CPS 1 - Spring, 1997 - Ramm 1/27/97 #5

Chapter 1. An Intro to Programming: Coding Decision Trees

• Turbo Pascal Summary (Syntax Description)
  • need a precise notation
  • more formal method
  1. <identifier> -> //a sequence of letters and/or digits that begin with a letter//

  2. <string expression> -> <identifier>

  3. <string expression> -> '//any string of printable chars//'

  4. <statement> -> writeln(<string expression>)

  5. <statement> -> readln(<identifier>)

  6. <statement> -> readln

  7. <compound statement>	->	begin
     		//a sequence of <statements>s
     		each followed by a semicolon//
     		end

• To Use: substitute, multiple times as necessary

• Example with compound statement: Want:

  begin

  readln(msg);

  writeln(msg);

  end

  #7	<compound statement>	->	begin
  			<statement>;
  			<statement>;
  			end
  	<compound statement>	->	begin
  #5			readln(<identifier>);
  			<statement>;
  			end
  	<compound statement>	->	begin
  #1			readln(msg);
  			<statement>;
  			end
  	<compound statement>	->	begin
  			readln(msg);
  #4			writeln(<string expression>) ;
  			end
  	<compound statement>	->	begin
  			readln(msg);
  #2			writeln(<identifier>) ;
  			end
  	<compound statement>	->	begin
  			readln(msg);
  #1			writeln(msg);
  			end

• Example involving compound statement
  • Want:

  begin

  writeln('Your name?'));

  readln(name);

  end

  • Do, In Class, as if we were taking a quiz.
  • Allow Some Shortcuts
  • Help from class required
  
  
  

• What about IF-THEN-ELSE:

  8.	<statement>	->	if <boolean expression> then
  			<compound statement>
  			else
  			<compound statement>

• IF-THEN:

  9.	<statement>	->	if <boolean expression> then
  			<compound statement>

• Boolean Expression:

  10.	<boolean expression>	->	if <identifier> =
  			<string expression>

  
  
  

• What about a Complete Program?

  11.	<variable declaration>	->	//nothing//
  12.	<variable declaration>	->	var //list of//
  			<identifier>s : <type>;
  13.	<type>	->	string
  14.	<program>	->	program <identifier>;
  			<variable declarations>
  			<compound statement>.

  
  
  

• Can use these rules to derive the following program.

  program train;

  var

  gate:string;

  begin

  writeln('Is crossing gate up or down?');

  readln(gate);

  if gate = 'down' then

  begin

  writeln('Watch the train go by.');

  end

  else

  begin

  writeln('We beat the train this time!');

  end;

  readln;

  end.

  
  
  

• Will start ...

  #14	<program>	->	program <identifier>;
  			<variable declarations>
  			<compound statement>.
  #1	<program>	->	program train;
  			<variable declarations>
  			<compound statement>.
  	<program>	->	program train;
  #12			var
  			<identifier>:<type>;
  			<compound statement>.
  	<program>	->	program train;
  			var
  #1			gate: <type>;
  			<compound statement>.
  	<program>	->	program train;
  			var
  #13			gate: string;
  			<compound statement>.
  	<program>	->	program train;
  			var
  			gate: string;
  #7			begin
  			<statement>;
  			<statement>;
  			<statement>;
  			<statement>;
  			end.

• Look at book for complete examples

• Shows program is syntactically correct except for choice of identifiers. Doesn't help logic.

• There are other system for describing syntax

Chapter 2. Text Manipulation and Algorithm Design