Unit 4 lesson 2 contrived examples considered harmful.

[Dan-TEALS]

Discussed in #365

^{Originally posted by jdonwells March 6, 2022}
There are two problems with contrived examples. The first is code that the students can't understand because it simply makes no sense. The second is most of these contrived examples are badly written Python. We want to show the students well written Python only.

I would even go so far as to say if you can't think of a well written Python example maybe you should skip what you want to show them entirely. Showing the students how to traverse a list using for i in range(0, len(list)) is probably not something to show them at all. There are no good examples because no one would do that.

In Unit 4 lesson 2 we show them this code:

for i in range(len(numbers)):
    numbers[i] = numbers[i] * 2

The correct way to do that is this:

doubled_numbers = [n * 2 for n in numbers]

We also show them this code:

def is_reverse(word1, word2):
    if len(word1) != len(word2):
        return False
    i = 0
    j = len(word2)
    while j > 0:
        if word1[i] != word2[j]:
            return False
        i = i + 1
        j = j - 1
    return True
print(is_reverse('pots', 'stop'))

The correct way to do that is this:

def is_reverse(word_a, word_b):
    return word_a == word_b[::-1]


print(is_reverse('pots', 'stop'))

Here are my new examples. I am going to show 4. One each for the major traversals (map, filter, reduce) and one for using range.

Traversing a list to transform it. (map)

def strings(list):
    all_strings = []
    for item in list:
        all_strings.append(str(item))
    return all_strings

Traversing a list to summarize the items. (reduce)

def sum(numbers):
    """Return a total of the numbers"""
    total = 0
    for number in numbers:
        total = total + number
    return total

Using a traversal to filter a list. (filter)

def only_primes(numbers):
    """Return a list of the primes in the numbers list"""
    primes = []
    for n in numbers:
        if is_prime(n):
            primes.append(n)
    return primes

Using range() to generate a list for traversal.

def is_prime(n):
    for divisor in range(2, n):
        if n % divisor == 0:
            return False
    return True
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