gistfile1.pas

var
	i : integer;
	inf, post : string;
	stack : array[1..255] of char;
	top : integer;

procedure die(error: string);
begin
	writeln(error);
	halt(1);
end;

procedure push(ch : char);
begin
	inc(top);
	stack[top] := ch;
end;

procedure pop();
begin
	post := post + stack[top];
	dec(top);
end;

function prio(ch : char) : integer;
begin
	case ch of
	'*', '/': prio := 1;
	'+', '-': prio := 0;
	'(': prio := -1; { Not an operator, but can be found on stack }
	else die('Unknown operator: ' + ch);
	end;
end;

begin
	write('Enter expression in infex form: ');
	readln(inf);
	for i := 1 to length(inf) do
	begin
		case inf[i] of
		'0'..'9': post := post + inf[i];
		'+', '-', '*', '/':
		begin
			while (top > 0) and (prio(inf[i]) <= prio(stack[top])) do
				pop();
			push(inf[i]);
		end;
		'(': push(inf[i]);
		')':
		begin
			while (top > 0) and (stack[top] <> '(') do
				pop();
			if (top > 0) then dec(top)
			else die('Unbalanced parentheses');
		end;
		else die('Unexpected character: ' + inf[i]);
		end;
	end;
	while top > 0 do
		case stack[top] of
		'+', '-', '*', '/': pop();
		else die('Unbalanced parentheses');
		end;
	writeln('Postfix form: ', post);
end.

var
	i : integer;
	inf, post : string;
	stack : array[1..255] of char;
	top : integer;

procedure die(error: string);
begin
	writeln(error);
	halt(1);
end;

procedure push(ch : char);
begin
	inc(top);
	stack[top] := ch;
end;

procedure pop();
begin
	post := post + stack[top];
	dec(top);
end;

function prio(ch : char) : integer;
begin
	case ch of
	'*', '/': prio := 1;
	'+', '-': prio := 0;
	'(': prio := -1; { Not an operator, but can be found on stack }
	else die('Unknown operator: ' + ch);
	end;
end;

begin
	write('Enter expression in infex form: ');
	readln(inf);
	for i := 1 to length(inf) do
	begin
		case inf[i] of
		'0'..'9': post := post + inf[i];
		'+', '-', '*', '/':
		begin
			while (top > 0) and (prio(inf[i]) <= prio(stack[top])) do
				pop();
			push(inf[i]);
		end;
		'(': push(inf[i]);
		')':
		begin
			while (top > 0) and (stack[top] <> '(') do
				pop();
			if (top > 0) then dec(top)
			else die('Unbalanced parentheses');
		end;
		else die('Unexpected character: ' + inf[i]);
		end;
	end;
	while top > 0 do
		case stack[top] of
		'+', '-', '*', '/': pop();
		else die('Unbalanced parentheses');
		end;
	writeln('Postfix form: ', post);
end.