CPS 1 - Spring, 1997 - Ramm 1/22/97 #3

Chapter 1. An Intro to Programming: Coding Decision Trees

• Let's Enter, Compile, & Run Program:

  program echo;

  var

  msg1:string;

  begin

  readln(msg1);

  writeln(msg1);

  readln;

  end.

  • Note program written as though talking to the computer
  • We ARE trying to tell it what to do!

• How to modify to make it produce in triplicate?

• What is minimal program?
  
  

• Program with multiple variables
  snoop.pas
  • Two variables
  • Note reuse of city
  • Almost as if we had three variables

• Review Syntax/Punctuation/Style
  • semicolons
  • indenting

• Back to Decision Trees
  
  

  Can't get car's engine to run; what is the problem?

  was car just running and quit now?	yes	is gas gauge on empty?	yes	get gas
  			no	call tow truck
  	no	does engine crank?	yes	check gas
  			no	check battery

  
  
  

• IF-THEN-ELSE statement
  
  

  IF <logical expression> THEN

  <Pascal code for "true" situation>

  ELSE

  <Pascal code for "false" situation>

  
  
  

• Example (part of decision tree):

  IF response = 'yes' THEN

  BEGIN

  writeln('suspect gas');

  END

  ELSE

  BEGIN

  writeln('suspect battery');

  END;

  
  
  • Where did response come from?
  • Why BEGIN ... END ?
  • Note indenting

• Compound Statement
  
  

  BEGIN

  <Pascal Statements>;

  ...

  <Pascal Statements>;

  END

  
  
  • Use wherever statement is allowed
  
  cara.pas

• More on Syntax
  • Semicolons at the end of complete statements.
  • BEGIN and END are not statements.

• Add second level to decision tree
  carb.pas

• Limitations: 'yes' vs 'YES' vs 'Y' vs 'y'

• More complicated examples: Using more than one variable.

• IF-THEN: can use for simplest cases (No ELSE)

• IF-THEN: can use for multi-way decision

  readln(color);

  if color = 'red' then

  writeln('Keep going.');

  if color = 'yellow' then

  writeln('You need to go faster!');

  if color = 'red' then

  writeln('I guess you should stop.');