IF Notes

Equality and Relational Operators

Most of these operators are represented by two characters, just as my name is represented by four characters. The characters must be adjacent to be interpreted correctly, that is, PA  UL might refer Pennsylvania products that are UL® approved, but we must remove the space if they refer to me.

For all examples assume the following declaration: var x=5, y=6;

JavaScript Text Symbol	Math Symbol	Description	True Expression	False Expression
<	<	less than	(x < y) (x < 2 * y) (3 < x)	(y < x) (x + 1 < y) (x < y / 2)
<=		less than or equal to	(x <= y) (x + 1 <= y) (x <= y -1) (x / 2 <= y + 1)	(y <= x) (x < y / 2) (7 <= y) (x + y <= 0)
>	>	greater than	(y > x) (y + 1 > x) (x > 0)	(x > y) (x > 100) (x > 0)
>=		greater than or equal to	(y >= x) (y >= x + 1) (y + 1 >= x + 1)	(x >= y) (y -2 >= x) (x >= 100)
==	=	equal to	(x + 1 == y) (x == y - 1) (x - 2 == y / 2)	(x / 2 == y / 2) (x + 2 == y) (x * 2 == y)
!=		not equal to	(x / 2 != y / 2) (x + 2 != y) (x * 2 != y)	(x + 1 != y) (x != y - 1) (x - 2 != y / 2)

• Common mistake: using = for ==
  • The first is assignment, the second is an equality comparison.
• Compare numbers using all of the relational operators (refresh page to run again)

Conditional Programming

• Not sequential
• When you need to choose between two (or more) alternatives
• IF syntax for single statements
  if ( condition )
       statement;
• example of JavaScript code
  if ( input_num < 65 )
        alert ( "Message");
• IF syntax for multiple statements
  if ( condition )
  {
             statement₁;
             statement₂;
  }
• example of JavaScript code
  if ( grade > 98 )
  {
            document.write( "Excellent job<br>");
            bonus = 1;
  }

IF-ELSE syntax

• single statements
  if ( condition )
       statement;
  else
       statement;
• example of JavaScript code
  if (x < 0 )
       document.write( "negative number<br>");
  else
       document.write( "non-negative number<br>");
• compound statements
  if ( condition )
  {
             statement₁;
             statement₂;
  }
  else
  {
             statement₃;
             statement₄;
  }
• example of JavaScript code

Nested IFs

• More than two possible actions
• Second if can be nested inside of the 1^st "if" part or the 1^st "else" part
• Example
  if ( x < 0 )
       document.write( "negative number<br>");
  else {
       if ( x == 0 )
             document.write( "zero<br>");
       else
             document.write( "positive number<br>");
       }

ELSE IFs

• Just another way to write the above
  • Note that the { } were not needed (even above)
• Example
  if (x < 0 )
       document.write( "negative number<br>");
  else if (x == 0)
       document.write( "zero<br>");
  else
       document.write(" "positive number<br>"");