Friday, December 15, 2006

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Friday, December 15, 2006

• When you come to a fork in the road, take it.
• - Yogi Berra.

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• Class on Saturday (Tomorrow)

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• How do we define a block in C?

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• A block in C is created by placing a sequence
  of statements between curly braces.

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Good programming practices

• Meaningful Variable names
• Code indentation
• Comments
• There will be marks for style in
  homeworks/assignments.

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Terminology

• Syntax errors
• reported by the compiler
• Linker errors
• reported by the linker
• Execution/Run-time errors
• reported by the operating system
• Logic errors
• not reported

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• Relational Operators
• gt
• gt
• lt
• lt
• !
• Logical Operators
• !

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Implementing the Branch

• if-else statement is used in C to perform a
  branch
• if (hours gt 40)
• gross_pay rate 40 1.5 rate
  (hours-40)
• else
• gross_pay rate hours

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Boolean expressions
Logical AND is represented by Example.
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Boolean expressions

• Pitfall Do not use strings of inequalities. They
  may be legal C code but will not do what you
  expect them to do.
• int x2, y9, z3
• //Is y is between x and z?
• coutltlt(x lt y lt z)ltltendl //WRONG!

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Boolean expressions

• Pitfall Do not use strings of inequalities. They
  may be legal C code but will not do what you
  (for now) expect them to do.
• int x2, y9, z3
• //Is y is between x and z?
• coutltlt(x lt y lt z)ltltendl //WRONG!
• Instead, you have to write
• coutltlt ((x lt y) (y lt z)) ltltendl

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Boolean expressions

• Given x3 and y4. What's the value of
• !(( (x lt y) (x gt y-2) ) ( x2 lt y ) )

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Boolean expressions

• Given x3 and y4. What's the value of
• coutltlt !(( (x lt y) (x gt y-2) ) ( x2 lt y )
  )
• prints 1

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Boolean expressions

• What's wrong with these?
• (y gt x) (z gt x)
• (z !gt y)
• (a lt x lt b)
• !(x 3)

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Boolean expressions
I want to put some balls in a box if the number
of balls in not 3 or 5. How do we write the
statement in C?
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If selective execution
Example Bank withdrawal checker int
balance // some other code here int
withdrawal cout ltlt "Please enter the withdrawal
amount " cin gtgt withdrawal if(withdrawal lt
balance) cout ltlt "Here is your money" ltlt
endl else cout ltlt "Sorry, insufficient
balance" ltltendl
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If selective execution

• What is wrong here?
• int age
• cout ltlt "Please enter your age "
• cin gtgt age
• if (age lt 0)
• cout ltlt "Wow, you are really young!"
• cout ltlt "Your age is negative."
• cout ltlt endl
• cout ltlt "Your age is " ltlt age ltlt endl

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If selective execution

• Pitfall Indentation by itself is not enough.
• int age
• cout ltlt "Please enter your age "
• cin gtgt age
• if (age lt 0)
• cout ltlt "Wow, you are really young!"
• cout ltlt "Your age is negative."
• cout ltlt endl
• cout ltlt "Your age is " ltlt age ltlt endl

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If selective execution

• What's wrong with this?
• if (x gt y)
• x y
• cout ltlt x

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If selective execution
Pitfall There is no semi-colon after the
if-condition!
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Stringing if-then-elses together

• If-then-else statements can be strung together.
• Exactly one of the alternatives will be executed.

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Stringing if-then-elses together
if(num_credits lt 0) cout ltlt "Come on,
get real" ltlt endl else if (num_credits lt
12) cout ltlt "Part-time student" ltlt
endl else if (num_credits lt 18)
cout ltlt "Full-time student" ltlt endl else
cout ltlt "Glutton for punishment" ltlt
endl
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Another way of writing this?
if(num_credits lt 0) cout ltlt "Come on,
get real" ltlt endl else if (num_credits lt
12) cout ltlt "Part-time student" ltlt
endl else if (num_credits lt 18)
cout ltlt "Full-time student" ltlt endl else
cout ltlt "Glutton for punishment" ltlt
endl
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If selective execution

• int main ()
• int age
• cout ltlt "Please enter your age "
• cin gtgt age
• if(age lt 0)
• cout ltlt "Age must be gt 0\n"
• return 0
• cout ltlt "Your age is " ltlt age ltlt endl
• return 0

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While loop

• int main ()
• int age
• cout ltlt "Please enter your age "
• cin gtgt age
• while(age lt 0)
• cout ltlt "Age must be gt 0\n"
• cout ltlt "Please enter your age again "
• cin gtgt age
• cout ltlt "Your age is " ltlt age ltlt endl
• return 0

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Nested if statements

• The ltstatementsgt inside the braces can contain
  any valid C statements, including if
  statements!
• // some other code here
• char answer
• if (withdrawal gt balance)
• cout ltlt "Insufficient funds." ltlt endl
• cout ltlt "Do you want to see your balance?
  "
• cin gtgt answer
• if (answer 'y')
• coutltlt "Your balance is "ltltbalanceltlt
  endl
• else
• cout ltlt "Here is your money." ltlt endl
• cout ltlt "Good bye." ltlt endl

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Nested if statements
if (xgty) if (xgtz) statement1 if
(xgtp) statement2 else statement3 else statemen
t4 //bad style- no indentation (code with proper
indentation on next slide)
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Nested if statements
if (xgty) if (xgtz) statement1 if
(xgtp) statement2 else
statement3 else statement4 /else statement
always refers to nearest if statement that is
within same block as else and not already
associated with another else/
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Nested if statements // what is wrong here?
int main() int x10, y2, z12, p13 if
(xgty) if (xgtz) coutltlt1ltltendl//statement1
if (xgtp) coutltlt2ltltendl//statement2 els
e coutltlt3ltltendl//statement3 else
coutltlt4ltltendl//statement4 else
coutltlt5ltltendl//statement5 return 0
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Nested if statements // what is wrong here?
int main() int x10, y2, z12, p13 if
(xgty) if (xgtz) coutltlt1ltltendl//statement1
if (xgtp) coutltlt2ltltendl//statement2 els
e coutltlt3ltltendl//statement3 else
//Error coutltlt4ltltendl//statement4 else
coutltlt5ltltendl//statement5 return 0
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Nested if statements // what is output here?
int main() int x10, y2, z12, p13 if
(xgty) if (xgtz) coutltlt1ltltendl//statement1
if (xgtp) coutltlt2ltltendl//statement2 els
e coutltlt3ltltendl//statement3 els
e coutltlt4ltltendl//statement4 else
coutltlt5ltltendl//statement5 return 0
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• Output is 4