The Array filter method
One very common operation in programming is to iterate through an Array's contents, apply a test function to each item, and create a new array containing only those items the passed the test. For example, let's say you wanted to loop through an array of stocks and select only those with the price larger than a certain value. In this lesson we will demonstrate how to use the Array's filter method to easily perform this operation with less code than a loop would require.
[00:01] In the previous video, we learned about the Array's map function. The Array's map function is great if you want to apply some transformation to every item in an Array and collect up the results in a new Array. Now, sometimes map isn't good enough.
[00:15] Sometimes what you'd like to do is filter the items in an Array, returning only those items that match a certain test. Let's keep going with the stocks example that we've set up thus far.
[00:27] Let's imagine that we want to create a function called "get stocks over" which accepts an Array of stocks, a minimum price, and then returns only those stocks with a price larger than or equal to that minimum price.
[00:44] What we'll do is we will run get stocks over which will return a new Array containing only those stocks that are expensive, will pass in an Array of stocks, and then a minimum price, let's say $150. Then we're going to log the expensive stocks to the console.
[01:04] In order to make this function run, we're going to use the forEach function that we learned about earlier. First, we're going to create a new Array to hold all of the results, those stocks with a price larger than or equal to the minimum price, and we're going to use the forEach method on the stocks' Array.
[01:24] Remember, the forEach method accepts a function which it calls once for each stock in the Array. All we're going to do, this forEach function gives us the opportunity to run some operation for each stock in that Array.
[01:38] That operation we're going to run is we're going to check to see if the stock's price is larger than or equal to the minimum price and, if so, we're going to add it to the results Array. Finally, get stocks over is going to return its results Array.
[01:53] I want you to notice that get stocks over doesn't modify the stock's Array, it just creates a new Array, adds only those stocks that match the test to that Array and then finally returns it. If we run this, we should see only two stocks, XFX and TNZ, both of which have prices larger than or equal to $150. Sure enough, we see it.
[02:12] It turns out that this operation, this filtering operation, is really common. That's why the ArrayObject has a special filter method custom designed to do exactly this, apply a test to each item in an Array and create a new Array containing only those items that pass the test.
[02:32] Array already has a filter method, but I'm going to go ahead and redefine it here so that we can see how it works. We're going to add a filter method to the ArrayObject. The filter method accepts a function called a predicate. A predicate is really simple. All it is is a function that accepts a value and returns true or false.
[02:50] We're going to see that this structure, this code, is going to mimic the operation that we're doing in get stocks over. First, we're going to create a results Array. We're going to use this to accumulate up those items that pass that test. Then we're going to call forEach on this. Now, notice that this, in this context, is actually the Array because this filters a method on Array.
[03:15] We're going to use this forEach to run an operation on every item in the Array. Now, what we're going to do is we're going to apply the predicate function to the item and if it returns true, we're going to add it to the results. We're not going to translate the item like we did in map. We're just going to add it exactly as it is.
[03:34] Finally, all the filter function needs to do is return the new Array of the filtered results. Now, we can use this filter function to replace all this code with something much, much shorter. I can simply go return stocks filter.
[03:52] We pass in the predicate function. The predicate function gets past the current stock, and it returns true or false depending on whether we want to keep a stock or not. Now, in order to decide if we want to keep a stock, we're going to compare the price to the min price and if true, it's going to end up in the results.
[04:10] Here, if I run this, we should see the exact same result. Sure enough we do. Only XFX and TNZ are included. I can go ahead and delete this code right here because, as I said, this filter method already exists on the Array. All we've done is redefine it so that we can see how it works. If I delete this and run it again, we'll still see that we get the exact same result.
[04:33] In summation, filter is great if you want to apply a test function to every item in Array and create a new Array containing only those items that pass the test.