CS 111 - 2/28/17

If Statements - one form of Selection Statements

  We select whether a block of code is executed or not.


Syntax: of the if-then statement

    if    <condition>  :
        <bodyStatements>

If the <condition> is true, then execute the <bodyStatements>.
   If the <condtion> is false, skip the <bodyStatements>.

Most <conditions> use the relational operations:
    >  >=  <  <=  ==  !=


Example: Absolute Value

   if ( value1 < 0 ) :
      value1 = -value1  

Syntax: of the if-then-else statement

    if <condition>  :
        <bodyStatements1>
    else :
        <bodyStatements2>

If the <condition> is true, execute <bodyStatements1>
  Otherwise, execute <bodyStatement2>

Example: Determine the larger of two values

  val1 =    6
  val2 =   17

  if ( val1 > val2 ) :
     max = val1
  else :
     max = val2

==================================

when in the "main()" function, if we need to quit a program
  we can use the return statement:


    if ( x != y ) :
       print "Error message"
       return

    ....

When not in the "main()" function, if we need to quit a
   program, we can use the sys.exit() function.

   Normally a parameter is given such as:
     sys.exit(0)

   This parameter could be used "report" the kind of termination.
   We don't worry about this, so any parameter value is fine.

   Note: on some python systems, the entire system will terminate 
     when sys.exit( ) is used (not just your program).