INV DSZ in its natural habitat

[Dave Britten]

Yesterday there was some question as to whether or not the INV DSZ (decrement and skip if not zero) instruction present on some TIs was at all useful. I maintain that it's useful in any situation where you decrement a counter and break out of the middle of a loop, rather than at the end of the loop body.

I put together a little example by way of a cumulative binomial distribution program. First, a BASIC version for the 71B that makes the algorithm a little easier to read:

Code:

0001 DESTROY ALL 
0010 INPUT "N?";N
0020 INPUT "P?";P
0030 INPUT "X?";X
0040 T=0 @ R=0
0050 F=P/(1-P)
0060 K=X+1
0070 B=(1-P)^N
0080 T=T+B
0090 K=K-1 @ IF K=0 THEN GOTO 120 @ REM INV DSZ!
0100 B=B*(N-R)/(R+1)*F @ R=R+1
0110 GOTO 80
0120 PRINT T

To use it, run the program, enter the number of trials N, the probability of success in a single trial P, and the number of successful trials X. The program will show the cumulative lower tail binomial probability, i.e. probability of the number of successes being between 0 and X.

Note line 90, where the program decrements a counter, and breaks out of the middle of the loop (lines 80 to 110) when the counter reaches zero.

Now, here are printouts of both an HP 97 version, and a TI-59 version. On the HP version, you have to do something like using an extra LBL and GTO to invert the DSZ test behavior. On the TI-59, you can simply use INV DSZ directly.

To run either of these, store N in R01, P in R02, X in R03, and press A.

https://i.imgur.com/rxENjWN.jpg

Ignoring the fact that the TI program is about twice the size of the HP one it's clear that this is a case where INV DSZ can be used to save a couple of steps, and make the program flow easier to read.

[BruceH]

I assume these two statements are used to implement the following types of loop (expressed in C syntax), in which case both have their place.

Code:

/* DSZ */
do {
...
} while (--x != 0)

whereas

Code:

/* INV DSZ (or DSNZ) */
while (--x != 0) {
...
}

[Dave Britten]

Kind of. It's more akin to this, at least in my example:

Code:

 c = 10;
while (true) {
  //do stuff
  if (--c == 0) break;
  //do more stuff
}

[Ross Barnes]

Dave,
Someone at TI found INV DSZ useful since it was used in steps 28-29 of their Battleship program in the SR-56 Application Library book, p. 188.
Ross

[xerxes]

INV DSZ/ISZ can be usefull in some cases. Here a size optimized code for the TI-62 to calculate the factoral:

Code:

00 *
01 INV DSZ
02 R/S
03 RCL 0

Usage example: 69 STO 0 RST R/S

[Dave Britten]

(10-18-2021 06:38 PM)xerxes Wrote:  INV DSZ/ISZ can be usefull in some cases. Here a size optimized code for the TI-62 to calculate the factoral:

Code:

00 *
01 INV DSZ
02 R/S
03 RCL 0

Usage example: 69 STO 0 RST R/S

Nice, didn't realize the lower-end machines had it too. I wonder if it works on the TI-65.

[xerxes]

(10-19-2021 06:54 PM)Dave Britten Wrote:  Nice, didn't realize the lower-end machines had it too. I wonder if it works on the TI-65.

INV DSZ is also present on the TI-65, but I'm not sure, if the code works on the TI-65 too, because it based on the circular program execution ability of the TI-62.