• Syntax of the if-statement :

  if ( boolean expression ) one-statement // Then Body if ( boolean expression ) { // Then Body more-than-one-statement }

• Meaning of the if-statement :
  If the boolean expression is true , the statement in the then body is executed .

  Otherwise the statement in the then body is SKIPPED.

• Syntax of the if-else-statement :

  if ( boolean expression ) one-statement // Then Body else one-statement // Else Body if ( boolean expression ) { // Then Body more-than-one-statement } else { // Else Body more-than-one-statement }

• Meaning of the if-else-statement :
  If the boolean expression is true , the statement in the THEN body is executed .

  Otherwise the statement in the ELSE body is executed.

• A boolean expression is an expression that evaluates to true or false

  Examples:

  x > y x*x + y*y <= z*z

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• A boolean expression comprise of
  • relational operators and
  • boolean (logic) operators

Operate (compares) on numerical values and returns a Boolean (logical) result value

Relational Operator	Description
>	Greater than
>=	Greater than or equal
<	Less than
<=	Less than or equal
==	Equal
!=	Not equal

Operate (compares) on Boolean (logical) values and returns a new Boolean (logical) result value

Logical Operator	Description
&&	Logical AND
||	Logical OR
!	Logical NOT

• If-statement used to find the absolute value of x

  if ( x < 0 ) x = -x;

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• Boolean expression to test if the value of a variable x is in the interval [10,20):

  if ( (x >= 10 && x < 20) ) ... else ...

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• Boolean expression to test if the value of a variable x is outside the interval [10,20):

  if ( !(x >= 10 && x < 20) ) ... else ... which is equivalent to: if ( x < 10 || x >= 20 ) ... else ...

• Conditional statements (if and if-else statements) can be nested

• Example:

  if ( x < 10 ) { // Fact: x < 10 if ( x > 2 ) { ... executed when x < 10 and x > 2 } else { ... executed when (x < 10 and) x <= 2 } } else { // Fact: x >= 10 if ( x > 17 ) { ... executed when (x >= 10 and) x > 17 } else { ... executed when x < 10 and x <= 17 } }

• A commonly used nested if-else statement is tail nesting

• Example:

  if ( avg >= 90 ) { printf("Grade is A"); // x >= 90 } else { // Fact: x < 90 if ( avg >= 80 ) { printf("Grade is B"); // Fact: x in [80,90) } else { // Fact: x < 80 if ( avg >= 70 ) { printf("Grade is C"); } else { if ( avg >= 60 ) { printf("Grade is D"); } else { printf("Grade is F"); } } } }

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• The tail-nested if-else statements are commonly written as follows:

  if ( avg >= 90 ) { printf("Grade is A"); } else if ( avg >= 80 ) { printf("Grade is B"); } else if ( avg >= 70 ) { printf("Grade is C"); } else if ( avg >= 60 ) { printf("Grade is D"); } else { printf("Grade is F"); }

• A multi-way selection statement is used to execute at most ONE of the choices of a set of statements presented.

• Syntax of the multi-way select statement:

  switch ( EXPRESSION ) { case CONSTANT1: one or more statements; break; case CONSTANT2: one or more statements; break; ... [default: one or more statements;] }

• Example:

  switch ( Avg ) { case 90: case 91: ... case 100: printf("Grade is A"); break; case 80: case 81: ... case 89: printf("Grade is B"); break; case 70: case 71: ... case 79: printf("Grade is C"); break; case 60: case 61: ... case 69: printf("Grade is D"); break; default: printf("Grade is F"); }