Selection in C

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• Selection in C

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If Statement

• One way decision
• Referred to as Open Branch in the book.
• Syntax Flowchart
• if ( condition)
• statement(s)

Condition?
true
Statement(s)
false
Rest of the program
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If / else statement

• 2-way decision
• Syntax Flowchart
• if (condition)
• statement(s)
• else
• statement(s)

Condition?
false
true
Statement(s)
Statement(s)
Rest of the program
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Exercise Write a C program to..

• Ask the user to enter his/her age.
• Check the age, if it is gt 21, display You may
  drink., otherwise, display Do not drink
• regardless of the age, Display Do not Drink and
  Drive.

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Explain the solution, using flowchart

• Flowchart

Start
Declare variable age
Get users age
Age gt 21?
true
false
You may drink
May not drink
Dont drink drive
end
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Explain the solution using pseudo code

• Pseudo code
• Declare variable to store age.
• Prompt the user to enter age,
• Read the entered value into variable
• If age is gt 21 then
• Display you may drink
• Otherwise
• Display you may not drink
• 5. Display Do not drink and drive

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Translate the logic into C code
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Nested If Statement

• An if statement can be nested in the if part or
  the else part of the if statement.
• if (condition)
• if (condition)
• .
• else
• else
• if (condition)
• How can you tell where an if statement begins or
  ends?

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• Rule else binds with the closest if
• Ex1 Assume that int a 5, b 7, c 10
• if (a lt b)
• if (c lt a b)
• printf( " First place \n)
• else
• printf( "2nd place \n)
• else
• printf (3rd place \n)
• Output

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Dangling else

• Determine which if the else binds with?
• Ex2 Assume a 12, b 2, c 10
• if (a lt b)
• if (c lt a b)
• printf( " First place \n)
• else
• printf( "2nd place \n)
• Output

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Ex What is the outcome?

• 
  
  
  
  
  float score 75
• float gpa 2.5
• if (score gt 80)
  
• if (gpa gt 2.0)
• printf (xxx)
• else
• printf( )

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If / else if statement

• The else part of an if statement can be another
  if statement.
• if (condition)
• else
• if (condition)
• else
• if (condition)
• .
• else

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Improved style ? more clarity

• Such nested if / else can be written as
• if (condition)
• else if (condition)
• else if (condition)
• .
• else

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Example

• Examine the students test score and assign a
  letter grade
• int score
• char letter
• if (score gt 90)
• letter A
• printf( Great job! \n)
• else if (score gt 80)
• letter B
• else
• if (score gt 70)

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Rewrite the code with improved style

• if (score gt 90)
• letter A
• printf( Great job! \n)
• else if (score gt 80)
• letter B
• else if (score gt 70)
• letter C
• else if (score gt 60)
• letter D
• else
• letter F
• Printf( Your letter grade is c, letter)

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More complex conditions

• You may combine 2 or more conditions using
  logical operators
• Logical Operators
• Logical and

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Application

• Check the range of the data
• Ex A healthy heart rate is between 60 and
  85. Check the users heart rate, and inform the
  user whether s/he is healthy or not
• int rate
• printf ( Enter your heart rate )
• Scanf(d, rate)
• if ((rate gt 60) (rate lt 85) )
• else

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Logical or
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Applications

• 1) Checking for invalid rage
• Ex1 Check the range of the test score. If it is
  outside the range of 0 .. 100 end the program
• int test
• test 77
• if ( (test lt 0) (test gt 100) )
• printf ( Invalid test score..)
• printf ( Program will end now.)
• return 1

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Cont

• Ex2 Assume that a company is considering a
  promotion for employees older than 60, or those
  who are paid under 20,000. Assume variables
  salary and age are declared with some values
  stored in them.
• If ( (salary lt 20000) (age gt 60) )
• printf ( Give promotion..)
• else
• printf ( Does not qualify!)

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Cont

• Logical not !
• Operates on one condition.
• It negates the value of the condition.
• Ex1 ! (120 lt 100) ?
• Ex2 int veteran 1
• !veteran ?
• Notice
• ! (a lt b) is the same as (a gt b)
• ! (a b) is the same as (a ! b)

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Precedence Rule

• (..)
• Unary minus, !
• /
• -
• lt, lt, gt, gt
• , !
• assignment operator

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Evaluate the following expressions

• Assume int a 5, b 10, c 15, flag 1
• a lt b b lt c
• flag b gt 10
• !(b lt 12) (a 2 0)
• !flag c gt a
• int answer
• answer (a gt b) (c gt b)
• Printf (d,answer)
• C gt a b