Thus when **FN p(2,3)** is being evaluated, **a** has the value **10** because it has been initialised in line 10, **x** has the value **2** because it is the first argument, and **y** has the value **3** because it is the second argument. Both **x** and **y** albeit also defined in line **10** are treated as **LOCAL** variables and their value doesn't modify their global counterparts. The result is then, **10+2\*3=16**. When **FN q()** is being evaluated, on the other hand, there are no arguments. So **a**, **x** and **y** all still refer to the variables and have values **10**, **0** and **0** respectively. The answer in this case is **10+0\*0=10**. Now change line 20 to: ``` 20 DEF FN p(x,y)=FN q() ``` This time, **FN p(2,3)** will have the value **10** because **FN q** will still go back to the variables **x** and **y** rather than using the arguments of **FN p**. **DEF FN** can take parameters passed by **REF**erence as is obvious from the next example: ``` DEF FN ian$(REF jen$(),idx)=jen$(idx) ``` Some BASICs (but not *NextBASIC*) have functions called **LEFT$**, **RIGHT$**, **MID$** and **TL$**. **LEFT$** (*a$,n*) gives the substring of *a$* consisting of the first *n* characters. **RIGHT$** (*a$,n*) gives the substring of *a$* consisting of the characters from *nᵗʰ* on. **MID$** (*a$, n₁, n₂*) gives the substring of *a$* consisting of *n₂* characters starting at the *n₁ᵗʰ*. **TL$** (*a$*) gives the substring of *a$* consisting of all its characters except the first. You can write some user-defined functions to do the same. For example: ``` 10 DEF FN TL$(a$)=a$(2 TO ) 20 DEF FN LEFT$(a$, n)=a$( TO n) ``` Check that these work with strings of length **0** or **1**. Note that our **FN LEFT$** has two arguments, one a number and the other a string. A function *cannot* have integer arguments, nor use integer expressions in its definitions. ### NextBASIC functions within integer expressions We already discussed the usage of the **INT {...}** keyword which converts any floating point expression into an integer expression, but in many cases this can be slow. In other cases the values produced by a function are either plain 8 or 16 bit integers which means that integer-only versions of said function would provide significant boost over their standard counterparts. *NextBASIC* caters for these cases with special integer-only forms of the following functions:

ABS n	Return the ABSolute value of n – See this Chapter
IN n	Read value from Hardware Port n – See Chapter 22
INPUT n	Read/Define input controllers – See Chapters 17 and 22
REG n	Read value from Next Register n – See Chapter 22
PEEK a	Read byte from address a in memory – See Chapter 23