Guard let construction, still getting fatal error: Index out of range - swift

Question

I am trying to parse some data with this code:
func findDate(data: String?) -> String {
guard let date: String? = (data!.componentsSeparatedByString("T"))[0] else{
return "20000101"
}
return date!
}
I tried the guard structure to prevent errors when there is no data found, or it has a different structure, but I still get the error when I run it:
fatal error: Index out of range
Does somebody know how to fix this?

Answer 1

The access of the element at index zero always happens. If the result of the call to components(separatedBy:) returns an empty array, your code crashes. Also, you should avoid force unwrapping data.
A solution to these crashes is to use the first property of the array, which is optional, so you can safely unwrap it.
guard let date = data?.components(separatedBy: "T").first else {
return "20000101"
}
return date.
Also, this could then be simplified using the nil coalescing operator:
return data?.components(separatedBy: "T").first ?? "20000101"

Answer 2

Please check out this code:
func findDate(data: String?) -> String {
guard let date: String? = (data?.componentsSeparatedByString("T"))?[0] else{
return "20000101"
}
return date!
}
findDate(nil)
findDate("")
findDate("98588T99")

Answer 3

There are two issues here:
1) Trying to access an index of an array that isn't large enough doesn't produce nil, it just crashes. For example, this code will crash:
let strings: [String] = []
guard let firstString = strings[0] else {
// couldn't find firstString
}
Instead, use the first method:
let strings: [String] = []
guard let firstString = strings.first else {
// couldn't find firstString
}
first attempts to access the first element of the array, and it returns nil if the array doesn't have a first element.
2) The purpose of guard let is to guarantee that the value you get out isn't nil. So your date value should be of type String, not String?, and you shouldn't have to force-unwrap it. For example:
let strings: [String] = ["one", "two", "three"]
guard let firstString = strings.first else {
// no first string
}
print(firstString) // firstString is not nil, no need to unwrap it