Random number generation

Random Number Generation

Programs often need to produce unpredictable values - to simulate a dice roll, shuffle a playlist, generate a test question, or create a game event. Random number generation provides a way to obtain a number within a specified range each time the program runs, without knowing the result in advance.

All three languages generate numbers using a function or method that accepts a lower bound and an upper bound. The critical difference is how the upper bound is treated: Python's randint() includes both bounds; VB.NET's and C#'s Next() excludes the upper bound, so you must add 1 to get an inclusive range.

Language	Import / setup	Inclusive range a to b	Upper bound behaviour
Python	`import random`	`random.randint(a, b)`	Inclusive (b is possible)
VB.NET	`Dim rnd As New Random()`	`rnd.Next(a, b + 1)`	Exclusive (pass b+1)
C#	`Random rnd = new Random();`	`rnd.Next(a, b + 1)`	Exclusive (pass b+1)

Worked Examples

A standard six-sided die produces a whole number between 1 and 6 inclusive. This is the most common introductory use of random number generation.

AQA pseudocode: roll ← RANDOM_INT(1, 6)

# Python -- randint(a, b) is INCLUSIVE of both a and b
import random

roll = random.randint(1, 6)   # any of 1, 2, 3, 4, 5, 6 is possible
print("You rolled:", roll)

' VB.NET -- Next(a, b) is EXCLUSIVE of b, so pass 7 to include 6
Dim rnd As New Random()

Dim roll As Integer = rnd.Next(1, 7)   ' range 1..6 inclusive
Console.WriteLine("You rolled: " & roll)

// C# -- Next(a, b) is EXCLUSIVE of b, so pass 7 to include 6
Random rnd = new Random();

int roll = rnd.Next(1, 7);    // range 1..6 inclusive
Console.WriteLine("You rolled: " + roll);

Random numbers are not limited to 1-6. Here the program generates a mock quiz score between 0 and 100 inclusive - a custom range chosen to match the problem.

AQA pseudocode: score ← RANDOM_INT(0, 100)

# Python -- custom range 0 to 100 inclusive
import random

score = random.randint(0, 100)
print("Quiz score:", score)

' VB.NET -- custom range 0 to 100 inclusive (pass 101 as upper bound)
Dim rnd As New Random()

Dim score As Integer = rnd.Next(0, 101)   ' 0..100 inclusive
Console.WriteLine("Quiz score: " & score)

// C# -- custom range 0 to 100 inclusive (pass 101 as upper bound)
Random rnd = new Random();

int score = rnd.Next(0, 101);    // 0..100 inclusive
Console.WriteLine("Quiz score: " + score);

Random values become much more useful when combined with selection. Here the program rolls a die and uses an IF statement to decide whether the player wins - demonstrating how random number generation drives program behaviour.

AQA pseudocode:

roll ← RANDOM_INT(1, 6)
IF roll = 6 THEN
    OUTPUT "You win!"
ELSE
    OUTPUT "You rolled " + roll + ". Try again."
ENDIF

# Python -- use random result in a decision
import random

roll = random.randint(1, 6)
if roll == 6:
    print("You win!")
else:
    print("You rolled", roll, ". Try again.")

' VB.NET -- use random result in a decision
Dim rnd As New Random()
Dim roll As Integer = rnd.Next(1, 7)   ' 1..6 inclusive

If roll = 6 Then
    Console.WriteLine("You win!")
Else
    Console.WriteLine("You rolled " & roll & ". Try again.")
End If

// C# -- use random result in a decision
Random rnd = new Random();
int roll = rnd.Next(1, 7);    // 1..6 inclusive

if (roll == 6)
    Console.WriteLine("You win!");
else
    Console.WriteLine("You rolled " + roll + ". Try again.");

Key Takeaways

Python's random.randint(a, b) includes both a and b; VB.NET's and C#'s rnd.Next(a, b) excludes b, so pass b + 1 to get an inclusive range.
Always import or initialise the random module or object before calling the function: import random in Python; Dim rnd As New Random() in VB.NET; Random rnd = new Random(); in C#.
The result is a whole number (integer) stored in a variable and used like any other integer - in output, arithmetic, or selection statements.
You are not required to understand how pseudo-random numbers are generated - only how to call the function correctly and use the result.