one-liner   num = int(''.join(map(str,numList)))   # Functionally: s = reduce(lambda x,y: x+str(y), numList, '') num = int(s)   # Using some oft-forgotten built-ins: s = filter(str.isdigit, repr(numList)) num = int(s) 

Answer 2

Two solutions:

>>> nums = [1, 2, 3] >>> magic = lambda nums: int(''.join(str(i) for i in nums)) # Generator exp. >>> magic(nums) 123 >>> magic = lambda nums: sum(digit * 10 ** (len(nums) - 1 - i) # Summation ...     for i, digit in enumerate(nums)) >>> magic(nums) 123 

The map-oriented solution actually comes out ahead on my box -- you definitely should not use sum for things that might be large numbers:

import collections import random import timeit  import matplotlib.pyplot as pyplot  MICROSECONDS_PER_SECOND = 1E6 FUNS = [] def test_fun(fun):     FUNS.append(fun)     return fun  @test_fun def with_map(nums):     return int(''.join(map(str, nums)))  @test_fun def with_interpolation(nums):     return int(''.join('%d' % num for num in nums))  @test_fun def with_genexp(nums):     return int(''.join(str(num) for num in nums))  @test_fun def with_sum(nums):     return sum(digit * 10 ** (len(nums) - 1 - i)         for i, digit in enumerate(nums))  @test_fun def with_reduce(nums):     return int(reduce(lambda x, y: x + str(y), nums, ''))  @test_fun def with_builtins(nums):     return int(filter(str.isdigit, repr(nums)))  @test_fun def with_accumulator(nums):     tot = 0     for num in nums:         tot *= 10         tot += num     return tot  def time_test(digit_count, test_count=10000):     """     :return: Map from func name to (normalized) microseconds per pass.     """     print 'Digit count:', digit_count     nums = [random.randrange(1, 10) for i in xrange(digit_count)]     stmt = 'to_int(%r)' % nums     result_by_method = {}     for fun in FUNS:         setup = 'from %s import %s as to_int' % (__name__, fun.func_name)         t = timeit.Timer(stmt, setup)         per_pass = t.timeit(number=test_count) / test_count         per_pass *= MICROSECONDS_PER_SECOND         print '%20s: %.2f usec/pass' % (fun.func_name, per_pass)         result_by_method[fun.func_name] = per_pass     return result_by_method  if __name__ == '__main__':     pass_times_by_method = collections.defaultdict(list)     assert_results = [fun([1, 2, 3]) for fun in FUNS]     assert all(result == 123 for result in assert_results)     digit_counts = range(1, 100, 2)     for digit_count in digit_counts:         for method, result in time_test(digit_count).iteritems():             pass_times_by_method[method].append(result)     for method, pass_times in pass_times_by_method.iteritems():         pyplot.plot(digit_counts, pass_times, label=method)     pyplot.legend(loc='upper left')     pyplot.xlabel('Number of Digits')     pyplot.ylabel('Microseconds')     pyplot.show()