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Unit  4:    Selection  Statements  

Computer  Programming  I  

Mr.  Kindt  

Spring  2014  

George  Boole  

•  Born  1815,  London  to  low-­‐class  parents  

•  Elementary  school  teacher  from  ages  of  16-­‐33.  

•  Self-­‐taught  mathematician  •  Published  ‘The  

Mathematical  Analysis  of  Logic’  in  1848  

•  Became  Professor  of  Mathematics  the  next  year!  

•  Considered  the  forefather  of  formal  logic  

•  The  “Boolean  Expression”  is  a  data  type  named  after  him  that  led  to  extraordinary  things.  

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Relational  Operators  •  Familiar  arithmetical  operators,  in  Pascal.  •  These  will  always  produce  a  value  of  true  or  false.  •  Any  statement  with  these  operators  is  called  a  Boolean  

expression.  

Examples  of  Boolean  Expressions  

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Logical  Operators:  Connectives  

•  The  “Connectives”:  AND,  OR  •  You  may  put  these  reserved  words  between  any  two  relational  

expressions  to  return  a  true  or  false  value.  •  If  AND  is  used,  both  expressions  must  be  true  to  return  a  true.    

Otherwise,  it  will  return  false.  •  If  OR  is  used,  at  least  one  expression  must  be  true  to  return  a  

true.    If  both  are  false,  it  will  return  false.  

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(8 = 8) AND (7 < 9)TRUE

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(8 = 8) OR (7 < 9)TRUE

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(8 <> 8) AND (7 < 9)FALSE

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(8 <> 8) OR (7 < 9)TRUE

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(8 <> 8) AND (7 >= 9)FALSE

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(8 <> 8) OR (7 >= 9)FALSE

BOTH TRUE ONE TRUE, ONE FALSE BOTH FALSE

Logical  Operator:  Negation  

•  The  “Negation”:  NOT  

•  This  reserved  word,  placed  left  of  a  relational  

expression,  returns  the  opposite  true/false  value.  

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NOT (14 > 22)NOT (FALSE)

TRUE

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NOT (14 <= 22)NOT (TRUE)FALSE

NEGATING TRUE NEGATING FALSE A COMPLEX ONE!

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NOT (6 < (12 + 3) AND 12 <> (36 / 3)[ ]NOT (6 <15 AND 12 <>12[ ]

NOT FALSE[ ]TRUE

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Order  of  Operations  

•  AND  will  

precede  OR  

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NOT (3 < 5) OR (21 <>18) AND (−81 > 0)[ ]

NOT (TRUE) OR (TRUE) AND (FALSE)[ ]

NOT (TRUE) OR (FALSE)[ ]

NOT [TRUE]

FALSE

ORDER OF OPERATIONS

( )NOT* / MOD DIV AND+ − OR< <= > >= = <>

A  New  Problem  We  use  Boolean  expressions  in  programming  to  

evaluate  user  input  for  certain  conditions.      

Example:  You  are  an  accountant  calculating  how  much  tax  your  clients  owe  Uncle  Sam.    Your  program  will  ask  them  what  their  income  is,  but  their  tax  %  is  dependent  upon  how  much  they  earn!  

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The  IF…THEN  Statement  You  can  evaluate  the  user  input  for  

conditions  with  the  following:  

IF  (Boolean  expression)  

THEN    

 BEGIN  

   executable  statement;  

 END;  

writeln(‘Please enter your income.’); readln(income); IF (income < 8375) THEN BEGIN tax := 0.10*income; END;

writeln(‘Your tax is $’, tax:1:2);

Please enter your income. 5000 Your tax is $500.00

Proper  Habits  of  If-­‐Then  

•  When  writing  an  if-­‐then  statement,  follow  these  coding  standards  and  tips.  

IF (Boolean argument) THEN BEGIN

executable statement; executable statement; END;

1. Place the Boolean argument in parentheses. 2. Following THEN, use a BEGIN before your executable statements (commands) and an END when finished. This is known as “blocking”. 3. Capitalize IF, THEN, BEGIN, and END. 4. Indent BEGIN and END one tab, and all executable statements contained within them two tabs. 5. The semicolons within the “block” (between BEGIN and END) refer to each executable statement. The semicolon following the END terminates the IF-THEN command.

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AN  EXAMPLE  OF  A  PROPERLY  WRITTEN  IF-­‐THEN  STATEMENT  IF  (average  >  92)  THEN            BEGIN                      lettergrade:=‘A’;                      writeln(‘Congrats!’);                      writeln(‘You  got  an  ‘,  lettergrade);            END;  

AN  EXAMPLE  OF  A  POORLY  WRITTEN  IF-­‐THEN  STATEMENT    

If  average  >  92  then;  lettergrade:=‘A’;  writeln(‘Congrats!’);  writeln(‘You  got  an  ‘,  lettergrade);            End;  

IF…THEN  with  Connectives  

You  can  use  complex  Boolean  

expressions  with  AND,  OR,  

or  NOT  where  needed.  

IF (income >= 8375) AND (income < 34000) THEN ���

BEGIN tax := 837.50 + 0.15*(income – 8375); END;

writeln(‘Your tax is $’, tax:1:2);