Python Program to Convert a list of multiple integers into a single integer

Given a list of integers, write a Python program to convert the given list into a single integer. Examples:

Input : [1, 2, 3]
Output : 123

Input : [55, 32, 890]
Output : 5532890

There are multiple approaches possible to convert the given list into a single integer. Let’s see each one by one. Approach #1 : Naive Method Simply iterate each element in the list and print them without space in between. 

Python3

# Python3 program to convert a list
# of integers into a single integer

# creating a list
lst = [12, 15, 17]

# iterating each element
for i in lst:
    print(i, end="")

Output:

121517

Approach #2 : Using join() Use the join() method of Python. First convert the list of integer into a list of strings( as join() works with strings only). Then, simply join them using join() method. It takes a time complexity of O(n). 

Python3

# Python3 program to convert a list
# of integers into a single integer
def convert(list):

    # Converting integer list to string list
    s = [str(i) for i in list]

    # Join list items using join()
    res = int("".join(s))

    return(res)

# Driver code
list = [1, 2, 3]
print(convert(list))

Output:

123

Approach #3 : Using map() Another approach to convert a list of multiple integers into a single integer is to use map() function of Python with str function to convert the Integer list to string list. After this, join them on the empty string and then cast back to integer. 

Python3

# Python3 program to convert a list
# of integers into a single integer
def convert(list):

    # Converting integer list to string list
    # and joining the list using join()
    res = int("".join(map(str, list)))

    return res

# Driver code
list = [1, 2, 3]
print(convert(list))

Output:

123

Approach #4 : Multiplying by corresponding power of 10 A more mathematical way, which does not require to convert the integer list to string list is, to multiply each integer element with its corresponding power of 10, and then summing it up. It takes a time complexity of O(n). 

Python3

# Python3 program to convert a list
# of integers into a single integer
def convert(list):

    # multiply each integer element with its
    # corresponding power and perform summation

    res = sum(d * 10**i for i, d in enumerate(list[::-1]))

    return(res)

# Driver code
list = [1, 2, 3]
print(convert(list))

Output:

123

Method: Using list comprehension

Python3

lst=[1,2,3]
s=[str(i) for i in lst]
print("".join(s))

Output

123

A small variation to this program leads to less computation in calculation of sum, i.e. using reduce(). This makes use of Horner’s rule, which factors the polynomial representing the number to reduce the number of multiplications. 

Python3

res = functools.reduce(lambda total, d: 10 * total + d, list, 0)

Using list comprehension and the reduce method:

This approach first converts the elements of the list to strings using list comprehension, then concatenates the strings using the reduce function and a lambda function, and finally casts the resulting concatenated string back to an integer before returning it.

Python3

from functools import reduce

def convert(lst):
    # Use list comprehension to concatenate the elements of the list as strings
    concat_lst = [str(i) for i in lst]

    # Use the reduce function to concatenate the strings in the list
    res = reduce(lambda x, y: x + y, concat_lst)

    # Cast the resulting concatenated string back to an integer and return it
    return int(res)

# Test the function
lst = [1, 2, 3]
print(convert(lst)) # Output: 123
#This code is contributed by Edula Vinay Kumar Reddy

Output

123

The time complexity of the approach using list comprehension and the reduce method is O(n), where n is the length of the input list. This is because the reduce function iterates through the elements of the list once and performs a constant number of operations on each element.

The space complexity of this approach is also O(n), because a new list of strings is created and a new string is created to store the concatenated elements. In addition, the reduce function may require additional space to store intermediate results during the reduction process.

Note that the specific time and space complexity of this approach may depend on the implementation details of the particular Python version and environment being used.