Swift Combine reduce Operator

KD Knowledge Diet
2 min readMar 10


In the world of reactive programming in Swift, the Combine framework provides a powerful set of tools for working with streams of data. One of the most important sequence operators in the framework is “reduce”, which allows you to reduce a sequence or an array down to a single value.

Let’s start by creating an array of integers to work with:

let numbers = [1, 2, 3, 4, 5, 6]

Next, we’ll create a publisher from this array:

let publisher = numbers.publisher

Now, let’s use the “reduce” operator to add all the numbers in the array together

let sum = publisher.reduce(0) { accumulator, value in
return accumulator + value

The first argument to “reduce” is the initial value for the accumulator. In this case, we’re starting with zero. The second argument is a closure that takes two parameters: the accumulator, which is the running total of the reduce operation, and the value, which is the next element in the sequence.

In the closure, we’re simply adding the value to the accumulator and returning the new value of the accumulator. The “reduce” operator takes care of iterating through the sequence and performing the reduce operation on each element.

When the reduce operation is complete, the final value of the accumulator is returned as the result of the operation. In this case, the “sum” constant will be set to 21, which is the sum of all the numbers in the array.

The “reduce” operator is a powerful tool for manipulating and transforming streams of data in reactive Swift code. Whether you’re working with arrays, streams of data, or any other kind of sequence, the “reduce” operator can help you reduce your data down to a single value.

In conclusion, the “reduce” operator in the Combine framework is a valuable tool for reducing sequences and arrays down to a single value. By using closures to specify the reduce operation, you can customize the behavior of the operator to fit your specific use case. Try it out in your own projects to see how it can help you manipulate and transform streams of data in your reactive Swift code.



KD Knowledge Diet

Software Engineer, Mobile Developer living in Seoul. I hate people using difficult words. Why not using simple words? Keep It Simple Stupid!