randrange() in Python

The randrange() function in Python's random module is used to generate a random number within a specified range. It allows defining a start, stop, and an optional step value to control the selection of numbers. Unlike randint(), which includes the upper limit, randrange() excludes the stop value. Example:

import random

print("0-100:", random.randrange(100))
print("50-100:", random.randrange(50, 100))
print("50-100 (step 5):", random.randrange(50, 100, 5))

0-100: 48
50-100: 61
50-100 (step 5): 50

Explanation: The first call picks a random number between 0 and 99. The second call selects a random number between 50 and 99. The third call chooses a random number between 50 and 99, but only from numbers that are multiples of 5 (like 50, 55, 60, etc.).

Syntax of randrange()

random.randrange(start, stop, step)

Parameter:

• start (optional): The starting number of the range. Default is 0.
• stop (required): The upper limit (exclusive) for the generated number.
• step (optional): The step size. Default is 1 and cannot be zero (raises ValueError).

Return Value: This function returns a randomly selected number from the sequence [start, start+step, start+2*step, ..., stop). The stop value is never included.

Exceptions:

• Raises ValueError if stop <= start.
• Raises ValueError if non-integer values are provided as parameters.

Examples

Example 1: Generating Even or Odd Numbers

import random

print("10-50:", random.randrange(10, 50, 2))
print("11-51:", random.randrange(11, 51, 2))

10-50: 34
11-51: 37

Explanation:

• The first call generates a random even number within [10, 12, 14, ..., 48].
• The second call generates a random odd number within [11, 13, 15, ..., 49].

Example 2: Simulating a dice roll

import random

dice_roll = random.randrange(1, 7)
print(dice_roll)

5

Explanation: This function randomly selects an integer between 1 and 6, simulating a dice roll.

Example 3: Random selection from a list

import random

a = [10, 20, 30, 40, 50]
random_idx = random.randrange(len(a))
print(a[random_idx])

10

Explanation: randrange(len(a)) selects a random index from 0 to 4. The corresponding value from a is then printed.

Handling Exceptions

Case 1: Non-integer Arguments

The randrange() function does not accept float values. Instead of allowing a crash, we can handle it gracefully.

import random

start, stop, step = 14.5, 100, 1  # Example invalid input

try:
    if not all(isinstance(i, int) for i in [start, stop, step]):
        raise TypeError("All arguments must be integers.")
    print(random.randrange(start, stop, step))
except TypeError as e:
    print("Error:", e)

Error: All arguments must be integers.

Explanation: This function ensures start, stop, and step are integers using all(isinstance(i, int) for i in [start, stop, step]). If not, it raises a TypeError with a clear message. Upon valid input, random.randrange() generates a number. A try-except block catches TypeError and displays a user-friendly message.

Case 2: Invalid Range (start >= stop)

The start value must always be less than stop. Instead of allowing an error, we can handle it gracefully by swapping values if necessary.

import random

start, stop, step = 500, 100, 1  # Example invalid input

try:
    if start >= stop:
        raise ValueError("Start value must be less than stop value.")
    print(random.randrange(start, stop, step))
except ValueError as e:
    print("Error:", e)

Error: Start value must be less than stop value.

Explanation: This function validates that start < stop to ensure a proper range. If not, it raises a ValueError with a clear message. When valid, random.randrange() generates a number. A try-except block catches the ValueError and provides a user-friendly error message.

Practical application: Simple game simulation

Random number generation is widely used in games. Below is an example of a simple game where two players roll a dice until one reaches a score of 100.

import random

p1, p2, t = 0, 0, 0  # p1: Player 1 score, p2: Player 2 score, t: Turn counter

while True:
    t += 1  # Increment turn count
    r1, r2 = random.randrange(1, 11), random.randrange(1, 11)  # r1: Player 1 roll, r2: Player 2 roll  
    
    p1 += r1  # Add roll value to Player 1's score
    p2 += r2  # Add roll value to Player 2's score
    
    print(f"Turn {t}: P1 = {p1}, P2 = {p2}")  # Display scores
    
    if p1 >= 100:  # Check if Player 1 wins
        print("P1 wins!")
        break
    if p2 >= 100:  # Check if Player 2 wins
        print("P2 wins!")
        break

Turn 1: Player 1 = 7, Player 2 = 9
Turn 2: Player 1 = 15, Player 2 = 10
Turn 3: Player 1 = 19, Player 2 = 15
Turn 4: Player 1 = 28, Player 2 = 25
Turn 5: Player 1 = 29, Player 2 = 34
Turn 6: Player 1 ...

Explanation: In this game, both players take turns rolling a number between 1 and 10, adding it to their scores. The game continues until one player reaches 100 points and wins.