To return the sign of x.
x is any numeric expression.
If x is positive, SGN(x) returns 1.
If x is 0, SGN(x) returns 0.
If x is negative, SGN(x) returns -1.
This statement is similar to, but not the same as SIN(x), which returns a trigonometric function in radians, rather than in ones and zeros.
10 INPUT "Enter value", x 20 ON SGN(X)+2 GOTO 100, 200, 300
GW-BASIC branches to 100 if X is negative, 200 if X is 0, and 300 if X is positive.