Replacing the position of an element in Array of Array

Question

I have been trying to run this loop to change the position of an element in an array based but I get the error below. Not really sure what could be wrong. Any thoughts or Ideas!! Thanks.

var population = [[98, 8, 45, 34, 56], [9, 13, 65, 4, 90], [24, 5, 4, 56, 88], [3, 55, 22, 19, 10]]

private func randomIntergerInRange(low:Int, high:Int) -> Int {
    let randomGeneratedNumber = low + Int(arc4random()) % (high - low);
    return Int(randomGeneratedNumber)
}

for individual in population {
    var mutate = 0.01

    if mutate > Double(Float(arc4random()) / Float(UINT32_MAX)) {

        var positionToMutate = randomIntergerInRange(0, (individual.count - 1))

        var valueToAssign = randomIntergerInRange(minElement(individual), maxElement(individual))

        individual[positionToMutate] = valueToAssign
        print(individual)

    }
}

why your array look more like an Dictionary

Lamour
– Lamour

2015-08-30 16:17:55 +00:00
Commented Aug 30, 2015 at 16:17 — Lamour
– Lamour, Commented Aug 30, 2015 at 16:17
It's an array of Arrays @Lamar ;)

Gugulethu
– Gugulethu

2015-08-30 16:20:30 +00:00
Commented Aug 30, 2015 at 16:20 — Gugulethu
– Gugulethu, Commented Aug 30, 2015 at 16:20

Renzo · Accepted Answer · 2015-08-30 16:27:35Z

2

Array are passed by value, so when you use individual at each iteration it is assigned a copy of the subarray of population, and even if you use for var individual in population ... you mutate only a copy of the original array.

If you want to mutate the values of the original array you can do it by using indices, for instance:

for index in population.indices {
    var mutate = 0.01

    if mutate > Double(Float(arc4random()) / Float(UINT32_MAX)) {

        var positionToMutate = randomIntergerInRange(0, (population[index].count - 1))

        var valueToAssign = randomIntergerInRange(minElement(population[index]), maxElement(population[index]))

        population[index][positionToMutate] = valueToAssign
        print(population[index])

    }
}

edited Aug 30, 2015 at 16:27

answered Aug 30, 2015 at 16:22

Renzo

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1 Comment

Gugulethu Over a year ago

Oh I see. I wasn't aware of that. Thanks a lot. ;) @Renzo

Eric Aya · Accepted Answer · 2016-03-04 16:33:20Z

1

Reason for the error message: the individual object in your loop is immutable, it's a copy of the object, not a reference.

You could do something like this, using enumerate:

for (index, individual) in enumerate(population) {
    var mutate = 0.01

    if mutate > Double(Float(arc4random()) / Float(UINT32_MAX)) {

        var positionToMutate = randomIntergerInRange(0, (individual.count - 1))

        var valueToAssign = randomIntergerInRange(minElement(individual), maxElement(individual))

        population[index][positionToMutate] = valueToAssign

        print(population[index])

    }
}

You shouldn't modify a collection while iterating it, btw, it could have side effects.

Update for Swift 2

private func randomIntegerInRange(low:Int, _ high:Int) -> Int {
    let randomGeneratedNumber = low + Int(arc4random()) % (high - low)
    return Int(randomGeneratedNumber)
}

var mutate = 0.01
for (index, individual) in population.enumerate() {
    if mutate > Double(Float(arc4random()) / Float(UINT32_MAX)) {
        var positionToMutate = randomIntegerInRange(0, (individual.count - 1))
        if let min = individual.minElement(), max = individual.maxElement() {
            var valueToAssign = randomIntegerInRange(min, max)
            population[index][positionToMutate] = valueToAssign
            print(population[index])
        }
    }
}

edited Mar 4, 2016 at 16:33

answered Aug 30, 2015 at 16:19

Eric Aya

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1 Comment

Gugulethu Over a year ago

Thanks. ;). Stuff I wasn't really sure of!.

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