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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
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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