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Chapter 4

# Repetition: For Loops

To this point, our Python programs have been limited to tasks that could be accomplished by a calculator. Repetition, the ability to repeat any section of a program, adds a surprising amount of new power.

Listing 4.1: Harmonic Sum

` 1 # harmonic.py`
` 2`
` 3 def harmonic(n):`
` 4 # Compute the sum of 1/k for k=1 to n.`
` 5 total = 0`
` 6 for k in range(1, n + 1):`
` 7  total += 1 / k`
` 8 return total`
` 9`
`10 def main():`
`11 n = int(input(’Enter a positive integer: ’))`
`12 print("The sum of 1/k for k = 1 to", n, "is", harmonic(n))`
`13`
`14 main()`

This program computes the harmonic sum

$1+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}+\cdots +\frac{1}{n}=\sum _{k=1}^{n}\frac{1}{k}$

Type it in and run it a few times to get a feeling for what it does and how it works. Try ...

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