Not as a modifier

The $If statement can contain a Not modifier.  If so the GAMS statementtoexecute will be compiled and executed only when the conditional is false.  The basic form of the $If not command is

$If NOT conditional statementtoexecute

$Ifi NOT conditional statementtoexecute

Example:

(basicif.gms)

scalar x /1/;

*$ondollar

scalar y /1/;

$Setglobal gg 

$Setglobal tt doit

$If setglobal gg display x;

$If not setglobal gg display y;

$If "%tt%" == "doit" x=x*2;

$Ifi "%tt%" == "DOIT" x=y**2;

$If "%tt%" == "DOIT" x=x*100;

$If not "%tt%" == "doit" y=y/4;

$set aa yes

$If "%aa%" == yes y=x;

The resultant effect on the compiled code is

1  scalar x /1/;

2  *$ondollar

3  scalar y /1/;

6  display x;

8  x=x*2;

9  x=y**2;

13  y=x;

Note lines in blue are suppressed because the $If fails while the magenta, orange, red and violet lines occur since the conditionals are true.  The contrast between the lines

$Ifi "%tt%" == "DOIT" x=y**2;

$If "%tt%" == "DOIT" x=x*100;

where the second line fails since tt was set to "doit".  This shows the case sensitive nature of $If versus the case insensitivity of $Ifi.