Building Dictionaries

By now you are familiar with two important concepts: 1) counting with for loops and 2) the dictionary get method. These two can actually be combined to create a useful counter dictionary, something you will likely come across again. For example, we can create a dictionary, word_counter, that keeps track of the total count of each word in a string.

The following are a couple of ways to do it:

Method 1: Using a for loop to create a set of counters

Let’s start with a list containing the words in a series of book titles:

book_title =  ['great', 'expectations','the', 'adventures', 'of', 'sherlock','holmes','the','great','gasby','hamlet','adventures','of','huckleberry','fin']

Step 1: Create an empty dictionary.

word_counter = {}

Step 2. Iterate through each element in the list. If an element is already included in the dictionary, add 1 to its value. If not, add the element to the dictionary and set its value to 1.

for word in book_title:
    if word not in word_counter:
        word_counter[word] = 1
    else:
        word_counter[word] += 1

What’s happening here?

• The for loop iterates through each element in the list. For the first iteration, word takes the value ‘great’.
• Next, the if statement checks if word is in the word_counter dictionary.
• Since it doesn’t yet, the statement word_counter[word] = 1 adds great as a key to the dictionary with a value of 1.
• Then, it leaves the if else statement and moves on to the next iteration of the for loop. word now takes the value expectations and repeats the process.
• When the if condition is not met, it is because thatword already exists in the word_counter dictionary, and the statement word_counter[word] = word_counter[word] + 1 increases the count of that word by 1.
• Once the for loop finishes iterating through the list, the for loop is complete.

We can see the output by printing out the dictionary. Printing word_counter results in the following output.

{'great': 2, 'expectations': 1, 'the': 2, 'adventures': 2, 'of': 2, 'sherlock': 1, 'holmes': 1, 'gasby': 1, 'hamlet': 1, 'huckleberry': 1, 'fin': 1}

Feel free to try this out yourself in the code editor at the bottom of this page.

Method 2: Using the get method

We will use the same list for this example:

book_title =  ['great', 'expectations','the', 'adventures', 'of', 'sherlock','holmes','the','great','gasby','hamlet','adventures','of','huckleberry','fin']

Step 1: Create an empty dictionary.

word_counter = {}

Step 2. Iterate through each element, get() its value in the dictionary, and add 1.

Recall that the dictionary get method is another way to retrieve the value of a key in a dictionary. Except unlike indexing, this will return a default value if the key is not found. If unspecified, this default value is set to None. We can use get with a default value of 0 to simplify the code from the first method above.

for word in book_title:
    word_counter[word] = word_counter.get(word, 0) + 1

What’s happening here?

• The for loop iterates through the list as we saw earlier. The for loop feeds ‘great’ to the next statement in the body of the for loop.
• In this line: word_counter[word] = word_counter.get(word,0) + 1, since the key ‘great’ doesn’t yet exist in the dictionary, get() will return the value 0 and word_counter[word] will be set to 1.
• Once it encounters a word that already exists in word_counter (e.g. the second appearance of ‘the’), the value for that key is incremented by 1. On the second appearance of ‘the’, the key’s value would add 1 again, resulting in 2.
• Once the for loop finishes iterating through the list, the for loop is complete.

Printing word_counter shows us we get the same result as we did in method 1.

{'great': 2, 'expectations': 1, 'the': 2, 'adventures': 2, 'of': 2, 'sherlock': 1, 'holmes': 1, 'gasby': 1, 'hamlet': 1, 'huckleberry': 1, 'fin': 1}