Understanding swift functions

Question

I'm studying swift and I wonder why following code:

func isNumberDivisible(_ number: Int, by divior: Int) -> Bool {

    if number % divior == 0 {
        return true;
    }   else {
        return false;
    }
}

func isPrime (_ number: Int) -> Bool {

    var isPrimeNumber = false

    for index in 0..<number {

        if (isNumberDivisible(number, by:index )) {
            isPrimeNumber = false
        }   else {
            isPrimeNumber = true
        }
    }

    return isPrimeNumber
}

isPrime(10)

Output an error - Execution was interrupted, EXC_BAD_INSTRUCTION..

@Hamish Hes not dividing. Its a mod `%` operator. But its throwing error because you are using `%` with zero which is not defined I guess. — GoodSp33d, Feb 07 '17 at 16:54
@LucasPalaian I just meant it literally with the operator :) Sorry my bad ! But another thing OP: you need to start the loop from 2 — GoodSp33d, Feb 07 '17 at 16:58
Related ? : http://stackoverflow.com/questions/7370154/cant-mod-zero — GoodSp33d, Feb 07 '17 at 16:59
@GoodSp33d Technically, `%` is a *remainder* operator in Swift (not modulo), as it just negates the results for a negative dividend. And okay, OP *himself* isn't technically dividing by zero, but he's asking Swift to tell him the remainder when a given number is divided by 0 – which is invalid. — Hamish, Feb 07 '17 at 17:05
Note that Xcode comes with a decent *debugger.* If you single-step through the code then you should find the problem quickly. — Martin R, Feb 07 '17 at 17:47

Russell · Accepted Answer · 2017-02-07T17:03:45.440

4

Your for loop starts with zero. That's a meaningless test, and so is checking if you can divide by 1.

You should start your index at 2

for index in 2..<number {

and once you find it is NOT a prime number, you should stop - what this function actually prints out is whether or not the number is divisible by (number - 1). And as @rmaddy points out, you don't need to check every number - in your example 10 is divisible by 2 and 5 - but you don't need to check 5, because you will have already failed on 2

for index in 2..<Int(sqrt(Double(number))) {

        if (isNumberDivisible(number, by:index )){
            isPrimeNumber = false
            break
        }   else{
            isPrimeNumber = true
        }
    }

edited Feb 07 '17 at 17:03

answered Feb 07 '17 at 16:58

Russell

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And it should end at the `Int(sqrt(number))`. – rmaddy Feb 07 '17 at 16:59
1

It also is kind of backwards - for a non-prime it will loop more than necessary. The looping/flags should be refactored to improved efficiency asking if 10000 is Prime will loop 10000 times as is, and 10000-2=9998 times if started at 2 or 100 times with @rmaddy correction. If flagged properly, it would loop 1 time. – Feb 07 '17 at 17:03

score 3 · Answer 2 · answered Feb 07 '17 at 16:58

3

The remainder operator (%) performs a division
The first index is 0
number % divior does 10 / 0

Division by zero causes a runtime error.

By the way, the first prime number is 2

for index in 2..<number

and your algorithm doesn't work anyway.

answered Feb 07 '17 at 16:58

vadian

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Doesn't the algorithm work if you put a break in the first if ? – GoodSp33d Feb 07 '17 at 17:02
Looks like homework - they might have been told to follow this approach. A coding exercise may have different objectives than optimising runtime performance – Russell Feb 07 '17 at 17:16

Understanding swift functions

2 Answers2