The most Pythonic way to convert a list of strings to a list of ints is to use the list comprehension
[int(x) for x in strings]. It iterates over all elements in the list and converts each list element
x to an integer value using the
int(x) built-in function.
This article shows you the simplest ways to convert a one-dimensional list consisting only of strings to a list of ints.
Problem: Given a list of strings
["1", "2", "-3"]. How to convert it to a list of ints
[1, 2, -3]?
- Problem Variant: Given a list of strings with mixed representations
["1", "2.0", "-3.4"]. How to convert it to a list of ints
[1, 2, -3]?
We’ll dive into the easier base problem first and examine the problem variant in Method 5.
Method 1: List Comprehension
Suppose we have a list:
a = ["1", "2", "-3"]
Now, check the type of the first list element:
print(type(a)) # <class 'str'>
a = ["1", "2", "-3"] print([int(x) for x in a]) # [1, 2, -3]
? List comprehension is a compact way of creating lists. The simple formula is
[expression + context]. Expression: What to do with each list element? Context: What elements to select? The context consists of an arbitrary number of
You can watch me explain list comprehensions in this video:
Check the type of numbers in the new list:
A = [int(x) for x in a] print(type(A)) # <class 'int'>
The built-in function
int() converts a string to an integer. Thus, it helps us create a new list of ints from the list of strings in a single line of code.
Method 2: Map Function
The built-in function
map is well optimized and efficient, when it is called, the elements of the list are retrieved upon access. Therefore, one element is stored and processed in memory, which allows the program not to store the entire list of elements in the system memory.
Apply to the same list
a the following code:
a = ["1", "2", "-3"] print(list(map(int, a))) # [1, 2, -3]
map() function applies the first argument, a function, to each element in an iterable. It transforms each element in the original iterable to a new element and returns a new iterable
map object of transformed values. To obtain a list, you need to convert it using the built-in
You can watch my explainer video of the map function here:
Method 3: For Loop
Of course, you can also convert a list of strings to a list of ints using a simple for loop. This is what most people coming from a programming language such as Java and C++ would do as they don’t know the most Pythonic way of using list comprehension, yet (see Method 1).
a = ["1", "2", "-3"] ints =  for element in a: ints.append(int(element)) print(ints) # [1, 2, -3]
This basic method to convert a list of strings to a list of integers uses three steps:
- Create an empty list with
ints = .
- Iterate over each string element using a
forloop such as
for element in list.
- Convert the string to an integer using
int(element)and append it to the new integer list using the
Method 4: List Comprehension + eval()
You can also use the
eval() function in a list comprehension to convert a list of strings to a list of ints:
a = ["1", "2", "-3"] ints = [eval(x) for x in a] print(ints) # [1, 2, -3]
? Python’s built-in
eval(s) function parses the string argument
s into a Python expression, runs it, and returns the result of the expression. If the “expression” is a simple integer representation, Python converts the argument
s to an integer.
You can watch me introducing the ins and outs of the
eval() function in this short guide:
Method 5: Mixed String Representation with Rounding
Problem Variant: Given a list of strings with mixed representations
["1", "2.0", "-3.4", "3.6"]. How to convert it to a list of ints
[1, 2, -3, 4]?
The challenge is to convert each string to a float first and only then convert it to an integer. These two steps are integral and none can be skipped because you need the float to be capable of representing any number. But you also need the integer as this is the goal you set out to do: converting a list of strings to a list of ints.
❗ Convert a list of mixed string representations to a list of rounded integers this by chaining the built-in functions
float() in a list comprehension expression
[round(float(s)) for s in a], assuming that the list of mixed string representation is stored in the variable
a = ["1", "2.0", "-3.4", "3.6"] ints = [round(float(s)) for s in a] print(ints) # [1, 2, -3, 4]
You can learn everything about the
round() function in the following video:
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