9.8. Count and Index Methods

You’ve learned about methods before when drawing with the turtle module. There, you used .forward(50) and .color("purple") to complete actions. We refer to forward() and color() as methods of the turtle class. Objects like strings, tuples, and lists also have methods that we can use.

As you create more complex programs, you will find that some tasks are common and Python objects often have built-in methods to help you with these tasks. This page will cover two helpful methods that can be called on sequence objects like strings, lists and tuples: count() and index().

9.8.1. Count

The first method we’ll talk about is called count. You can call the count() method on any string, list, or tuple, by using variable_name.count(<itemtocount>). The count() method requires that you provide one argument, which is what you would like to count. The method then returns the number of times that the argument occured in the string/list/tuple the method was used on. There are some differences between count for strings and count for lists. When you use count on a string, the argument can only be a string. You can’t count how many times the integer 2 appears in a string, though you can count how many times the string “2” appears in a string. For lists and tuples, the argument can be any type: a string, an integer, a floating point number, a boolean value, etc.

The activecode window above demonstrates the use of count on a string. Just like with the turtle module when we had to specify which turtle was changing color or moving, we have to specify which string we are using count on.

When you run the activecode window above, you’ll see how count with a list works. Notice how “4” has a count of zero but 4 has a count of three. This is because the list z only contains the integer 4. There are n strings in this list that are “4”. Additionally, when we check the count of “a”, we see that the program returns zero. Though some of the words in the list contain the letter “a”, the program is looking for items in the list that are just the letter “a”.

9.8.2. Index

The other method that can be helpful for both strings and lists is the index method. The index method requires one argument, and, like the count method, it takes only strings when index is used on strings, and any type when it is used on lists/tuples. For any sequence type, index returns the leftmost index where the argument is found. If it is unable to find the argument in the string or list, an error will occur.

All of the above examples work, but were you surprised by any of the return values? Remember that index will return the left most index of the argument. Even though “Metatarsal” occurs many times in bio, the method will only return the location of one of them.

Here’s another example.

In the activecode window above, we’re trying to see where “autumn” is in the list of seasons. However, there is no string called autumn (though there is a string called “fall” which is likely what the program is looking for). Remember that an error occurs if the argument is not in the string or list. This seems problematic: it seems like you can’t ask for the index of something in a list/tuple without potentially causing your program to crash. The solution to this is to first use the in operator to check if the item exists in the sequence, and then, if it does, ask for the index. See the above example modified below:

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