Write a program that uses two threads to print the numbers from 1 to n.

Question

Anonymous · Accepted Answer

The idea would be to use either a semaphore or a lock. 
func speedyCounter(_ n: Int, _ numThreads: Int) {
    var currentNumber = 0
    let lock = NSLock()
    let sem = DispatchSemaphore(value: 1)
    let operationQueue = OperationQueue()
    operationQueue.maxConcurrentOperationCount = numThreads


    let lock_counter: (inout Int) -> Void = { currentNumber in
        while true {
            lock.lock()
            if currentNumber >= n {
                lock.unlock()
                return
            }
            print(currentNumber)
            currentNumber += 1
            lock.unlock()
        }
    }

    let sem_counter: (inout Int) -> Void = { currentNumber in
        while true {
            sem.wait()
            if currentNumber >= n {
                sem.signal()
                return
            }
            print(currentNumber)
            currentNumber += 1
            sem.signal()

        }
    }

    operationQueue.addOperation {
        sem_counter(&currentNumber)
    }

}

usage: speedyCounter(100, 5)

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