L3.8: More on range and for loop without range

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Learning outcomes

• use all range forms confidently
• handle reverse loops with negative step
• iterate without range over strings/lists
• avoid off-by-one loop errors

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range behavior recap

list(range(5))         # [0, 1, 2, 3, 4]
list(range(2, 6))      # [2, 3, 4, 5]
list(range(2, 10, 3))  # [2, 5, 8]

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Negative step examples

list(range(10, 0, -2))  # [10, 8, 6, 4, 2]
list(range(5, -1, -1))  # [5, 4, 3, 2, 1, 0]

list(range(1, 10, -1))  # []

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for loop without range

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String traversal

for ch in "hello":
    print(ch)

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List traversal

nums = [10, 20, 30]
for x in nums:
    print(x)

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Dictionary traversal

d = {"a": 1, "b": 2}
for k in d:
    print(k, d[k])

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Index + value using enumerate

fruits = ["apple", "mango", "kiwi"]
for i, fruit in enumerate(fruits):
    print(i, fruit)

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Exam hints and traps

• stop value in range is excluded
• wrong sign in step can produce empty iteration
• direct iteration gives values, not indexes

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Practice

1. Print indexes and characters for "python".
2. Reverse-print list [3, 5, 7, 9] using range indexes.
3. Predict output:

for i in range(3, 3):
    print(i)

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Answers

for i, ch in enumerate("python"):
    print(i, ch)

3. No output.