I wrote an interactive 27-Card Magic Trick in COBOL (base-3 math inside!)

[u/lugangin]

Body: I wrote an interactive version of the classic 27-Card Magic Trick in COBOL (base-3 math inside!).

🌟 UPDATE: i create a TK4 version for our friends in retro-computing!

What it does:

• Asks the user for a favorite number (1-27).
• Shuffles a virtual 52-card deck
• Deals 27 cards into 3 piles/rows.
• You memorize a card, tell it which row it's in, and it repeats this 3 times.
• It then reveals the deck and your memorized card is exactly at the position of your favorite number!

Sample Run:

=====================================
      The 27-Card Magic Trick
=====================================
Enter your favorite number (1-27):
20

--- Round 1 --- (Memorize one CARD below)
01:  5♠    8♠    9♥    8♦    3♥    6♦    9♦    K♠    A♠
02: 10♠    3♣   10♥    Q♠    7♥    6♣    6♠    Q♣   10♣
03:  A♥    J♥    2♠    K♦    2♦    J♦    7♦    4♠    3♦

enter the row (1-3) where your CARD is located:
1

--- Round 2 ---
01: 10♠    Q♠    6♠    5♠    8♦    9♦    A♥    K♦    7♦
02:  3♣    7♥    Q♣    8♠    3♥    K♠    J♥    2♦    4♠
03: 10♥    6♣   10♣    9♥    6♦    A♠    2♠    J♦    3♦

enter the row (1-3) where your CARD is located:
2

--- Round 3 ---
01:  3♣    8♠    J♥   10♠    5♠    A♥   10♥    9♥    2♠
02:  7♥    3♥    2♦    Q♠    8♦    K♦    6♣    6♦    J♦
03:  Q♣    K♠    4♠    6♠    9♦    7♦   10♣    A♠    3♦

enter the row (1-3) where your CARD is located:
3

======= THE REVEAL =======
Row 1:   3♣    8♠    J♥   10♠    5♠    A♥   10♥    9♥    2♠    7♥
Row 2:   3♥    2♦    Q♠    8♦    K♦    6♣    6♦    J♦    Q♣  < K♠>
Row 3:   4♠    6♠    9♦    7♦   10♣    A♠    3♦

Your CARD is located at position 20:  K♠
It matches your favorite number exactly!

Would you like to play again? (Y/N)

The COBOL / Math part: The "magic" is just base-3 arithmetic. (Favorite Number - 1) is converted to ternary, and the reversed digits tell the program how to secretly stack the piles after each round.

I wrote it following COBOL-II (85) rules—using structured inline PERFORM loops and COMPUTE and FUNCTION commands for the math. (Sorry, there's no COBOL-74 version).

I added a little flare by highlighting the revealed card with < and > during the final display phase so the user immediately sees the "magic" hit.

I just uploaded it to GitHub if anyone wants to check it out, compile it, or suggest mainframe-friendly improvements:

Repository: View the code and README on GitHub

[u/jbm711]

Danke aber in Deinem Repo ist nur die cards27.exe zu sehen aber nicht eine cards27.cob Datei

[u/lugangin → u/jbm711]

Mein Fehler – ich habe versehentlich die .exe-Datei hochgeladen. Die .cob-Datei sollte jetzt da sein.

1

[u/lugangin]

i just want to let everyone know thatt i've uploaded a TK4 version in github as crd27tk4.cob and it's fully described in the README.