DEF Statement ☆☆☆
DEF statements enables us to write functions defined by complicated expressions simply as f(x), g(x,y) and so on.
Example.

    10 DEF f(x,y)=x^2+3*y^2
    20 FOR x=-4 TO 4
    30    FOR y=-4 TO 4
    40        PRINT x,y,f(x,y)
    50    NEXT y
    60 NEXT x
    70 END

Parameters
Variables written in the parentheses succeeding DEF are called parameters.
Any variable which is written in other than the DEF line is a different variable, that is, even if it have the same name as that of a parameter, they have distinct locations on the memory.
Example.

    10 DEF f(x)=x^3
    20 LET x=4
    30 PRINT f(2)
    40 PRINT x
    50 END

Variables x in line 10 and x in line 40 are different.
Thus the PRINT statement in line 40 outputs 4.
Variables in Common
Any variables other than parameters are common among the program unit.
Example.

    10 DEF f(x)=a*x^2+b*x*c
    20 INPUT a,b,c
    30 PRINT f(1),f(2)
    40 END

On the above program, variables a, b, c in line 10 and variables a, b, c in line 20 are the same, respectively.
Define a function with no parameter
A function that have no parameter is written without parentheses.
Example.

    10 DEF spot=INT(6*RND+1)
    20 FOR i=1 TO 100
    30    PRINT spot
    40 NEXT i
    50 END

Note.
The result of a DEF statement has the precision of over 16 digits on the decimal operation mode. （not rounded to 15 digits as on a LET statement）
Note.
Functions can not be re-defined during execution as on some interpreter dialect.