Slice assignment

Slice assignment is a feature of Python that allows you to make bulk updates to a list.

The FizzBuzz test is described here: http://c2.com/cgi/wiki?FizzBuzzTest

"Write a program that prints the numbers from 1 to 100. But for multiples of three print “Fizz” instead of the number and for the multiples of five print “Buzz”. For numbers which are multiples of both three and five print “FizzBuzz”."

 I'll implement a solution using slice assignment to demonstrate how it works.

Solution

Start with a list of numbers.

fizzbuzz = list(range(101)) 

Replace every multiple of 3 with the word "Fizz'. The length of the slice tells us how many replacement values we need,

fizzbuzz[3::3] = ['Fizz'] * len(fizzbuzz[3::3])

Do the same for the multiples of 5 and 15.

fizzbuzz[5::5] = ['Buzz'] * len(fizzbuzz[5::5])

fizzbuzz[15::15] = ['FizzBuzz'] * len(fizzbuzz[15::15])

Now print the results (skipping the first value of 0)

for i in fizzbuzz[1:]:

    print(i)



You can paste this code into Python Tutor if you want to step through it line by line.

Other solutions

Compare this solutions to others you might have seen. For example

for n in range(1, 101):
      line = ""
      if n%3 == 0:
          line = line + "Fizz"
      if n%5 == 0:
          line = line + "Buzz"

      if line:
          print line
      else:
          print n

Which one do you prefer? Why?

What if I told you the slice based solution runs 6X faster? Does that change your opinion?

You can find more tips and tricks in my book Mastering Python Lists.