SEARCH AND DESTROY (16K ZX81)
by C Stewart, South Shields, Tyne and Wear
from Your Computer (April 1983), Page 133

This short machine-code routine produces a missile on line 18 on the
display. It proceeds upwards searching for an alien as it goes. The missile
leaves the display if no obstruction is found, or blots it out if an
obstruction is found. The machine code is stored in a REM in line 1
containing 75 bytes.

To operate the routine the print position must be set to:

    PRINT AT 18,A;

immediately before calling the routine, where A is the column you wish the
missile to appear in. To increase speed, POKE 16549  less than 255  and
for the missile character, POKE 16520  code character  and POKE 16533 
code character.

Run the program and move using the 5 and 8 keys. Try missing the alien and
then hitting it. Line 70 keeps the score and line 30 must be within the
main loop. Line 0 on the display is left untouched by the missile for
scores and titles, however more can be left by POKE 16518. The routine is
ideal for invader-type games.
