Randomize array? [duplicate] - delphi

Question

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Randomize StringList
(3 answers)
Closed 9 years ago.
In pascal, i want to randomly organize an array. Therefore the majority of the time the array should be organized differently.
Consider this array setup
const
ARRAY_ELEMENTS = 3;
SetLength(iIndex, ARRAY_ELEMENTS);
for i := Low(iIndex) to High(iIndex) do
begin
case i of
0: iIndex[i] := 0;
1: iIndex[i] := 1;
2: iIndex[i] := 2;
end;
end;
How is it possible for the iIndex[] that contains the value 0 not to always be in the first element of the array and for the iIndex[] that contains value 2 not to be always the last value of the array but generate the order of the array randomly so that the order of the array is not always the same on initialization?

Answer 1

This code permutes an integer array, but I'm not sure if it is optimal (probably isn't).
type
TDynIntegerArray = array of integer;
procedure PermuteArray(A: TDynIntegerArray);
var
B: TDynIntegerArray;
Z: TDynIntegerArray;
π: TDynIntegerArray;
i: Integer;
j: Integer;
k: Integer;
begin
B := Copy(A);
SetLength(Z, Length(A));
SetLength(π, Length(A));
for i := 0 to High(Z) do
Z[i] := i;
for i := 0 to High(π) do
begin
π[i] := RandomFrom(Z);
for j := 0 to High(Z) do
begin
if Z[j] = π[i] then
begin
for k := j to High(Z) - 1 do
Z[k] := Z[k+1];
SetLength(Z, length(Z) - 1);
break;
end;
end;
end;
for i := 0 to High(A) do
A[i] := B[π[i]];
end;
A much faster, but less cool approach is simply to randomly swap the items, one pair at a time:
procedure FastPermuteArray(A: TDynIntegerArray);
procedure Swap(n, m: integer);
var
tmp: integer;
begin
tmp := A[n];
A[n] := A[m];
A[m] := tmp;
end;
var
i: Integer;
begin
for i := High(A) downto 1 do
Swap(i, RandomRange(0, i));
end;

Answer 2

Try something like this:
uses
System.Generics.Collections;
const
ARRAY_ELEMENTS = 3;
var
iArray: array of Integer;
iIndex: TList<Integer>;
I, j: Integer;
begin
Randomize;
SetLength(iArray, ARRAY_ELEMENTS);
iIndex := TList<Integer>.Create;
try
iIndex.Count := ARRAY_ELEMENTS;
for i := 0 to Pred(ARRAY_ELEMENTS) do
iIndex[i] := i;
for i := Low(iArray) to High(iArray) do
begin
j := Random(iIndex.Count);
iArray[iIndex[j]] := i;
iIndex.Delete(j);
end;
finally
iIndex.Free;
end;
end;
If you don't have TList<T> available in your version of Delphi, you can use a plain TList instead:
uses
Classes;
const
ARRAY_ELEMENTS = 3;
var
iArray: array of Integer;
iIndex: TList;
I, j: Integer;
begin
Randomize;
SetLength(iArray, ARRAY_ELEMENTS);
iIndex := TList.Create;
try
iIndex.Count := ARRAY_ELEMENTS;
for i := 0 to Pred(ARRAY_ELEMENTS) do
iIndex[i] := Pointer(I);
for i := Low(iArray) to High(iArray) do
begin
j := Random(iIndex.Count);
iArray[Integer(iIndex[j])] := i;
iIndex.Delete(j);
end;
finally
iIndex.Free;
end;
end;