9.6. Common List Tasks

Lists provide a very flexible tool for storing and organizing data. As you use them again and again, you will often find yourself running the same type of task.

Let’s take a moment to look at some routine list techniques.

9.6.1. Switching Two Elements

The sort and reverse methods reorder all of the elements in a list. However, sometimes we only need to swap the positions of two items.

Let’s take a look at the long way first, so you can appreciate the shortcut that much more.

Example

Swapping two elements the long way. We need to use a variable to temporarily store one of the values we want to swap.

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my_list = ['r', 'a', 't']

temp_value = my_list[0]    # Assigns the value 'r' to temp_value
my_list[0] = my_list[2]    # Changes ['r', 'a', 't'] to ['t', 'a', 't']
my_list[2] = temp_value    # Finishes the exchange

print(my_list)

Console Output

['t', 'a', 'r']

  1. Line 3 assigns the value at index 0 ('r') to the temp_value variable.

  2. Line 4 assigns the value at index 2 to index 0. Printing my_list at this point would return ['t', 'a', 't'].

  3. Line 5 assigns temp_value to index 2.

Switching the order of two list elements is so common that Python gives us a shortcut.

Example

Swapping two elements the short way.

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my_list = ['r', 'a', 't']

my_list[0], my_list[2] = my_list[2], my_list[0]

print(my_list)

Console Output

['t', 'a', 'r']

The idea behind the shortcut is that, if we have two list elements—a and b—then the syntax:

a, b = b, a

switches the positions of the two elements inside the list.

9.6.2. Accumulating List Elements

Just like strings, we can use the accumulator pattern to add items to a list. Since the append method only adds one element at a time, placing the statement inside a loop allows us to continually increase the size of a list.

As we did when keeping a running total, we need to define an accumulator variable, but in this case it will be assigned an empty list, [].

Example

Let’s begin by adding even numbers to an empty list:

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evens = []     # evens is the accumulator variable

for num in range(0, 21, 2):  # num takes the values 0, 2, 4...20.
   evens.append(num)         # Add the value of num to the end of the list.

print(evens)

Console Output

[0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20]

We can also use the accumulator pattern to add selected elements from one list to another.

Example

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words = ['It', 'was', 'a', 'bright', 'cold', 'night', 'in', 'April', 'and', 'all', 'the', 'clocks', 'were', 'striking', 'thirteen']
a_words = []        # Accumulator list

for word in words:            # word takes the value of each element from words.
   if word[0].lower() == 'a': # True if word starts with 'a' or 'A'.
      a_words.append(word)    # Add word to the list.

print(a_words)

Console Output

['a', 'April', 'and', 'all']

Note

One benefit of using the accumulator pattern is that it preserves the original list.

9.6.2.1. Multiple List Options

In the same accumulator loop, we can use a conditional to decide which list receives a value.

Example

Let’s divide a list of mixed data types into strings and integers:

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mixed_types = [5, 7, 3.14, 'rutabaga', 'integer', True]
integers = []
strings = []

for item in mixed_types:
   if type(item) == str:      # Check if item is a string data type.
      strings.append(item)
   elif type(item) == int:    # Check if item is an int data type.
      integers.append(item)

print(mixed_types)
print(integers)
print(strings)

Console Output

[5, 7, 3.14, 'rutabaga', 'integer', True]
[5, 7]
['rutabaga', 'integer']

Note that the values 3.14 and True are not placed into either list, since they are of the float and bool data types, respectively.

We could easily extend the if/elif block to deal with other data types.

9.6.2.2. Try It!

In the code editor below, practice using the accumulator pattern to add selected elements to different lists.

Try It!

Assign elements from the strings list into either vowel_start, digit_start, or other_start.

  1. In line 7, use a for statement to loop through strings by element.

    for item in strings:

  2. In line 8, use an if statement to check if item starts with a vowel. If so, append item to the vowel_start list.

  3. Add an elif statement to check if item starts with a digit (0-9). If so, append item to digit_start.

  4. Add an else block to deal with strings that do NOT start with a digit or a vowel.

  5. Print the lists after the loop to check your work.

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strings = ['apple', 'banana', '1-to-1', '@launchcode', 'everyone can code', ':-)', '4EVR']
vowel_start = []
digit_start = []
other_start = []

# Add your loop here:

repl.it

Expected Results

vowel_start: ['apple', 'everyone can code']
digit_start: ['1-to-1', '4EVR']
other_start: ['banana', '@launchcode', ':-)']

9.6.3. Finding Max and Min

Often, we want to find the largest or smallest value from the elements in a list. We can accomplish this two different ways.

  1. Sorting a list arranges the elements from the smallest to largest value. The maximum (or minimum) value can then be accessed with bracket notation.

    Example

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    numbers = [42, 27, 30, 46, -36, 30, -28, 53, 53, 32]
output = "Minimum = {0}, Maximum = {1}"

numbers.sort()
print(numbers)
print(output.format(numbers[0], numbers[-1]))
# Index 0 is the first element in the list, and index -1 is the last.

    Console Output

    [-36, -28, 27, 30, 30, 32, 42, 46, 53, 53]
Minimum = -36, Maximum = 53

  2. Python also has two functions, max() and min(), that return the largest and smallest values from a collection.

    Example

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    numbers = [42, 27, 30, 46, -36, 30, -28, 53, 53, 32]
output = "Minimum = {0}, Maximum = {1}"

largest = max(numbers)
smallest = min(numbers)
print(numbers)
print(output.format(smallest, largest))

    Console Output

    [42, 27, 30, 46, -36, 30, -28, 53, 53, 32]
Minimum = -36, Maximum = 53

    Since the max and min functions return a value, we could easily use the expressions inside format.

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    print(numbers)
print(output.format(min(numbers), max(numbers)))

Note

Finding the maximum and minimum values also works with strings.

max(['apple', 'bear', 'zebra', 'display']) returns 'zebra', and min('telescope') returns 'c'.

9.6.4. Check Your Understanding

Question

What does the following program print?

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my_list = []

for num in range(7):
   if num%2 == 0:
      my_list.append(num)

print(my_list)

  1. []

  2. [0, 1, 2, 3, 4, 5, 6]

  3. [0, 2, 4, 6]

  4. [1, 3, 5]

Question

What does the following program print?

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my_list = [8, 2, 7, 4, 10]

my_list[2], my_list[4] = my_list[4], my_list[2]

print(my_list)

  1. [8, 2, 7, 4, 10]

  2. [8, 4, 7, 2, 10]

  3. [10, 4, 7, 2, 8]

  4. [8, 2, 10, 4, 7]

Question

What does the following program print?

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word = 'bookkeeper'
some_letters = []
other_letters = []

for char in word:
   if char not in 'aeiou':
      some_letters.append(char)
   else:
      other_letters.append(char)

print(some_letters)

  1. [‘b’, ‘k’, ‘k’, ‘p’, ‘r’]

  2. [‘o’, ‘o’, ‘e’, ‘e’, ‘e’]

  3. [‘bkkpr’]

  4. [‘ooeee’]