从随机字典键值创建数组

Question

I want to make a function which returns array of strings and function parameter is array size. Array values are random strings from dictionary keys. I made this code

import UIKit

var cards: [String:Int] = [

"crvenaSedmica":0,
"crvenaOsmica":0,
"crvenaDevetka":0,
"crvenaDesetka":0,
"crvenaDecko":0,
"crvenaDama":0,
"crvenaKralj":0,
"crvenaAs":0,
"bundevaSedmica":0,
"bundevaOsmica":0,
"bundevaDevetka":0,
"bundevaDesetka":0,
"bundevaDecko":0,
"bundevaDama":0,
"bundevaKralj":0,
"bundevaAs":0,
"zelenaSedmica":0,
"zelenaOsmica":0,
"zelenaDevetka":0,
"zelenaDesetka":0,
"zelenaDecko":0,
"zelenaDama":0,
"zelenaKralj":0,
"zelenaAs":0,
"zirSedmica":0,
"zirOsmica":0,
"zirDevetka":0,
"zirDesetka":0,
"zirDecko":0,
"zirDama":0,
"zirKralj":0,
"zirAs":0,
]

func createRandomCards(number:Int) -> [String] {
    let cardTypes = Array(cards.keys) 
    var randomCards: [String] = []

    for i in 0..<number {  
        randomCards[i] = cardTypes[Int.random(in: 0..<32)]    
    }
    return randomCards
}

createRandomCards(number: 5)

but I'm getting:

Execution was interrupted, reason: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0 error.

Answer 1

The real error is actually:

Swift/ContiguousArrayBuffer.swift:593: Fatal error: Index out of range 2021-05-16 09:59:14.866263-0700 YOURAPPNAME[2540:97613] Swift/ContiguousArrayBuffer.swift:593: Fatal error: Index out of range

The problem is that you are accessing randomCards by index, but the element doesn't exist yet.

var randomCards: [String] = []
...
randomCards[i] = cardTypes[Int.random(in: 0..<32)]
            ^ right here!

Here, randomCards is still empty. randomCards[0] , randomCards[1] , randomCards[2] , etc all wouldn't work.

Instead, you want to append . Accessing by index is only for when you want to modify an existing element.

randomCards.append(cardTypes[Int.random(in: 0..<32)])

---

Also you are throwing away the function's return value. Instead of...

createRandomCards(number: 5)

... you should assign it to a constant.

let cards = createRandomCards(number: 5)

Answer 2

Do not use that for loop. It has been abstracted away into the map method. Writing it yourself is error-prone, as you've seen.

Also, do not hard-code the keys' count.

func createRandomCards(number: Int) -> [String] {
  AnyIterator { }
    .prefix(number)
    .map { cards.keys.randomElement()! }
}