Put the threshold at 0 and make the thing you test reach it

Say you want to do something five times.

You'd probably count from 0 up to 5 and check at the top of each pass whether you're still under 5. That's how it looks in most languages, and it works here, but it costs more than it looks. The check is a SUB and a cell to hold the 5, and because TAC only jumps when the answer is negative, you need a JMP to get out of the loop and another at the bottom to get back in.

cell  contents  notes
 10      130    CLA 30  cells 10-13: still below 5? TAC into the body, else JMP out
 11      731    SUB 31
 12      314    TAC 14
 13      819    JMP 19
 14      532    OUT 32  cells 14-19: the body, then count one more and go back to the test
 15      130    CLA 30
 16      200    ADD 00
 17      630    STO 30
 18      810    JMP 10
 19      900    HRS 00
 30      000    the count
 31      005    the limit
 32      042    what the body prints

Step through the natural way in Cardigan

Here's the trick: count from -5 up to 0 instead, and test at the bottom. TAC only ever asks "is this below zero?", and while your count is still negative the answer is yes, so TAC's jump is what takes you round again. There's no limit to subtract, and both JMPs disappear.

cell  contents  notes
 10      532    OUT 32  cells 10-15: the body, then count up; TAC goes round while the count is negative
 11      130    CLA 30
 12      200    ADD 00
 13      630    STO 30
 14      310    TAC 10
 15      900    HRS 00
 30     -005    the count, from -5
 32      042    what the body prints

Step through the CARDIAC way in Cardigan

Both print 42 five times. The first takes 13 cells and runs 45 instructions; the second takes 8 cells and runs 26. Same loop, a lot less work, and you'll find this shape all over good CARDIAC code.

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