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 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 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 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 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

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.

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