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Practice 5: Map, Filter, Reduce, Enumerate, Zip

This practice problems set will guide you through Python’s useful functions map, filter, reduce, enumerate and zip.

Map

  1. Use map to convert a list of strings into a list of their lengths. For example, ['cat', 'window', 'defenestrate'] should become [3, 6, 12].
  2. Given a list of Cartesian coordinates represented by a list [x, y]. Use map to convert them into a list of Polar coordinates represented by [r, theta]. How to convert X, Y to Polar. For example, [[3, 4], [1, 1]] becomes [[5, 53.1...], [1.414..., 45]].
  3. Write a function that takes a list of functions and a list of values and uses map to apply each function to the corresponding value.

Filter

  1. Use filter to remove all the numbers in a list that are not multiples of 3.
  2. Given a list of strings, use filter to return only those strings that are palindromes.
  3. Use filter to implement a simple text censor function that takes in a list of words and a list of offensive words and returns the list of words not in the offensive list.

Reduce

Note: For using reduce, make sure to write from functools import reduce in the header of the file.

  1. Use reduce to compute the product of a list of numbers.
  2. Use reduce to implement your own version of Python’s max function.
  3. A 3D vector is represented by a list [x, y, z]. Given a list of 3D vectors, write a function that uses reduce to produce a sum of all vectors in the list. For example, [[1, 2, 3], [3, 1, 5], [0, -1, 0]] produces [4, 2, 8].

Enumerate

  1. Use enumerate to modify the elements of a list to include their index. For example, [‘a’, ‘b’, ‘c’] becomes [‘0: a’, ‘1: b’, ‘2: c’].
  2. Given a string, use enumerate to print indices of lowercase characters.
  3. Write a function that takes a list of numbers and returns a list of only those numbers that have an even index. Implement this using enumerate.

Zip

  1. Given two lists, use zip to create a new list of lists where each list contains the elements of the original lists at the corresponding positions. For example, given the lists ['a', 'b', 'c'] and [1, 2, 3], your code should produce the list [['a', 1], ['b', 2], ['c', 3]].
  2. Use zip and map to compute the element-wise product of two lists of numbers. For example, given the lists [1, 2, 3, 4] and [5, 6, 7, 8], your code should produce the list [5, 12, 21, 32].
  3. Given 2 lists representing 2 coordinate vectors, e.g., [w1, x1, y1, z1] and [w2, x2, y2, z2]. Use zip to compute a dot product of 2 vectors. In this case w1*w2 + x1*x2 + y1*y2 + z1*z2.
  4. From the last problems, try to do it without loop. Hint: use reduce.
  5. Given a matrix represented by a list of lists. Use zip to help you write a transpose function. You need only 1 loop.