The Computer Tutor: Simulating the DEF FN Function

Good morning Class! I’m wondering if the poor attendance is due to these late season snow storms or the phasing out of the TS-1000. Oh, well – today’s lesson will be on simulating the <DEF FN> or Define Function. This is a powerful programming tool that is poorly promoted and even less understood.

On many other computers, you will find this function wherever mathematical formulae need to be repeated. An example of a routine using <DEF FN> might be:

10 DFN A(X) = 17*(18.5 - X/5)
20 PRINT FNA(12)

The result of RUNning these 2 lines would be 273.7. This function, of course, could easily be written with a BASIC routine as follows:

10 INPUT X
20 GOSUB 1000
30 PRINT Y
40 STOP
1000 Y = 17*(18.5 - X/5)
1010 RETURN

For most purposes, this is easy to understand and works quite nicely. But there is a more memory efficient method. Enter the following program into your computer.

10 LET A$ = "17*(18.5 - X/5)"
20 INPUT X
30 PRINT VAL A$

Look at the advantages of this technique! We never have to access a subroutine. Enter line “40 GOTO 20” and the program recycles where we can enter different values and get the proper results each time.

The purpose of <DEF FN> is to define a function or formula where variables may keep changing but the formula that uses these variables stays intact. The TS-1000 actually has the capability to define more complex formulae than the <DEF FN> function can because formulae can be custom designed to include additional variables and be much longer.

Once the string variable (A$ in our example) has been defined, we never need encounter that program line again. Simply finding the value of that string will do the job.

A short lesson today, but one that could have important repercussions in your future programs.

Class dismissed!