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Full Version: Sieve of Eratosthenes
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The program SIEVEMIN shows a miniature version of the famous Sieve of Eratosthenes. The Sieve of Eratosthenes is a Greek algorithm that determines prime numbers from 2 to N through eliminating multiplies.

Code:
```sub1(); // subroutine EXPORT SIEVEMIN() BEGIN // 2018-12-18 EWS // Mini Sieve HFormat:=0; // standard PRINT(); PRINT("Mini Sieve"); WAIT(1); LOCAL M0,T,R,C,K; T:=1; M0:=MAKEMAT(0,7,7); FOR R FROM 1 TO 7 DO FOR C FROM 1 TO 7 DO M0(R,C):=T; T:=1+T; END; END; sub1(M0); M0(1,1):=0; FOR K FROM 2 TO 7 DO FOR R FROM 1 TO 7 DO FOR C FROM 1 TO 7 DO IF FP(M0(R,C)/K)==0 AND  M0(R,C)>K THEN M0(R,C):=0; sub1(M0); END; END; END; END; // end of program END; sub1(M1) BEGIN PRINT(); LOCAL I; FOR I FROM 1 TO 7 DO PRINT(row(M1,I)); END; WAIT(0.5); END;```

I remember doing something similar to solve one of the early puzzles on the Project Euler website
(12-19-2018 02:30 PM)Eddie W. Shore Wrote: [ -> ]FOR K FROM 2 TO 7 DO
FOR R FROM 1 TO 7 DO
FOR C FROM 1 TO 7 DO
IF FP(M0(R,C)/K)==0 AND M0(R,C)>K THEN M0(R,C):=0; ...

I don't think the code is really a prime sieve.
A true sieve does not need any divisibility test.
Also, looping count is very high = 6x7x7 = 294

I think sieve should be like below code.
To get your 7x7 matrix, do list2mat(sieve(7*7), 7)

PHP Code:
`sieve(n) := {    local a, c, p, pmax;    a := range(1, n+1); a(1) := 0;       // 1 is not prime    pmax := floor(sqrt(n));    for p from 2 to pmax do        if (a(p) == 0) continue;         // skip composite        for(c:=p*p; c<=n; c+=p) a(c):=0; // mark composite    end;    return a;} `
There is a program written by C.Ret for the HP-29C and also for the HP-41C that uses a min-heap as data-structure instead of an array due to memory limitations.

Cheers
Thomas
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