Let negative mean "none"

Say you want to add up a stack of cards, and you don't know how many there are.

The usual answer is to put a count on the first card and count it down as you go. That works, but it costs you a cell for the count and four instructions every pass just to take one off it and test it.

cell  contents  notes
 10      030    INP 30  cells 10-21: the first card says how many follow; count it down
 11      130    CLA 30
 12      700    SUB 00
 13      320    TAC 20
 14      630    STO 30
 15      031    INP 31
 16      132    CLA 32
 17      231    ADD 31
 18      632    STO 32
 19      811    JMP 11
 20      532    OUT 32
 21      900    HRS 00
 30      000    how many cards are left
 31      000
 32      000

Step through the natural way in Cardigan

Instead, end the stack with a -1. You're already loading each card into the accumulator to add it, so spotting the end costs you nothing but the TAC, and -1 can never be mistaken for a real card. Once you start doing this you'll see uses everywhere: -1 for "not found", "off the edge", "no more". Whatever you're testing for, make it the negative case.

cell  contents  notes
 10      030    INP 30  cells 10-17: add up cards until a negative one
 11      130    CLA 30
 12      316    TAC 16  -1 means no more cards
 13      231    ADD 31
 14      631    STO 31
 15      810    JMP 10
 16      531    OUT 31
 17      900    HRS 00
 30      000
 31      000

Step through the CARDIAC way in Cardigan

Both add up 3, 4 and 5 and print 12. The counted version takes 15 cells and runs 33 instructions; the -1 version takes 10 cells and runs 23.

From the tips in cardiac-code on GitHub. All the tips.