Python How to Convert List of Strings to Integers

You can convert a list of strings to integers in Python using list comprehension like [int(x) for x in list_of_strings] or the map(int, list_of_strings) function.

Examples

Input["1", "2", "3"]

Output[1, 2, 3]

Input["10", "20", "30", "40"]

Output[10, 20, 30, 40]

Input[]

Output[]

How to Think About It

To convert a list of strings to integers, think about changing each string one by one into a number. You can do this by going through the list and applying the conversion to each item, then collecting the results back into a list.

Algorithm

1

Get the list of strings as input.

2

For each string in the list, convert it to an integer.

3

Collect all converted integers into a new list.

4

Return the new list of integers.

Code

python

list_of_strings = ["1", "2", "3"]
list_of_integers = [int(x) for x in list_of_strings]
print(list_of_integers)

Output

[1, 2, 3]

Dry Run

Let's trace converting ["1", "2", "3"] to integers using list comprehension.

1

Start with list

list_of_strings = ["1", "2", "3"]

2

Convert each string

int("1") = 1, int("2") = 2, int("3") = 3

3

Create new list

list_of_integers = [1, 2, 3]

Original String	Converted Integer
"1"	1
"2"	2
"3"	3

Why This Works

Step 1: Why use int()

The int() function changes a string that looks like a number into an actual number.

Step 2: Why list comprehension

List comprehension applies int() to each string quickly and collects results in a new list.

Step 3: Alternative with map

The map() function can also apply int to each item, returning a map object that can be converted to a list.

Alternative Approaches

Using map() function

python

list_of_strings = ["4", "5", "6"]
list_of_integers = list(map(int, list_of_strings))
print(list_of_integers)

This method is concise and uses built-in map, but returns a map object that needs conversion to list.

Using a for loop

python

list_of_strings = ["7", "8", "9"]
list_of_integers = []
for s in list_of_strings:
    list_of_integers.append(int(s))
print(list_of_integers)

This is more verbose but clear for beginners and easy to debug.

Complexity: O(n) time, O(n) space

Time Complexity

The code processes each string once, so time grows linearly with the list size.

Space Complexity

A new list of integers is created, so space also grows linearly with input size.

Which Approach is Fastest?

List comprehension and map() have similar speed; map() may be slightly faster in some cases but both are efficient.

Approach	Time	Space	Best For
List Comprehension	O(n)	O(n)	Readability and Pythonic style
map() Function	O(n)	O(n)	Concise code and functional style
For Loop	O(n)	O(n)	Beginners and step-by-step debugging