Ruby Exception Handling

Ruby Exception Handling: IndexError

Today we’ll be continuing our journey through our Ruby Exception Handling series by exploring the wonderful world of the Ruby IndexError. The IndexError is raised when there’s an attempted call to an element within an array, where the given index is out of bounds or invalid in some way.

Throughout today’s article we’ll examine exactly what the IndexError means, see where it fits within the Ruby Exception class hierarchy, and detail how to properly handle any IndexErrors you might experience in your own coding endeavors. Let’s get going!

The Technical Rundown

  • All Ruby exceptions are descendants of the Exception class, or a subclass therein.
  • StandardError is a direct descendant of the Exception class, and is also a superclass with many descendants of its own.
  • IndexError is a direct descendant of the StandardError class, and is a superclass to a few descendants of its own.

When Should You Use It?

The IndexError is directly linked to the use of Arrays in Ruby, so let’s briefly cover arrays and how they work.

We can create an array by calling the Array.new method: Array.new(3) would create an array with three (nil) elements. We can also create an array inline by specifying the values as part of the array assignment: names = ["Alice", "Bob", "Christine", "Dan"]

Once an array is created, there are numerous methods to manipulate it. If we want to fetch a value from the array, we can use the square bracket syntax and include the index we are trying to get:

We can also use something like the Array.fetch method, which expects the first argument provided to be the index of the element to fetch:

As with most programming languages, arrays use zero-based numbering, so the first element is at index 0, the second element at index 1, and so forth. Therefore, in both examples above, when we pass the index value of 3, it returns the fourth element in our array.

With only four elements total, let’s see what happens if we try to retrieve the fifth index with the direct square bracket syntax:

Ruby recognizes that no value exists, and thus returns nil. However, what if we pass the same syntax with the Array.fetch method:

Perhaps unsurprisingly, this raises an explicit IndexError, indicating that the index of 5 is outside the bounds of our array:

There are a couple ways to solve this and prevent IndexError from occurring. The first, and most obvious, is to improve our code logic so that we don’t try to retrieve any indices that don’t exist with the Array.fetch method.

The second option is to pass a second argument to the Array.fetch method, which acts as a default value if the provided index is out of bounds:

Now when we execute our code, rather than producing an IndexError, the Array.fetch method recognizes the second argument of "No name found." and returns that value as our output instead:

A third use for Array.fetch, which is somewhat interesting if not directly related to the IndexError, is that we can attach a code block after the Array.fetch call, if we want the block to execute only when the index value is invalid:

Here’s our output:

This functionality can be rather useful as a predetermined escape sequence for when we aren’t sure of a provided index will be valid, so we can perform other actions if necessary.

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