CS 192

1
CS 192

• Lecture 8
• Winter 2003
• December 17-18, 2003
• Dr. Shafay Shamail

2
The if Statement

• Its a selection or decision statement
• if (test-condition) statement
• test-condition is an expression that evaluates to
  true or false
• If the condition is true, then the statement will
  execute otherwise not
• if (10 lt 11) cout ltlt hello ltlt endl
• cout ltlt I am after if
• Can have a block of statements instead of just
  one
• Pitfall if(marks 100) cout ltlt Congrats!
• Write if(100 marks). Now compiler catches
  single

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The if else Statement

• Choice between two alternatives
• Pseudocode
• if(ltexpressiongt)
• lttrue_statementsgt
• else
• ltfalse_statementsgt
• If ltexpressiongt is TRUE (non-zero), then
  lttrue_statementsgt are executed
• If ltexpressiongt is FALSE (zero), then
  ltfalse_statementsgt are executed

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The if else Statement

• Example ATM Withdrawal program
• include ltiostream.hgt
• int main()
• int balance 10000
• 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
• return 0

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Nested ifs

• An if statement that is the target of another if
  or an else
• An else refers to the nearest if statement within
  the same block that doesnt have an else
• if(i)
• if(j) statement1
• if(k) statement2 // this if
• else statement3 // is associated with //
  this else
• else
• statement4 // associated with?

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Nested ifs

• The ltstatementsgt inside the braces can contain
  any valid C statements, including other if-else
  statements!
• if (withdrawal gt balance)
• cout ltlt "Insufficient funds." ltlt endl
• cout ltlt "Do you want to see your
  balance?(y/n) "
• cin gtgt answer
• if (answer 'y')
• cout ltlt "Your balance is "ltltbalanceltlt endl
• else
• cout ltlt "Ok then. "
• else

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Nested ifs

• Another example
• include ltiostream.hgt
• int main()
• int a, b, c
• cout ltlt Enter three numbers a, b, c\n
• cin gtgt a gtgt b gtgt c
• if (ab)
• if(bc)
• cout ltlt a, b, c are equal\n
• else
• cout ltlt a and b are different\n
• return 0
• Let a2, bc3. Whats printed?

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The if-else-if Construction

• More than 2 choices. if else is itself a
  statement, so can follow an else
• if (ch A)
• agrade //choice 1
• else
• if (ch B) //choice 2
• bgrade
• else
• SoSo //choice 3
• Easier to read format
• if (ch A)
• agrade //choice 1
• else if (ch B)
• bgrade //choice 2
• else
• SoSo //choice 3

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The if-else-if Ladder

• Conditional expressions evaluated top-down The
  first true conditions statements evaluated rest
  of the ladder skipped
• int num_credits
• coutltlt"Please enter workload "
• cingtgtnum_credits
• 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

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The switch Statement

• If a large number of choices that all depend on
  one expressions value, better to use switch than
  the if-else-if construct
• switch (expression)
• case constant1
• statement(s)
• break
• case constant2
• statement(s)
• break
• default
• statement(s)
• break // not required
• expression must evaluate to an integer value
  (includes characters) constant must be integer
  (or character) constant

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The switch Statement

• int main()
• int x1
• switch (x)
• case 0
• coutltlt"zero \n"
• break
• case 1
• coutltltone \n"
• break
• default
• coutltltsorry! \n"
• break
• return 0
• char c M switch(c). What case will match
  now?

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The switch Statement

• int main()
• int x1
• switch (x)
• case 0
• coutltlt"zero \n"
• break
• case 1
• coutltltone \n"
• default
• coutltltsorry! \n
• return 0
• break necessary to exit from switch, otherwise
  all subsequent executed

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The switch Statement

• break and default are optional but important.
• No two cases can have same value
• Can have same statements for multiple cases
• int medal
• coutltlt"Enter your rank in class "
• cingtgtmedal
• switch (medal)
• case 1 case 2 case 3
• coutltlt"Wow! Medal winner \n"
• break
• default
• coutltlt"Better luck next time \n"

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The switch Statement

• int main()
• int i
• for(i0 ilt5 i)
• switch(i)
• case 0 cout ltlt "less than 1\n"
• case 1 cout ltlt "less than 2\n"
• case 2 cout ltlt "less than 3\n"
• case 3 cout ltlt "less than 4\n"
• case 4 cout ltlt "less than 5\n"
• cout ltlt '\n'
• return 0
• Output?