290121 Computer Programming

1
290121 Computer Programming

• 5. ???????????????????????????
• CONDITIONS

?. ????????? ????????? ????????????????
???????????????? ????????
2
??????????

• if, if-else
• comparison
• combining
• DeMorgans Laws
• switch

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Conditional Execution

• The power of computers lies to a large extent
  with conditionals.
• A conditional statement allows the computer to
  choose a different execution path depending on
  the value of a variable or expression, e.g.
• Example
• if the withdrawal is more than the bank balance,
  print an error
• if today is my birthday, add one to my age
• if my grade is greater than 3.5, go to party
• if x is bigger than y then store x in z otherwise
  story y in z

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Conditional ( if ) Statement
????????????????????????? condition ????????
if (condition) statement

• if (x lt 100) xx1
• if (withdraw gt balance) print_error()
• if (temperature gt 98.6) printf (You have a
  fever.\n)
• if (month birthmonth day birthday)
  my_agemy_age1

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Condition Flow Chart
if (x lt 100) x x1 else y y1
yes
x lt 100 ?
x x1
no
y y1
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Condition Expressions

• ???????? logical ???? Boolean expressions
• ????????????????, ????????, arithmetic
  expressions ??? relational operators ???? lt ,
  lt, gt, gt, , !
• ?????????????????????????
• air_temperature gt 0.0
• 98.6 lt body_temperature
• marital_status M
• divisor ! 0

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Boolean Operators in Conditional Expressions

• ???????????? Boolean
• !
• and or not

???????? temp gt 90.1 humidity gt 50.1 !
(salary lt 30000 work_year lt 4)
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Value of conditional expressions

• ?????????????????????????? TRUE ???? False
• ?????? C ????????? boolean ??????????????????
  (integer)
• FALSE ??????? 0
• TRUE ??????????????????? 0
• ?????????? 1
• 1 ???????????????? relational operator
  ( lt, lt,gt,gt,,! ) ???????????????????? true

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Formatting if statements
if (condition) statement else
statement
if (condition) statement
if (condition) statement1
statement2 else statement1
statement2
if (condition) statement1 statement2
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if statements

• if (condition)
• statement

??? condition ???????? (TRUE) ????????????????????
???????? if (????? if-statement)
?????????????????????????????? if
In pseudocode if denominator is not equal to
zero result is numerator /
denominator
In C if (denom ! 0)
result num / denom
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Flow of Control
temperature 98.1 if (temperature lt 98.6)
printf (You have a fever) pulse 80
temperature 99.7 if (temperature lt 98.6)
printf (You have a fever) pulse 80
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Multiple actions

• ????????????????????????????????????????????????
• ??? compound statement
• ????????????????????? ???????? block

statement1 statement2 ...
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Multiple actions

• ???????? ?????? compound statement

if (temperature gt 98.6) printf ( You have a
fever. \n) aspirin 2
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if-else

• if (balance gt withdrawal)
• balance balance-withdrawal
• printf (Withdraw Complete.\n)
• else
• printf (Insufficient Fund.\n)

???????????????????????
???????????????????????
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if-else Control Flow
yes
no
balance gt withdraw ?
balancebalance withdrawal printf (Withdraw
Complete.\n)
printf (Insufficient Funds.\n)
/ arrive here whether condition is TRUE of FALSE
/
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Flow of Control
bal 25 withdrawal 40 if (bal gt
withdrawal) bal bal withdrawal printf
(Withdraw Complete.\n) else printf
(Insufficient Fund.\n) eject_card()
bal 25 withdrawal 20 if (bal gt
withdrawal) bal bal withdrawal printf
(Withdraw Complete.\n) else printf
(Insufficient Fund.\n) eject_card()
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Nested if

• define BILL_SIZE 20
• if (balance gt withdrawal)
• balance balance withdraw
• else
• if (balance gt BILL_SIZE)
• printf (Try a smaller amount. \n)
• else
• printf (Go away! \n)

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Nested if

• if (x 5)
• if (y 5)
• printf (Both are 5. \n)
• else
• printf (x is 5, but y is not.\n)
• else
• if (y 5)
• printf (y is 5, but x is not.\n)
• else
• printf (Neither is 5. \n)

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Tax Example
Tax 0 18 22 28 31

• Income
• lt15,000
• 15,000 - lt30,000
• 30,000 - lt50,000
• 50,000 - lt100,000
• gt100,000

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Simple Solution

• if (income lt 15000)
• printf(No tax.)
• if (income gt 15000 income lt 30000)
• printf(18 tax.)
• if (income gt 30000 income lt 50000)
• printf(22 tax.)
• if (income gt 50000 income lt 100000)
• printf(28 tax.)
• if (income gt 100000)
• printf(31 tax.)

