I have setup a simple json import to hold my list of words for concentration. Using the simplified technique, I am storying each pair of words as a class :

class Pair: NSObject, Codable {
    var word1: String
    var word2: String

    init(word1: String, word2: String) {
        self.word1 = word1
        self.word2 = word2
    }
}

I am trying to search into an array of Pairs to find the word from the first card and to get it's word2, and then see if that matches the second card, but I am unsure how to do that lookup.

I receive a "Cannot convert value of type 'String?' to expected argument type 'Pair' if I try pairs.firstIndex(of: checkValue)

What is the correct way to search the array of Pairs, I really don't want to brute force it (even thought the number of values is pretty small.

firstIndex(of:) will get you the first index in an array, but that's not really what you want here. What you really want is the first Pair that has a word1 that matches your criteria. So you should be using first(where:) instead...

import Foundation

class Pair: NSObject, Codable {
    var word1: String
    var word2: String

    init(word1: String, word2: String) {
        self.word1 = word1
        self.word2 = word2
    }
}

let pairs = [
    Pair(word1: "kiwi", word2: "grapefruit"),
    Pair(word1: "bread", word2: "butter"),
    Pair(word1: "brick", word2: "mortar"),
    Pair(word1: "leather", word2: "lace"),
    Pair(word1: "mental", word2: "physical"),
]

let checkValue = "leather"
if let firstWord = pairs.first(where: { $0.word1 == checkValue }) {
    print(firstWord.word2)
} else {
    print("not found")
}