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Range Function

Generating sequences with range()

Interview: Fundamental for loops and algorithm problems — tests understanding of lazy sequences

Last Updated: June 12, 2026 • 9 min read

The range() function generates immutable sequences of integers. In Python 3, it returns a range object (not a list), which generates values on demand — making it extremely memory-efficient even for billions of elements.

Three Forms of range()

  • range(stop): Generates 0, 1, 2, ..., stop-1. Example: range(5) → 0,1,2,3,4
  • range(start, stop): Generates start, start+1, ..., stop-1. Example: range(2, 7) → 2,3,4,5,6
  • range(start, stop, step): Generates start, start+step, start+2*step, ... Example: range(0, 10, 2) → 0,2,4,6,8
  • Negative step: Counts down: range(5, 0, -1) → 5,4,3,2,1. Note: stop value is exclusive
  • Empty range: range(0) or range(5, 0) (start > stop with positive step) produces no values

Range Object Properties

  • Lazy evaluation: range(10**9) uses the same memory as range(10) — it computes values on demand
  • Supports len(): len(range(0, 10, 2)) → 5 (computed in O(1))
  • Supports indexing: range(10)[3] → 3, range(10)[-1] → 9
  • Supports 'in' operator: 5 in range(10) → True (O(1) check, not iteration)
  • Immutable: Cannot modify elements after creation
  • Hashable: Can be used as dictionary keys (unlike lists)

Performance Insight

In Python 2, range() created a full list in memory. Python 3's range is a sequence object with O(1) memory. For Python 2 compatibility code, use xrange() instead of range().

Common Patterns

  • Repeat N times: for _ in range(n): — use underscore when index isn't needed
  • Index-based access: for i in range(len(lst)): — but prefer enumerate() instead
  • Slicing with range: list(range(10))[::2] → [0, 2, 4, 6, 8]
  • Creating lists: list(range(5)) → [0, 1, 2, 3, 4]

Common Pitfall

range() only accepts integers. Using floats raises TypeError. For float ranges, use numpy.arange() or a list comprehension: [i * 0.1 for i in range(10)].

Use Cases

Looping a specific number of times (repeat N times)

Generating index sequences for list/array traversal

Creating evenly-spaced integer sequences

Memory-efficient large-range iteration in algorithms

Common Mistakes

Using range(len(lst)) instead of enumerate() when you need both index and value

Forgetting that the stop value is exclusive (range(5) gives 0-4, not 0-5)

Trying to use floats with range() — it only accepts integers

Using range() in Python 2 thinking it is lazy — use xrange() instead