Optional value in array

Question

Can someone explain to me why this code here returns the error: "fatal error: unexpectedly found nil while unwrapping an Optional value"

if let steps = legs[0]["steps"] {
    for i in 0...steps.length {
        print(steps[i])
    }
}

while this code:

let steps = legs[0]["steps"]!
print(steps[0])

returns the desired output? I am very confused as I have not been able to get all the values of steps contained in an array somehow..

Similarly:

for i in 0...legs[0]["steps"]!.length {
    print(legs[0]["steps"]![i]["start_location"])
}

gets fatal error while:

print(legs[0]["steps"]![0]["start_location"])

returns an optional value

You have to show us the original declaration of legs and any subtypes that that refers to. — BaseZen
– BaseZen, Commented Aug 4, 2016 at 15:41
Better use "for in" loops. See stackoverflow.com/documentation/swift/1186/loops/3839/… — Eric Aya
– Eric Aya, Commented Aug 4, 2016 at 15:43
But he didn't get an Array out of bounds error. He got a nil Optional error — BaseZen
– BaseZen, Commented Aug 4, 2016 at 15:43
Am I missing something? The length of an array in Swift is count, not length. — BaseZen
– BaseZen, Commented Aug 4, 2016 at 15:44
@BaseZen lol as usual I'm cursing everything from the makers of swift to xcode and in the end it's me who is 100% a dummy. Thanks man, that was it. Too much python. — tryingtolearn
– tryingtolearn, Commented Aug 4, 2016 at 15:46

Luca Angeletti · Accepted Answer · 2016-08-04 15:59:30Z

2

length??

First of all what is the type of steps? If it's an array it does not have a length property but count.

What is happening?

Lets take a look at this example snippet of code

let words = ["Hello", "world"]

for i in 0...words.count {
    print(words[i])
}

Here words.count is 2 so the for is being executed 3 times (i=0, i=1, i=2). Since arrays indexes begin from 0, the following elements are available

words[0] // "Hello"
words[1] // "world"

As you can imagine the last execution of the loop (when i=2) does access words[2] which does not exists! And this produces a crash.

Accessing the right index

Now let's take a look at your for loop

for i in 0...steps.length {
    print(steps[i])
}

As described in the previous paragraph, over the last loop execution you are accessing an element that does not exists. It should be

for i in 0..<steps.count {
    print(steps[i])
}

For in

Even better you could get rid of the indexes and simply write

for step in steps {
    print(step)
}

For each

Another syntax, same result of the previous block of code

steps.forEach { step in
    print(step)
}

edited Aug 4, 2016 at 15:59

answered Aug 4, 2016 at 15:47

Luca Angeletti

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1 Comment

tryingtolearn Over a year ago

Yep, as you and BaseZen have pointed out to me it's not length lol it's count. I've been coding in python too much. Thanks!

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Optional value in array

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length??

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