# Map Index to 2D Array

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I was reading a recent post about converting a 2D array into a 1D array.  The OP, from my understanding, more-or-less wanted to extract a single column of data and format it in a 1D array.  The solutions were proper and efficient compared to what I wanted to share, but I figured I'd share as it's a good lesson and a neat, purely mathematical approach to mapping an index to a 2D array.  As-is, this only works for 2D arrays.

By knowing how big a 2D array is, an index can be calculated for each indices/subscript.  The functions below define a random sized 2D array, and calculate an index that matches the indices on the 2D array.

```#include "Array.au3"
\$iRow = Random(2,10,1)
\$iCol = Random(2,10,1)
\$iRandomIndex = Random(0,(\$iRow*\$iCol)-1,1)

\$sIndicies = _IndexTo2DIndicies(\$iRandomIndex, \$iRow, \$iCol)
_2DArrayTo1DIndexes(\$iRow, \$iCol, \$iRandomIndex & " = \$aArr" & \$sIndicies)

Func _2DArrayTo1DIndexes(\$iRow = 1, \$iCol = 1, \$sMsg = Default)
\$iRow = Int(\$iRow)
\$iCOl = Int(\$iCol)

For \$iY = 0 to \$iRow - 1
For \$iX = 0 to \$iCol - 1
Local \$iIndex
;Index = (\$iColMax * \$iY) + \$iX
\$aData[\$iY][\$iX] = (\$iCol * \$iY) + \$iX
Next
Next

If \$sMsg = Default Then \$sMsg = "1D Indexes of 2D Array Indicies"
EndFunc

Func _IndexTo2DIndicies(\$iIndex = 0, \$iRow = 0, \$iCol = 0)
\$iIndex = Int(\$iIndex)
\$iRow = Int(\$iRow)
\$iCol = Int(\$iCol)

If \$iIndex = 0 Then Return "[0][0]"
If \$iIndex >= \$iRow * \$iCol Then Return SetError(1,0,0)

Local \$iX = Mod(\$iIndex, \$iCol)
;Local \$iY = (\$iIndex-\$iX) / \$iCol
Local \$iY = Int(\$iIndex / \$iCol)    ;Thanks Chimp

Return "[" & \$iY & "][" & \$iX & "]"
EndFunc```

I used this type of approach before with a pixel array.  I had a 1D array for storing individual pixel information, then mapped the pixels to an X|Y grid (2D array or XY coords).

Now if the purpose is just to flatten an array, the most efficient way I am aware of would be to create a new 1D array the size of the total number of indices in the 2D array (RowMax * ColMax), and loop through the array.  I suppose these methods (above) would be particularly useful if you need to randomly or programmatically access indices in a non-linear fashion.

Anyways, just wanted to share a fun exercise with arrays.

Edited by spudw2k

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\$iY can also be calculated like this:

`Local \$iY = Int(\$iIndex / \$iCol)`

• 1

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You're right, and it seems slightly faster.  Good idea.  Implemented.

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