Writing Multiple Test Cases Together

In this lesson you will learn how to run several test cases together in Swift, and understand what a body of unit tests looks like. This is for anyone searching for "Swift unit test how to write".

Testing one function against several input patterns at once is called unit testing. Preparing a spread of cases - normal values, boundary values, special values - helps you find bugs early.

The sample code collects several cases in an array of (a, b, expected) tuples, feeds each one to add() in a for loop, checks the result, and counts the successes in passed. Keeping the cases in an array of tuples also makes adding new ones easy later.

A common stumbling block for beginners is testing only the happy path and forgetting boundary values such as zero or negative numbers. Covering the cases that are easy to overlook is what matters.

In real development it is standard to run these tests automatically after every change using a dedicated framework such as XCTest.

📖 Reference code

func add(_ a: Int, _ b: Int) -> Int {
    return a + b
}

let testCases: [(a: Int, b: Int, expected: Int)] = [
    (2, 3, 5),
    (-1, 1, 0),
    (0, 0, 0),
]

var passed = 0
for (i, tc) in testCases.enumerated() {
    let result = add(tc.a, tc.b)
    let ok = result == tc.expected
    if ok { passed += 1 }
    print("Test \(i + 1): \(ok ? "OK" : "FAIL")")
}
print("\(passed)/\(testCases.count) passed")

✍️ Your code

Type your code, then press "Run"

🧪 This site can't compile or run Swift directly, so it checks on the spot whether what you typed matches the reference code (scoring happens entirely in your browser — nothing is sent anywhere).