program ReverseVectorChars;
// Compiler directives: modern Object Pascal mode and long/dynamic strings enabled
{$mode objfpc}{$H+}
// Define a dynamic character array type (acts as a vector of characters)
type
TCharVector = array of Char;
{
Checks if a character is alphabetic using Pascal's efficient set membership operator.
}
function IsAlpha(C: Char): Boolean;
begin
Result := C in ['a'..'z', 'A'..'Z'];
end;
{
Reverses only the alphabetic characters in a character vector in-place,
leaving special characters, spaces, and digits in their original positions.
}
procedure ReverseOnlyLetters(var Vec: TCharVector);
var
Left, Right: Integer;
Temp: Char;
begin
if Length(Vec) = 0 then
Exit; // Guard against empty vectors
Left := Low(Vec); // 0-indexed for dynamic arrays
Right := High(Vec);
// Use a two-pointer approach moving inward from both ends
while Left < Right do
begin
// Advance left pointer until it points to an alphabetic character
while (Left < Right) and not IsAlpha(Vec[Left]) do
Inc(Left);
// Move right pointer backward until it points to an alphabetic character
while (Left < Right) and not IsAlpha(Vec[Right]) do
Dec(Right);
// Swap the valid letters found at both pointers
if Left < Right then
begin
Temp := Vec[Left];
Vec[Left] := Vec[Right];
Vec[Right] := Temp;
Inc(Left);
Dec(Right);
end;
end;
end;
{ Helper function to convert a standard string into a TCharVector }
function StringToCharVector(const S: string): TCharVector;
var
I: Integer;
begin
SetLength(Result, Length(S));
for I := 1 to Length(S) do
Result[I - 1] := S[I]; // Free Pascal strings are 1-indexed, dynamic arrays are 0-indexed
end;
{ Helper function to convert a TCharVector back into a standard string }
function CharVectorToString(const Vec: TCharVector): string;
var
I: Integer;
begin
SetLength(Result, Length(Vec));
for I := 0 to High(Vec) do
Result[I + 1] := Vec[I];
end;
var
InputStr: string;
CharVec: TCharVector;
begin
// Input string specified
InputStr := 'a#b$%c&*(def!';
Writeln('Original: ', InputStr);
// Convert the string into a vector of characters (array of Char)
CharVec := StringToCharVector(InputStr);
// Apply the reversal function in-place
ReverseOnlyLetters(CharVec);
// Convert back to string for neat display output
Writeln('Reversed: ', CharVectorToString(CharVec));
end.
(*
run:
Original: a#b$%c&*(def!
Reversed: f#e$%d&*(cba!
*)
