Finding a value in an array of arrays (similar to VLOOKUP function in Excel) in Swift

Question

I am quite new to Swift 3 and to programming languages in general. I have the following arrays inside an array and a variable income:

let testArray: [[Double]] = [
    [0,0],
    [1000,20.5],
    [3000,21],
    [4000,22.5],
   ]

var income: Double = 3500

What I want to do is something similar to the VLOOKUP function in Excel: I want to find in the first column of the arrays (ie 0, 1000, 3000, 4000) a number which is equal or immediately smaller than my variable. In this case, as income = 3500, the program should return 3000. I tried using filter() but I don't know how to work with the arrays inside the array. Any help appreciated.

Answer 1

You can proceed as follows.

Get the first column of the array:

let firstColumn = testArray.map { $0[0] }
print(firstColumn) // [0.0, 1000.0, 3000.0, 4000.0]

Restrict to those elements which are less than or equal to the given amount:

let filtered = firstColumn.filter { $0 <= income }
print(filtered) // [0.0, 1000.0, 3000.0]

Get the maximal element of the filtered array. If the elements are sorted in increasing order then you can use last instead of max() :

let result = filtered.max()!
// Or: let result = filtered.last!
print(result) // 3000.0

Putting it all together:

let result = testArray.map { $0[0] }.filter { $0 <= income }.max()!
print(result) // 3000.0

A possible optimization is to combine map and filter into flatMap :

let result = testArray.flatMap { $0[0] <= income ? $0[0] : nil }.max()!
print(result) // 3000.0

This code assumes that there is at least one matching element, otherwise last! or max()! would crash. If that is not guaranteed:

if let result = testArray.flatMap( { $0[0] <= income ? $0[0] : nil }).max() {
    print(result) // 3000.0
} else {
    print("no result")
}

Or provide a default value ( 0.0 in this example):

let result = testArray.flatMap( { $0[0] <= income ? $0[0] : nil }).max() ?? 0.0
print(result) // 3000.0

Answer 2

Something like this:

let testArray: [[Double]] = [
        [0,0],
        [1000,20.5],
        [3000,21],
        [3500,22.5],
        [3300,21],
]

let income: Double = 3500

var closest = testArray[0][0]
var closestDif = closest - income

for innerArray in testArray {
    let value = innerArray[0]

    let thisDif = value - income

    guard thisDif <= 0 else {
        continue
    }

    if closestDif < thisDif {
        closestDif = thisDif
        closest = value

        guard closestDif != 0 else {
            break
        }
    }
}

print(closest)