Local reference to internal array in fast

Question

I came from the C # world and got used to the fact that arrays are reference types. As I understand it, there are value types in fast arrays, but they try to play as reference ones.

I really don’t know how to ask what I need (I think this is the case when I need to know the answer in order to be able to ask a question), but in C # I would say that I need to store a link to the internal array jagged array into a local variable.

Consider the following code snippet:

// a function to change row values in-place
func processRow(inout row : [Int], _ value : Int)
{
    for col in 0..<row.count
    {
        row[col] = value;
    }
}

// a function to change matrix values in-place
func processMatrix(inout matrix : [[Int]])
{
    for rowIdx in 0..<matrix.count
    {
        // (1) Works with array in-place
        processRow(&(matrix[rowIdx]), -1)

        // (2) Creates local copy
        var r = matrix[rowIdx]; // <--- What to write here to not make a copy but still have this local variable?
        processRow(&r, -2);
    }
}

var matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

processMatrix(&matrix)

print(matrix) // outputs [[-1, -1, -1], [-1, -1, -1], [-1, -1, -1]]

The quick sandbox is here http://swiftlang.ng.bluemix.net/#/repl/8608824e18317e19b32113e1aa08deeb4ec5ab96ed8cdbe1dbfb4e753e87d528

Here I want to process a multidimensional array in place so that I don't create a copy of the array or part of it.

(1) "-1", , .

(2) matrix[rowIdx], - , ; , .

, (1), ? , ?

" ", , Apple.

Answer 1

, . , Swift. Apple:

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, , - r , matrix:

// (2) Creates local copy
var r = matrix[rowIdx]   
processRow(&r, -2)
matrix[rowIdx] = r

, , - : UnsafeMutablePointer, UnsafePointer .. , Swift . , :

func processRow(ptr: UnsafeMutablePointer<Int>, _ value : Int, count: Int) {
    for col in 0..<count {
        ptr.advancedBy(col).memory = value
    }
}

func processMatrix(inout matrix : [[Int]]) {
    for rowIdx in 0..<matrix.count {

        let r = UnsafeMutablePointer<Int>(matrix[rowIdx])
        processRow(r, -2, count: matrix[rowIdx].count)
    }
}

var matrix = [
    [1, 2, 3],
    [4, 5, 6],
    [7, 8, 9]
]

processMatrix(&matrix)
print(matrix)   // [[2, 2, 2], [2, 2, 2], [2, 2, 2]]

Answer 2

, . :

1) NSMutableArray

import Foundation

func processRow(_ row : NSMutableArray, _ value : Int) {
    for col in 0..<row.count {
        row[col] = value;
    }
}

func processMatrix(_ matrix : inout [NSMutableArray]) {
    for rowIdx in 0..<matrix.count {
        let r = matrix[rowIdx]
        processRow(r, -2);
    }
}

var matrix = [
    NSMutableArray(array: [1, 2, 3]),
    NSMutableArray(array: [4, 5, 6]),
    NSMutableArray(array: [7, 8, 9])
]

processMatrix(&matrix)

print(matrix) // outputs , <__NSArrayM 0x6000027a1560>(-2,-2,-2)]

2)

class Wrapper<T: CustomDebugStringConvertible>: CustomDebugStringConvertible {
    var value: T

    init(_ value: T) {
        self.value = value
    }

    var debugDescription: String {
        return value.debugDescription
    }
}

func processRow(_ row : Wrapper<[Int]>, _ value : Int) {
    for col in 0..<row.value.count {
        row.value[col] = value;
    }
}

func processMatrix(_ matrix : inout [Wrapper<[Int]>]) {
    for rowIdx in 0..<matrix.count {
        let r = matrix[rowIdx]
        processRow(r, -2);
    }
}

var matrix = [
    Wrapper([1, 2, 3]),
    Wrapper([4, 5, 6]),
    Wrapper([7, 8, 9])
]

processMatrix(&matrix)

print(matrix) // outputs [[-2, -2, -2], [-2, -2, -2], [-2, -2, -2]]

.