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33. The range() Function

1. Introduction​

The range() function is commonly used in for loops to generate a sequence of numbers.
It doesn’t actually create a full list in memory but instead produces numbers on demand, making it efficient.

2. Basic Usage​

range(stop) generates numbers from 0 up to stop (exclusive).

for i in range(5):
    print(i)

Output:

0
1
2
3
4

3. Specifying Start and Stop​

You can also give a starting value:

for i in range(2, 6):
    print(i)

Output:

2
3
4
5

4. Using Step​

The third parameter is the step (increment).

for i in range(0, 10, 2):
    print(i)

Output:

0
2
4
6
8

5. Negative Steps​

You can loop backwards with a negative step.

for i in range(10, 0, -2):
    print(i)

Output:

10
8
6
4
2

6. Using range() Outside of Loops​

You can convert a range to a list or tuple:

print(list(range(5)))   # [0, 1, 2, 3, 4]
print(tuple(range(3)))  # (0, 1, 2)

7. Checking Membership in range​

You can check if a number is inside a range:

print(5 in range(10))    # True
print(15 in range(10))   # False

8. Large Ranges are Efficient​

Even huge ranges don’t take up memory:

r = range(1, 1_000_000)
print(len(r))     # 999999
print(r[100])     # 101

⚡ The values are generated on demand.

9. Common Mistakes​

  • Forgetting that the stop value is excluded: 
    for i in range(5):
        print(i)
    # Outputs 0–4, not 0–5
  • Using the wrong step (causes empty loops): 
    for i in range(5, 0, 1):  # ❌ won't run
        print(i)
    # Step should be -1 if going down

10. Practical Examples​

  • Printing even numbers:
for i in range(0, 11, 2):
    print(i)
  • Countdown:
for i in range(5, 0, -1):
    print(i)
print("Go!")

11. Next Steps​

✅ You now understand how to use range() with start, stop, and step values.
In the next chapter, we’ll look at loop control statements like break, continue, and pass.