L3.5: Introduction to for loop

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Learning outcomes

• write basic for loops with range
• iterate over strings/lists directly
• compare for and while usage

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Syntax

for variable in iterable:
    # body

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First examples

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

for ch in "python":
    print(ch)

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for vs while

• number of iterations is known
• iterating over sequence elements

• loop depends on dynamic condition
• iterations are unknown in advance

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range quick forms

• range(stop) -> 0 to stop-1
• range(start, stop) -> start to stop-1
• range(start, stop, step) -> controlled step

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Dry run

for i in range(2, 8, 2):
    print(i)

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Exam hints and traps

• range(1, 5) does not include 5
• negative step requires descending boundaries
• variable updates automatically in for

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Practice

1. Print numbers 1 to 10 using for.
2. Print characters of "code" one per line.
3. Predict output:

for i in range(5, 0, -2):
    print(i)

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Answers

for i in range(1, 11):
    print(i)

for ch in "code":
    print(ch)

3. 5 3 1