To replace a slice of array in Swift, we can use subscript assignment or replaceSubrange method on array. But, I wonder how can I change the elements of named subrange so that original array change as well?
var array = Array(0..<10)
var slice = array[0..<3]
slice[...] = [-1,-1,-1]
print(slice) // [-1, -1, -1]
print(array) // [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] does not change
Related
I am trying to understand Sets in Swift and how to declare them correctly but I have found the following a little confusing.
I understand that this works:
let elements = [ 1, 2, 3, 4, 5, 1, 2, 6, 7]
let setFromElements = Set(elements)
But I don't understand why the following doesn't:
let setFromElements : Set = elements
Or even:
let setFromElements : Set<Int> = elements
When the following is valid:
let setFromArray : Set = [ 1, 2, 4, 5]
Can someone please help me understand why this is the case?
let setFromArray: Set = [ 1, 2, 4, 5] works because Set conforms to ExpressibleByArrayLiteral and hence has an initializer that takes an ArrayLiteral. See Set.init(arrayLiteral:). This conformance gives syntactic sugar for not having to explicitly call the init.
On the other hand, once you save the array literal into a variable using let elements = [ 1, 2, 3, 4, 5, 1, 2, 6, 7], elements becomes an Array, not an ArrayLiteral and hence another initializer of Set has to be called that takes an Array. This init does not provide syntactic sugar like ExpressibleByArrayLiteral does, so you explicitly have to call the init by doing Set(array).
Set has an initializer that takes an array, and that makes a set, by taking the unique items in the array. But a set is not an array, two different types, so you can't just use = to assign one to the other.
I'm using IndexSet and I'm trying to access some indexes which at times are consecutive and at other times are not.
For example, my set may contain [1, 2, 3, 5, 6, 7, 13, 31]
I want to pull out of the set a range of 3...13, but am having difficulty with the syntax. I've learned how to use the function commands from Apple documentation, by using myIndexSet.sorted(). However, the Apple Documentation does not give an example of how to access a range of elements in the set. The Apple Documentation for accessing elements in the index set are the following:
subscript(Range<IndexSet.Index>)
I've tried a number of ways to write this but can't figure out how to do it right. Can someone show me how to access a range of elements in the set to create a new set? I've tried things such as:
let subset = subscript(Range: myLargerSet.3...13)
but it doesn't seem to work.
Thanks
What you're looking for is the intersection of your IndexSet ([1, 2, 3, 5, 6, 7, 13, 31]) with another IndexSet ([3, 4, ..., 12, 13]):
let yourIndexSet: IndexSet = [1, 2, 3, 5, 6, 7, 13, 31]
let desiredIndexRange = IndexSet(3...13)
let indicesOfInterest = yourIndexSet.intersection(desiredIndexRange)
print(indicesOfInterest.sorted()) // => [3, 5, 6, 7, 13]
One possible solution is to use a filter to create a new IndexSet.
let set = IndexSet(arrayLiteral: 1,2,3,5,6,7,13,31)
let subset = set.filteredIndexSet { (index) -> Bool in
index >= 3 && index <= 13
}
You can access a slice of your original set as follows:
let slice = indexSet[indexSet.indexRange(in: 3...13)]
slice accesses the existing elements in place, so creation of the slice is O(1)
I have a array of dictionary.
I need to add a new key and a value to all the elements of array of dictionary.
Please guide.
You need to extract the dictionary from the array, modify it and put back to the array, since arrays and dictionaries in Swift are value types.
Something like this should work for you:
var arrayofDict = [["1": 1], ["2": 2], ["3": 3]]
for i in 0..<arrayofDict.count {
var dict = arrayofDict[i]
dict["random"] = Int(arc4random_uniform(8)) //random integer value
arrayofDict[i] = dict
}
print(arrayofDict) //prints "[["1": 1, "random": 2], ["2": 2, "random": 0], ["random": 2, "3": 3]]\n"
I am not able to print array values in swift. My code is:
var array = 1...10
println(array)
The result is:
VSs5Range (has 2 children)
But when I try to print the following array, it works:
var array = [1,2,3,4,5,6,7,8,9,10]
println(array)
Result is:
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
Why isn't the first one printing correctly?
The expression 1...10 returns a Range, not an Array. Internally, a Range stores two values (a start and an end); an Array, on he other hand, is a dynamic structure containing "n" values.
As explained,
var array = 1...10
array, in this case, is a Range object, not an array
If you want to print its content do this (changed the name to something more suitable)
var range = 1...10
for value in range
{
println(value)
}
I read swift handbook and was trying to do some exercises. But I run into a problem and I do not know if I do something wrong or if xCode 6 beta is just buggy.
// Playground - noun: a place where people can play
import Cocoa
let interestingNumbers = [
"Prime": [2, 3, 5, 7, 11, 13],
"Fibonacci": [1, 1, 2, 3, 5, 8],
"Square": [1, 4, 9, 16, 25],
]
var largest = 0
var lastLargest = Integer[]()
var index = 0
for (kind, numbers) in interestingNumbers {
for number in numbers {
if number > largest {
//lastLargest[index] = number
index++
largest = number
}
}
}
index
lastLargest
largest
As soon as I uncomment lastLargest[index] = number I do not get any results on right side in playground. Nor I get any infos about index, lastLargest or largest.
Following example does not work either:
var index2 = 0
var lastLargest2 = Integer[]()
lastLargest2[index2] = 1
index2++
lastLargest2[index2] = 2
You are appending using an out of bound array-index. Don't do that. Instead, use append:
lastLargest.append(number)
From Apple's documentation:
You can’t use subscript syntax to append a new item to the end of an array. If you try to use subscript syntax to retrieve or set a value for an index that is outside of an array’s existing bounds, you will trigger a runtime error.
When you're using explicit indexes (subscript notation) to set values in a mutable array, some value must already exist in that array at that index. When you use subscript notation, you're essentially using a 'set', rather than a 'set and add if necessary'.
As a result, you should be using:
lastLargest.insert(number, atIndex: index)
If you want to insert a new item. This will let you insert an item at the specified index, assuming your collection's size is already greater than or equal to the index you're trying to replace.