String9°. Given an even integer N (> 0) and two characters C₁, C₂, output a string that is of length N, begins with C₁ and contains alternating characters C₁ and C₂.

Creating a template and acquaintance with the task

To create a template of the required task one should use PT4Load tool:

{$D+,I+,L+,Q+,R+,S+}
program String9;
uses PT4;

begin
  Task('String9');
end.

When the program is launched you will see the Programming Taskbook window:

Character and string data are enclosed by quotation marks. It allows to distinguish between numbers and their string representations (for example, 2 and '2' ). Also it gives opportunity to display leading and trailing blank characters of the string.

Initial data input

Before solving the task it is necessary to input initial data in the program:

var
  n: integer;
  c1, c2: char;
begin
  Task('String9');
  GetC(c1);
  GetC(c2);
  GetN(n);
end.

We have tried to input data in wrong order, therefore after the program running the status bar will contain the error message "Invalid type is used for an input data item."

To correct this error it is enough to change order of input statements:

  GetN(n);
  GetC(c1);
  GetC(c2);

Now data input is performed correctly, but the program does not output results. Therefore we shall see the following message: "Some data are not output.".

String processing and output

To add characters to a string we shall use "+" operator, to output the string we shall use the PutS procedure:

var
  n, i: integer;
  c1, c2: char;
  s: string;
begin
  Task('String9');
  GetN(n);
  GetC(c1);
  GetC(c2);
  s := '';
  for i := 1 to n do
    s := s + c1 + c2;
  PutS(s);
end.

When the program is running you will see the string terminated with a red asterisk. For example,

'a1a1a1a1a1*

It means that the length of the obtained string exceeds the length of a control string (i.e., the "correct" string). The status bar will contain the message "Wrong solution". To see the full contents of the obtained string you should move the mouse cursor to the panel of results. After 1-2 seconds the full text will be displayed as a hint.

Remark. The red asterisk can appear also if you output incorrect numeric data. For example, if the expected integer is in the range 1 to 99, then in case of output the result value 1000 it will be displayed as 1*.

Right solution, its testing and browsing results

The error in the previous program is due to incorrect amount of loop iterations. Indeed, two symbols are added to string at each iteration. So the string will contain 2n symbols instead of the required n symbols.

To correct the error you should reduce the amount of iterations as follows:

  for i := 1 to n div 2 do

After running the changed program we shall see the message "Right solution. The test 1 of 5", and after 5 test runnings we shall receive the message "The task is solved!".

Using the PT4Results tool we can browse information about all test runnings of our program (the letter "d" means that the task was solved in Delphi Pascal):

String9   d03/05 17:23 Acquaintance with the task.
String9   d03/05 17:25 Invalid type is used for an input data item.
String9   d03/05 17:27 Some data are not output.
String9   d03/05 17:30 Wrong solution.
String9   d03/05 17:34 The task is solved!

Last revised:
04.06.2011