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Jeremy Schuurmans
Jeremy Schuurmans

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Ruby Method Spotlight: Slice

Just a quick note before we begin: this post assumes a basic knowledge of Ruby data structures, since I can't really think of a reason why you would want to read about this otherwise. Specifically, you should know what arrays and strings are, and you should probably have some understanding of return value.


#slice is a method that operates on arrays, strings, and (since Ruby 2.5.0) hashes. We'll just focus on arrays for now, since the logic is basically the same regardless, but keep in mind that you can call #slice on strings and hashes as well. #slice allows you to cut into an array and select specific elements.


Let's say you have an array of fruits:

fruit = ["apple", "banana", "orange", "grapefruit", "tomato"]

You can call #slice on the array, pass it a specific index, and it will return the object at that index. Calling fruit.slice with an argument of (2) will return the string object "orange" because "orange" is at index 2 of the fruit array (note that array elements are indexed starting at 0, so "apple" is at index 0, "banana" is at index 1, etc). Let's drop into IRB and check it out:

2.5.1 :001 > fruit = ["apple", "banana", "orange", "grapefruit", "tomato"]
 => ["apple", "banana", "orange", "grapefruit", "tomato"]
2.5.1 :002 > fruit.slice(2)
 => "orange"

If you want the banana, the orange, and the grapefruit, you can call #slice with a starting index and a length of elements and it will return those objects in an array.

2.5.1 :001 > fruit = ["apple", "banana", "orange", "grapefruit", "tomato"]
 => ["apple", "banana", "orange", "grapefruit", "tomato"]
2.5.1 :002 > fruit.slice(1,3)
 => ["banana", "orange", "grapefruit"]

This selects all elements starting with index 1 and ending with index 3.

Similarly, we can use a range:

2.5.1 :002 > fruit.slice(1..3)
 => ["banana", "orange", "grapefruit"]

Another way to utilize this method is to call the array, and then simply select the elements you want, like so:

fruit[2], which returns
=> "orange"

This is useful if you need to work with specific parts of a specific element. If your baby really wants a "nana", for instance, not a "BAnana", you can select the "banana" at index 1 by calling fruit[1], and chain [2,5] onto it, which will select the string object at index 1 of the array, and then select the letters at indices 2 through 5 of the string object itself. Like this:

2.5.1 :001 > fruit = ["apple", "banana", "orange", "grapefruit", "tomato"]
 => ["apple", "banana", "orange", "grapefruit", "tomato"]
2.5.1 :002 > fruit[1][2,5]
 => "nana"

I hope you're starting to see how powerful this method is. You can do so much with it, especially when you use it with regular expressions, or other methods.
With a chef knife you can take food and change it in so many ways, and the same goes for the #slice method, but without the risk of cutting yourself. Drop into IRB, play around for awhile, and see for yourself what it can do. Ruby is designed to make programmers happy, and so with Ruby,

happiness is a piece of cake

#slice of cake?

A quick note on hashes: like I said above, you can use #slice on hashes, but only in the newest Ruby versions. You use it like so:

2.5.1 :003 > basket = {fruit: "apples, bananas",  vegetables: "carrots, onions, celery", grains: "rice, cornmeal, quinoa"}
 => {:fruit=>"apples, bananas", :vegetables=>"carrots, onions, celery", :grains=>"rice, cornmeal, quinoa"}
2.5.1 :004 > basket.slice(:vegetables)
 => {:vegetables=>"carrots, onions, celery"}
2.5.1 :005 > basket.slice(:vegetables, :grains)
 => {:vegetables=>"carrots, onions, celery", :grains=>"rice, cornmeal, quinoa"}

For more on #slice, see the ruby docs, particularly look at concepts we didn't cover, like what situations cause #slice to return [] or nil, and what #slice! does.
For more on using #slice with strings, click here
For more on using #slice with hashes, click here

If you have any questions or comments, just drop me an email, or comment below.

Until next time, happy coding!

Top comments (1)

jeremy profile image
Jeremy Schuurmans

Absolutely worth mentioning. Thank you!