# Generate N unique Random numbers within a Range in Python

Last updated: Feb 22, 2023
6 min

## #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.
main.py
import random def gen_random_numbers_in_range(low, high, n): return random.sample(range(low, high), n) # ๐๏ธ [6, 9, 4, 8, 1] print(gen_random_numbers_in_range(1, 10, 5)) # ๐๏ธ [8, 4, 1, 3, 5] print(gen_random_numbers_in_range(1, 10, 5))

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

main.py
print(list(range(1, 5))) # ๐๏ธ [1, 2, 3, 4] print(list(range(1, 8))) # ๐๏ธ [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:

NameDescription
startAn integer representing the start of the range (defaults to 0)
stopGo up to, but not including the provided integer
stepRange will consist of every N numbers from start to stop (defaults to 1)

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

The first argument the method takes is a sequence and the second is the number of random elements to be returned.
main.py
import random # ๐๏ธ [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.

Without replacement means that the same element cannot be returned multiple times.

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

main.py
import random # โ๏ธ ValueError print( random.sample( range(1, 5), 5 ) )

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

main.py
import random try: random_numbers = random.sample( range(1, 5), 5 ) except ValueError: # ๐๏ธ 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.
main.py
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] # ๐๏ธ [5, 7, 9, 3, 6] print(gen_random_numbers_in_range(1, 10, 5)) # ๐๏ธ [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.

main.py
import random a_list = list(range(1, 10)) random.shuffle(a_list) print(a_list) # ๐๏ธ [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).

If the start index is omitted, it is considered to be 0, if the stop index is omitted, the slice goes to the end of the list.

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.
main.py
import numpy as np list_of_numbers = np.random.choice( range(1, 15), 6, replace=False ).tolist() print(list_of_numbers) # ๐๏ธ [13, 14, 9, 6, 11, 12]

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

shell
pip install numpy # ๐๏ธ or with pip3 pip3 install numpy

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

The replace argument determines whether a value of the array-like object can be selected multiple times. The argument defaults to True.

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.
main.py
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] # ๐๏ธ 2 print(gen_random_number(1, 10, numbers_to_exclude)) # ๐๏ธ 19 print(gen_random_number(1, 100, numbers_to_exclude))

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

List comprehensions are used to perform some operation for every element or select a subset of elements that meet a condition.

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

main.py
print(list(range(1, 5))) # ๐๏ธ [1, 2, 3, 4] print(list(range(1, 3))) # ๐๏ธ [1, 2]

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

main.py
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.

x not in l returns the negation of x in l.

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.

main.py
import random print(random.choice(['a', 'b'])) # ๐๏ธ "a"

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

If you need to generate N random numbers in a range, excluding a list of numbers, use a list comprehension.
main.py
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) # ๐๏ธ [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.
main.py
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)) # ๐๏ธ 4 result = [ gen_random_number(1, 10, numbers_to_exclude) for _ in range(3) ] print(result) # ๐๏ธ [5, 2, 5]

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

Set objects are an unordered collection of unique elements and implement a difference() method.

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

main.py
print({1, 2, 3} - {2, 3}) # ๐๏ธ {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.