Bug in Swift array subscript indexing?

Question

I've isolated some Swift code from my project that can be pasted into a Playground. It produces an error "Could not find an overload for '+' that accepts the supplied arguments" both in normal Xcode editing and the Playground. The error refers to the last (non-trivial) line.

import UIKit

let points = 40
let max = points-1

let L = 10.0
let Deltat = 0.01
let Deltax = L/Double(points)

var a = [Double](count: points, repeatedValue: 0.0)
var b = [Double](count: points, repeatedValue: 0.0)
var c = [Double](count: points, repeatedValue: 0.0)

for i in 1..<max-1
{   //let iPlus1 = i+1
    //let temp = 0.5*Deltat/Deltax
    c[i] = 0.5*(a[i+1] + a[i-1]) + 0.5*Deltat/Deltax * (b[i+1] - b[i-1])
}

If I uncomment the line "let iPlus1..." and make the following edit, Swift accepts the code.

{   let iPlus1 = i+1
    //let temp = 0.5*Deltat/Deltax
    c[i] = 0.5*(a[iPlus1] + a[i-1]) + 0.5*Deltat/Deltax * (b[i+1] - b[i-1])
}

If I uncomment the line "let temp..." and make the following edit, Swift again accepts the code.

{   //let iPlus1=i+1
    let temp = 0.5*Deltat/Deltax
    c[i] = 0.5*(a[i+1] + a[i-1]) + temp * (b[i+1] - b[i-1])
}

Neither of these edits make any sense to me, as both are seemingly trivial substitutions. I am aware that Swift will never implicitly typecast for me. There doesn't seem to be any implicit typecasting attempted in the original code -- all Ints and Doubles are declared as intended. I'm starting to believe this is a bug with the Swift array subscript indexing.

What happens if you put parentheses around these additions? e.g. a[(i+1)] or + (0.5*Deltat/Deltax) ? — Ian MacDonald
– Ian MacDonald, Commented Oct 7, 2014 at 20:09
I had actually tried things like your suggestion prior to posting the original question. Neither of those suggestions fix the problem. Even if they did, it would still suggest a bug. — Brandon Murakami
– Brandon Murakami, Commented Oct 7, 2014 at 21:58
Oh, I definitely agree that there's a bug, I was just curious. :) — Ian MacDonald
– Ian MacDonald, Commented Oct 8, 2014 at 13:52

Community · Accepted Answer · 2017-05-23 12:05:49Z

2

This is a known swift bug: long statements generate strange compilation errors. Just split your line in 2 lines, such as:

c[i] = 0.5*(a[i+1] + a[i-1])
c[i] += 0.5*Deltat/Deltax * (b[i+1] - b[i-1])

I found out that happens with more than 4 or 5 arithmetic operations in the same line, but that's not a rule, just a number found with some expression types - it may be different in other cases.

Look for example at this Q&A: Xcode Beta 6.1 and Xcode 6 GM stuck indexing for weird reason and Xcode 6 with Swift super slow typing and autocompletion (this last one actually causes slowness, but solved in the same way, so the root is probably the same)

edited May 23, 2017 at 12:05

CommunityBot

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answered Oct 7, 2014 at 20:11

Antonio

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4 Comments

Brandon Murakami Over a year ago

In my debugging, I had actually come across your suggested solution. For the sake of brevity, I did not list it as a solution though. I also was very aware that Swift was taking unusually long to compile my very short and simple code. Verifying these bugs is very helpful, though it forces the difficult question of whether I should continue using Swift or not.

Antonio Over a year ago

About your concern, you have no other choice than using Swift if you want to develop for iOS or Mac in the future. The question might instead be: should I develop now or wait for it to be more stable?

Brandon Murakami Over a year ago

Yes, that is my true question. You are correct that Swift isn't going away. ...Back to the original question -- it seems any attempt to reduce the number of operations in the line in question is a potential workaround solution. The iPlus1 solution is bizarre in that it reduces complexity independent of the Double arithmetic. This suggests the problem is a fundamental parsing problem -- which is hard to believe survived to v1.0.

Antonio Over a year ago

Yeah that is strange - it is a known problem, experienced myself from beta 3, but I suspect it's been around since day 0. Maybe it's just too complicated to solve in a timely manner, or it requires substantial changes to the compiler, and they are just deferring from beta to beta (including 1.0, which is, in my opinion, still a beta :))

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