Only one will be true
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Cascaded if

• if (income lt 15000)
• printf(No tax.)
• else
• if (income lt 30000)
• printf(18 tax.)
• else
• if (income lt 50000)
• printf(22 tax.)
• else
• if (income lt 100000)
• printf(28 tax.)
• else
• printf(31 tax.)

if (income lt 15000) printf(No tax.) else if
(income lt 30000) printf(18 tax.) else
if (income lt 50000) printf(22 tax.)
else if (income lt 100000) printf(28
tax.) else printf(31
tax.)
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The First Character

• / read 3 character print the smallest /
• printf (Enter 3 chars )
• scanf (c c c, c1, c2, c3)
• first c1
• if (c2ltfirst)
• first c2
• if (c3 lt first)
• first c3
• printf (Alphabetically,
• the first is c, first)

c1 c2 c3 first h a t
? h a t h
(true) h a t a
(false) (printf a)
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Sort 2 Characters

• Input two characters
• Rearrange them in sorted order
• Output them in sorted order

Example Input ra Output ar Input
nt Output nt
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Pseudocode

• Input c1,c2
• if c2 comes before c1 in alphabet
• swap c1 and c2
• Output c1,c2

swap
c1
c2
Input c1,c2 if c2 comes before c1 in
alphabet Save c1 in temporary Assign c2 to
c1 Assign temporary to c2 Output c1,c2
2
c1
c2
1
3
temp
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Program

• / sort two characters and print in sorted order
  /
• char c1,c2,temp
• printf(Enter 2 chars )
• scanf(c c,c1,c2)
• if (c2 ltc1) / swap if out of order /
• temp c1
• c1c2
• c2temp
• printf (In alphabetical order, they are
  cc,c1,c2)

c1 c2 temp d a ?
TRUE d a d a a d a d d printf
ad
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Complex Conditionals

• AND (), OR (), NOT (!)
• Example

if (age lt 25) if (sex M)
insurance_rate insurance_rate 2
if (age lt 25) (sex M)
insurance_rate insurance_rate 2
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if (age lt 25) (sex M)
insurance_rate insurance_rate 2
int high_risk ... high_risk ( age lt 25 sex
M) if (high_risk) insurance_rate
insurance_rate 2
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Truth Tables for ,

• P Q PQ PQ
• T T T T
• T F F T
• F T F T
• F F F F

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Not (!)

• int high_risk
• ...
• high_risk (age lt25 sex M)
• if (high_risk)
• printf(Expensive rates.\n)
• if (!high_risk)
• printf(Cheap rates.\n)

P !P T F F T
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DeMorgans Laws

• if ( ! (age lt 25 sex M))
• printf (Cheap rates.\n)

is equivalent to
if (age gt 25 sex ! M)) printf (Cheap
rates.\n)
More generally ! (P Q) is equivalent to (!P
!Q) !(P Q) is equivalent to (!P !Q)
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????????????????????

• if (xgt10) /no action or null
  statement/
• printf(xgt10) /Always done (why?)/

Recall that any non-zero integer value is
true if (x) printf(x is nonzero)
/works,but bad style/
if (x 10) /should be , but its not a
syntax error/ printf(xgt10)
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• No if (0 lt x lt 10)
• printf( x is between 0 and 10. \n)
• Yes if (0lt x x lt10)
• printf(x is between 0 and 10. \n)

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switch

• switch (control expression)
• case constant1
• statement1
• break
• case constant2
• statement2
• break
• .
• .
• .
• default
• statement

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Longwinded if

• / How many days in a month? /
• if (month 1) days 31
• else if (month 2) days 28
• else if (month 3) days 31
• else if (month 4) days 30

/Jan/
/Feb/
/Mar/
/Apr/
/need 12 of these/
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Clearer Style

• if (month 9 month 4 month 6
  month 11)
• days 30
• else if (month 2)
• days 28
• else
• days 31

/Sep,Apr/
/Jun,Nov/
/All of the rest/
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Clearest switch

• / How many days in a month? /
• switch (month)
• case 2
• days 28 break
• case 9
• case 4
• case 6
• case 11
• days 30 break
• default
• days 31
• printf( There are d days in that month.\n,
  days)

/Feb/
/Sep/
/April/
/June/
/Nov/
/All of the rest/
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switch Flow of control
month 6 switch (month) case 2 days 28
break case 9 case 4 case 6 case
11 days 30 break default days
31 printf( There are d days in that
month.\n, days)
/Feb/
/Sep/
/April/
/June/
/Nov/
/All of the rest/
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????????????????????? switch
month 6 switch (month) case 2 days 28
case 9 case 4 case 6 case 11 days
30 default days 31 printf( There are
d days in that month.\n, days)
break missing where?
/Feb/
/Sep/
/April/
/June/
/Nov/
/All of the rest/
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switch on char

• char marital_status
• printf(Enter marital status (M,S) )
• scanf(c, marital_status)
• switch (marital_status)
• case m
• case M
• printf(Married\n) break
• case s
• case S
• printf(Single\n) break
• default
• printf(Sorry, I dont recognize that
  code.\n)

int or char expression