Conditionals

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Conditionals
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Conditional Execution

• A conditional statement allows the computer
• to choose an execution path depending on the
  value of a variable or expression
• if the withdrawal is more than the bank balance,
  then print an error
• if today is my birthday, then add one to my age
• if my grade is greater than 3.5, then give A as
  final grade

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Conditional statements in C

• C supports several forms for conditionals
• if
• if-else
• if-elsif-elsif--else
• And more
• You as a programmer must decide which form to
  use.
• General semantics the conditional statement
  depends on a boolean condition, the guard if
  this condition is true the statement that depends
  on it will get executed

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Conditional Statement in C

• if (condition)
• statement
• The statement is executed if and only if the
  condition is true.
• if (x lt 100)
• x x 1
• if (withdrawalAmount gt balance)
• printf( NSF check.\n)
• if (temperature gt 98.6)
• printf(You have a fever.\n)

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Conditional statements in C

• if (condition)
• statement1
• else
• statement2
• Semantics if condition is true, statement1 gets
  executed, otherwise statement2 gets executed.
• if (value gt 70)
• passed passed 1
• else
• notPassed notPassed 1

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Conditional statements in C

• if (condition1)
• statement1
• else if (condition2)
• statement2
• else if (condition3)
• statement3
• else if (conditionK)
• statementK
• else
• statement

The first condition that is true (from top to
bottom) will make its statement execute. If all
conditions are false last statement executes.
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Conditional Example

• if (grade gt 90)
• countAs countsAs 1
• else if (grade gt 80)
• countBs countBs 1
• else if (grade gt 70)
• countCs countCs 1
• else if (grade gt 60)
• countDs countDs 1
• else
• countFs countFs 1

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Flow of execution

• x y 10 (s1)
• if ( x lt 100)
• x x 1 (s2)
• y y 10 (s3)
• S2 depend on the condition (X lt 10)
• S1 and S3 do not depend on the condition (x lt 10)

Executes s1
true
xlt100
executes s2
false
Does not execute s2 Executes s3
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Conditional expressions

• Also called "logical" or "Boolean" expressions
• Made up of variables, constants, arithmetic
  expressions, and the "relational operators"
• Math symbols lt , ,gt , , ,
• in C lt , lt, gt , gt , , !
• Conditional expression examples
• airTemperature gt 0.0
• 98.6 lt body_temperature
• maritalStatus M
• divisor ! 0

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Value of conditional expressions

• In C conditional expressions are evaluated to
  yield an integer.
• If the resulting integer value is 1, it is
  equivalent to TRUE.
• If the resulting integer value is 0 it is
  equivalent to FALSE.
• An arithmetic relational operator ( lt, lt, gt,
  gt, !) will yield 1 when the relation is true
  and 0 when false.
• Any integer expression can be used as a
  conditional expression
• If its value is not zero, it is true
• Otherwise it its false.
• Thus we can write
• if (value) equivalent to if (value !0)
• statement statement

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Formation of complex conditional expressions

• Boolean operators
• stands for the and
• stands for the or
• ! Stands for the negation operator Not
• Examples
• (x gt 0) (x lt10)
• (ylt 0) (y gt 10)
• !(x 0)
• ! ( (xgt0) (x lt10))

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Boolean Tables for , , !

• Be1 be2 be1 be1
• 1 1 1 1 1
  1
• 1 0 0 1 1
  0
• 0 0 1 0 1
  1
• 0 0 0 0 0
  0
• !0 yields 1
• !1 yields 0

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DeMorgans Laws

• ! ( be1 be2) (!be1) (!2be)
• ! ( be1 be2) (!be1) (!2be)

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Precedence rules of expressions with boolean
operators

• Boolean ! Has higher precedence than
• has higher precedence than
• , and are left associative.
• !x y equivalent to (!x) y
• y z u equivalent to (y z) u
• z1 z2 z3 equivalent to (z1 z2) z3
• z1 z2 z3 equivalent to (z1 z2) z3
• ! Has higher precedence than lt, gt, lt, gt
• , have lower precedence than lt, gt, lt, gt
• !x lt y equivalent to (!x) lt y
• X y lt z equivalent to x (y lt z)

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Warning

• Notice that when we write
• if (be)
• statement1
• statement2
• the boolean expression is only affecting
  statement1. Statement2 will always execute after
  the if is executed.
• What if we need to perform more than one
  statement based on a condition?

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Blocks

• Use a block to form a group of statements, which
  will groups together statements so that they are
  treated as a single statement
• statement1
• statement2
• .
• With this notion we can write more complex
  conditional statements.

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Example

• If (withdrawal lt balance)
• balance balance - withdrawal
• monthyCharge monthlyCharge 1.0
• Else
• monthlyPenalty monthlyPenalty 25.0
• To note
• A single statement ends with semicolon
• A block does not have semicolon after
• A block statement can have zero or more
  statements
• Its best to use block statements in
  conditionals, it helps you reduces the number of
  errors

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Pitfalls with ifs

• Using instead of
• It will not yield a compilation error
• And accompaning statement might or might not
  execute.
• In any case, it is a BUG hard to find!!!!!!!!!!!
• if ( value average)
• statement

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Pitfalls with ifs

• Althought the following statement will not give a
  syntactic error it is incorrect
• if ( low lt value lt high)
• value value 2
• You should write it as
• if (low lt value value lt high)
• value value 2

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Pitfalls with ifs

• the operator is different from
• the operator is different from
• and are not boolean operators!!!!!
• if used by mistake, you will get no syntax
  errors, but program will most likely produce
  incorrect results.

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Pitfalls with ifs

• When using doubles do not compare them for
  equality () or inequality (!)
• When using either of these two operators with
  doubles you might get incorrect results due to
  accuracy of doubles!
• 30.0 (1.0/3.0) 10.0
• May yield false!
• 1 ½. 1/3. ¼. .. 1/n. Will most likely
  not be equal to 1/n. 1/(n.-1) ½. 1
  for a large value of n.