Generate N unique Random numbers within a Range in Python

# Table of Contents

1. Generate N unique random numbers within a range in Python
2. Generate N unique random numbers within a range using random.shuffle()
3. Create list of random numbers without duplicates using numpy
4. Generate random number in range excluding some numbers

# Generate N unique random numbers within a range in Python

To generate N unique random numbers within a range:

1. Use the range() class to create a range object.
2. Use the random.sample() method to get a list of N unique random numbers.
3. The random.sample() method returns a list of N unique elements from the provided sequence.

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import random


def gen_random_numbers_in_range(low, high, n):
    return random.sample(range(low, high), n)


# 👇️ [6, 9, 4, 8, 1]
print(gen_random_numbers_in_range(1, 10, 5))

# 👇️ [8, 4, 1, 3, 5]
print(gen_random_numbers_in_range(1, 10, 5))

We used the range() class to get a range object.

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print(list(range(1, 5)))  # 👉️ [1, 2, 3, 4]
print(list(range(1, 8)))  # 👉️ [1, 2, 3, 4, 5, 6, 7]

The range() class is commonly used for looping a specific number of times.

The range() class takes the following arguments:

The random.sample() method returns a list of N unique elements chosen from the provided sequence.

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import random


# 👇️ [5, 3, 8, 4, 7]
print(
    random.sample(
        range(1, 10),
        5
    )
)

The random.sample() method returns an N-sized list of elements chosen from the sequence without replacement.

The random.sample() method raises a ValueError if the sample is larger than the sequence.

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import random

# ⛔️ ValueError
print(
    random.sample(
        range(1, 5),
        5
    )
)

If you need to handle this scenario, use a try/except block.

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import random


try:
    random_numbers = random.sample(
        range(1, 5),
        5
    )
except ValueError:
    # 👇️ this runs
    print('The sample is larger than the sequence')

Trying to retrieve more elements than are present in the range caused a ValueError which was then handled by the except block.

Alternatively, you can use the random.shuffle() method.

# Generate N unique random numbers within a range using random.shuffle()

This is a four-step process:

1. Use the range() class to get a range object.
2. Use the list() class to convert the range object to a list.
3. Use the random.shuffle() method to shuffle the list.
4. Use list slicing to get N unique random numbers from the list.

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import random


def gen_random_numbers_in_range(low, high, n):
    a_list = list(range(low, high))

    random.shuffle(a_list)

    return a_list[:n]


# 👇️ [5, 7, 9, 3, 6]
print(gen_random_numbers_in_range(1, 10, 5))

# 👇️ [9, 4, 6, 3, 8]
print(gen_random_numbers_in_range(1, 10, 5))

The random.shuffle() method takes a sequence and shuffles it in place.

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import random

a_list = list(range(1, 10))

random.shuffle(a_list)

print(a_list)  # 👉️ [4, 7, 3, 9, 1, 2, 8, 6, 5]

The last step is to use list slicing to select N unique random elements from the list.

The syntax for list slicing is my_list[start:stop:step].

The start index is inclusive and the stop index is exclusive (up to, but not including).

Python indexes are zero-based, so the first item in a list has an index of 0, and the last item has an index of -1 or len(my_list) - 1.

The slice list[:n] returns the first n elements of the shuffled list.

# Create list of random numbers without duplicates using NumPy

This is a three-step process:

1. Use the range() class to create a range object.
2. Use the numpy.random.choice() method to create a list of numbers without duplicates.
3. Set the replace argument to False to select unique numbers.

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import numpy as np


list_of_numbers = np.random.choice(
    range(1, 15),
    6,
    replace=False
).tolist()

print(list_of_numbers)  # 👉️ [13, 14, 9, 6, 11, 12]

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

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pip install numpy

# 👇️ or with pip3
pip3 install numpy

The numpy.random.choice method generates a random sample from the given array-like object.

We set the replace argument to False, so no duplicates are selected.

You can use the tolist() method if you need to convert the NumPy array to a native Python list.

# Generate random number in range excluding some numbers

To generate a random number in a range, excluding some numbers:

1. Use a list comprehension to iterate over a range object.
2. Use the not in operator to exclude the numbers from the list.
3. Use the random.choice() method to generate a random number in the range.

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from random import choice


def gen_random_number(low, high, exclude):
    return choice(
        [number for number in range(low, high)
         if number not in exclude]
    )


numbers_to_exclude = [1, 3, 7]

# 👇️ 2
print(gen_random_number(1, 10, numbers_to_exclude))

# 👇️ 19
print(gen_random_number(1, 100, numbers_to_exclude))

We used a list comprehension to iterate over a range object.

The range() class is commonly used for looping a specific number of times.

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print(list(range(1, 5))) # 👉️ [1, 2, 3, 4]
print(list(range(1, 3))) # 👉️ [1, 2]

On each iteration, we use the not in operator to exclude a list of numbers from the result.

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from random import choice


def gen_random_number(low, high, exclude):
    return choice(
        [number for number in range(low, high)
         if number not in exclude]
    )

The in operator tests for membership. For example, x in l evaluates to True if x is a member of l, otherwise, it evaluates to False.

The last step is to use the random.choice() method.

The random.choice method takes a sequence and returns a random element from the non-empty sequence.

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import random

print(random.choice(['a', 'b'])) # 👉️ "a"

If the sequence is empty, the method raises an IndexError.

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from random import choice


def gen_random_number(low, high, exclude):
    return choice(
        [number for number in range(low, high)
         if number not in exclude]
    )


numbers_to_exclude = [1, 3, 7]

result = [
    gen_random_number(1, 10, numbers_to_exclude) for _ in range(3)
]
print(result)  # 👉️ [5, 2, 5]

We used a list comprehension to iterate over a range object of length N and called the get_random_number() function on each iteration.

Alternatively, you can use set objects.

# Generate random number in range excluding some numbers using a set

This is a four-step process:

1. Get a set containing all numbers in the range.
2. Convert the list of numbers to exclude to a set object.
3. Get the difference between the set objects.
4. Use the random.choice() method to generate a random number in the range.

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from random import choice


def gen_random_number(low, high, exclude):
    return choice(
        list(
            set(number for number in range(low, high)) -
            set(exclude)
        )
    )


numbers_to_exclude = [1, 3, 7]

print(gen_random_number(1, 10, numbers_to_exclude))  # 👉️ 4

result = [
    gen_random_number(1, 10, numbers_to_exclude) for _ in range(3)
]
print(result)  # 👉️ [5, 2, 5]

We used the set() class to convert the range object and the list of numbers to exclude to set objects.

The minus sign is a shorthand for calling the difference() method on the set.

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print({1, 2, 3} - {2, 3}) # 👉️ {1}

The difference() method returns a new set with elements in the set that are not in the provided iterable.

We used the list() class to convert the set object to a list and used the random.choice() method to pick a random number from the list.

# Additional Resources

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