Subtract a Value from every Number in a List in Python

# Table of Contents

1. Subtract a value from every number in a List in Python
2. Subtract a value from every number in a List using a for loop
3. Subtract a value from every number in a List using NumPy
4. Subtract a value from every number in a List using map()
5. Subtract a value from every number in a List using enumerate

# Subtract a value from every number in a List in Python

To subtract a value from every number in a list:

1. Use a list comprehension to iterate over the list.
2. Use the subtraction - operator to subtract a value from every number.
3. The new list will contain the results.

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a_list = [5, 15, 25, 35, 45]

a_list = [item - 5 for item in a_list]

print(a_list)  # 👉️ [0, 10, 20, 30, 40]

We used a list comprehension to iterate over the list and subtracted 5 from each number.

On each iteration, we subtract 5 from the current list item and return the result.

Alternatively, you can use a simple for loop.

# Subtract a value from every number in a List using a for loop

This is a four-step process:

1. Declare a new variable that stores an empty list.
2. Use a for loop to iterate over the original list.
3. On each iteration, subtract the number from the current item.
4. Append the result to the new list.

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a_list = [5, 15, 25, 35, 45]


new_list = []

for item in a_list:
    new_list.append(item - 5)

print(new_list)  # 👉️ [0, 10, 20, 30, 40]

The list.append() method adds an item to the end of the list.

# Subtract a value from every number in a List using NumPy

If you use the numpy module, you can also subtract the number directly from a numpy array.

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import numpy as np


arr = np.array([5, 15, 25, 35, 45])

arr = arr - 5

print(arr)  # 👉️ [ 0 10 20 30 40]

a_list = arr.tolist()
print(a_list)  # 👉️ [0, 10, 20, 30, 40]

Make sure you have NumPy installed to be able to run the code sample.

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pip install numpy

# 👇️ or with pip3
pip3 install numpy

Subtracting a number from a numpy array effectively subtracts the number from each element in the array.

Note that this only works with numpy arrays, you'd get a TypeError if you try to subtract a number from a native Python list.

The tolist method converts a numpy array to a list.

Alternatively, you can use the map() function.

# Subtract a value from every number in a List using map()

This is a three-step process:

1. Pass a lambda function and the list to the map() function.
2. The lambda function should subtract the specific value from the provided list element.
3. Use the list() class to convert the map object to a list.

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a_list = [5, 15, 25, 35, 45]

new_list = list(map(lambda item: item - 5, a_list))

print(new_list)  # 👉️ [0, 10, 20, 30, 40]

The map() function takes a function and an iterable as arguments and calls the function with each item of the iterable.

The last step is to use the list() class to convert the map object to a list.

Which approach you pick is a matter of personal preference. I'd use a list comprehension because I find them quite direct and easy to read.

# Subtract a value from every number in a List using enumerate

You can also use the enumerate() function to subtract a value from every number in a list.

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a_list = [5, 15, 25, 35, 45]

for index, item in enumerate(a_list):
    a_list[index] = item - 5

# 👇️ [0, 10, 20, 30, 40]
print(a_list)

The enumerate() function takes an iterable and returns an enumerate object containing tuples where the first element is the index and the second is the corresponding item.

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my_list = ['bobby', 'hadz', 'com']

for index, item in enumerate(my_list):
    print(index, item)  # 👉️ 0 bobby, 1 hadz, 2 com

On each iteration, we use the index to assign to the current list item, subtracting 5 from its value.

I've also written an article on how to sum all values in a dictionary or list of dictionaries.

# Additional Resources

You can learn more about the related topics by checking out the following tutorials:

• Sum the elements of a Tuple or List of Tuples in Python
• How to sum in a For or a While Loop in Python
• How to Sum all Numbers in a Range in Python
• How to sum a List of Strings in Python