Two-dimensional array in Swift

Question

I get so confused about 2D arrays in Swift  Let me describe step by step  And would you please correct me if I am wrong   First of all  declaration of an empty array   class test      var my2Darr   Int           Secondly fill the array   such as my2Darr i  j    0 where i  j are for-loop variables   class test       var my2Darr   Int           init             for var i Int 0 i lt 10 i                  for var j Int 0 j lt 10 j                      my2Darr i  j  18       Is this correct                                        And Lastly  Editing element of in array  class test       var my2Darr   Int           init                     same as up code           func edit number Int index Int           my2Darr index  index    number            Is this correct  and What if index is bigger            than i or j    Can we control that like          if  my2Darr i  j     nil

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In Swift 4  var arr   Array repeating  Array repeating  0  count  2   count  3       0  0    0  0    0  0

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Make it Generic Swift 4  struct Matrix lt T gt        let rows  Int  columns  Int     var grid   T      init rows  Int  columns  Int defaultValue  T            self rows   rows         self columns   columns         grid   Array repeating  defaultValue  count  rows   columns  as   T            func indexIsValid row  Int  column  Int  - gt  Bool           return row  gt   0  amp  amp  row  lt  rows  amp  amp  column  gt   0  amp  amp  column  lt  columns           subscript row  Int  column  Int  - gt  T           get               assert indexIsValid row  row  column  column    Index out of range               return grid  row   columns    column                    set               assert indexIsValid row  row  column  column    Index out of range               grid  row   columns    column    newValue                     var matrix Matrix lt Bool gt    Matrix rows  1000  columns  1000 defaultValue false   matrix 0 10    true   print matrix 0 10

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According to Apple documents for swift 4 1 you can use this struct so easily to create a 2D array   Link  https   developer apple com library content documentation Swift Conceptual Swift Programming Language Subscripts html  Code sample   struct Matrix       let rows  Int  columns  Int     var grid   Double      init rows  Int  columns  Int            self rows   rows         self columns   columns         grid   Array repeating  0 0  count  rows   columns            func indexIsValid row  Int  column  Int  - gt  Bool           return row  gt   0  amp  amp  row  lt  rows  amp  amp  column  gt   0  amp  amp  column  lt  columns           subscript row  Int  column  Int  - gt  Double           get               assert indexIsValid row  row  column  column    Index out of range               return grid  row   columns    column                    set               assert indexIsValid row  row  column  column    Index out of range               grid  row   columns    column    newValue

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Define mutable array     2 dimensional array of arrays of Ints  var arr     Int        OR      2 dimensional array of arrays of Ints  var arr    Int           OR if you need an array of predefined size  as mentioned by  0x7fffffff in comments       2 dimensional array of arrays of Ints set to 0  Arrays size is 10x5 var arr   Array count  3  repeatedValue  Array count  2  repeatedValue  0          and for Swift 3   var arr   Array repeating  Array repeating  0  count  2   count  3    Change element at position  arr 0  1    18   OR  let myVar   18 arr 0  1    myVar   Change sub array  arr 1     123  456  789     OR  arr 0     234   OR  arr 0      345  678    If you had 3x2 array of 0 zeros  before these changes  now you have         0  0  234  345  678      5 elements     123  456  789      0  0      So be aware that sub arrays are mutable and you can redefine initial array that represented matrix    Examine size bounds before access  let a   0 let b   1  if arr count  gt  a  amp  amp  arr a  count  gt  b       println arr a  b       Remarks  Same markup rules for 3 and N dimensional arrays

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This can be done in one simple line   Swift 5  var my2DArray    0   lt 4  map     in Array 0   lt       You could also map it to instances of any class or struct of your choice  struct MyStructCouldBeAClass       var x  Int     var y  Int    var my2DArray    MyStructCouldBeAClass      0   lt 2  map   x in     Array 0   lt 2  map   MyStructCouldBeAClass x  x  y   0

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Before using multidimensional arrays in Swift  consider their impact on performance  In my tests  the flattened array performed almost 2x better than the 2D version   var table    Int  repeating  0  count  size   size  let array    Int  1   size  for row in 0   lt size       for column in 0   lt size           let val   array row    array column             assign         table row   size   column    val           Average execution time for filling up a 50x50 Array  82 9ms  vs   var table     Int   repeating   Int  repeating  0  count  size   count  size  let array    Int  1   size  for row in 0   lt size       for column in 0   lt size              assign         table row  column    val           Average execution time for filling up a 50x50 2D Array  135ms  Both algorithms are O n 2   so the difference in execution times is caused by the way we initialize the table   Finally  the worst you can do is using append   to add new elements  That proved to be the slowest in my tests   var table    Int        let array    Int  1   size  for row in 0   lt size       for column in 0   lt size           table append val            Average execution time for filling up a 50x50 Array using append    2 59s  Conclusion  Avoid multidimensional arrays and use access by index if execution speed matters  1D arrays are more performant  but your code might be a bit harder to understand   You can run the performance tests yourself after downloading the demo project from my GitHub repo  https   github com nyisztor swift-algorithms tree master big-o-src Big-O playground

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From the docs   You can create multidimensional arrays by nesting pairs of square brackets  where the name of the base type of the elements is contained in the innermost pair of square brackets  For example  you can create a three-dimensional array of integers using three sets of square brackets   var array3D     Int         1  2    3  4      5  6    7  8      When accessing the elements in a multidimensional array  the left-most subscript index refers to the element at that index in the outermost array  The next subscript index to the right refers to the element at that index in the array that   s nested one level in  And so on  This means that in the example above  array3D 0  refers to   1  2    3  4    array3D 0  1  refers to  3  4   and array3D 0  1  1  refers to the value 4

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You should be careful when you re using Array repeating  Array repeating   value   count  80   count  24    If the value is an object  which is initialized by MyClass    then they will use the same reference   Array repeating  Array repeating  MyClass    count  80   count  24  doesn t create a new instance of MyClass in each array element  This method only creates MyClass once and puts it into the array   Here s a safe way to initialize a multidimensional array   private var matrix    MyClass     MyClass newMatrix    private static func newMatrix   - gt    MyClass         var matrix    MyClass             for i in 0   23           matrix append                for   in 0   79               matrix i  append  MyClass                          return matrix

[arrays] Two-dimensional array in Swift

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