Fold the multiply into the shift

Say you want to reverse a three-digit number, so 123 becomes 321.

You'd probably cut out the three digits one at a time, then use another shift on each to put it in its new place, and add them up.

cell  contents  notes
 10      040    INP 40  cells 10-19: cut out each digit: c, then b, then a
 11      140    CLA 40
 12      433    SFT 33
 13      643    STO 43
 14      140    CLA 40
 15      423    SFT 23
 16      642    STO 42
 17      140    CLA 40
 18      413    SFT 13
 19      641    STO 41
 20      143    CLA 43  cells 20-29: then put each in its place and add them up
 21      420    SFT 20
 22      644    STO 44
 23      142    CLA 42
 24      410    SFT 10
 25      244    ADD 44
 26      241    ADD 41
 27      644    STO 44
 28      544    OUT 44
 29      900    HRS 00
 40      000
 41      000
 42      000
 43      000
 44      000

Step through the natural way in Cardigan

Here's what makes SFT so handy: SFT xy shifts left x places and then right y, and whatever falls off either end is gone. That means one shift can cut a digit out and drop it where it belongs, all at once. From abc, SFT 31 gives you c00, and SFT 13 gives you 00a. Add each piece to a running total as you go. This is how the manual's own Program No. 6 does it.

cell  contents  notes
 10      040    INP 40  cells 10-24: each shift cuts a digit out and puts it in its new place at once
 11      140    CLA 40
 12      431    SFT 31  c00
 13      641    STO 41
 14      140    CLA 40
 15      423    SFT 23  00b
 16      410    SFT 10  0b0
 17      241    ADD 41
 18      641    STO 41
 19      140    CLA 40
 20      413    SFT 13  00a
 21      241    ADD 41
 22      641    STO 41
 23      541    OUT 41
 24      900    HRS 00
 40      000
 41      000

Step through the CARDIAC way in Cardigan

Both turn 123 into 321. Cutting and placing separately takes 25 cells and runs 20 instructions; folding them together takes 17 cells and runs 15.

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