45 lines
2.3 KiB
Text
45 lines
2.3 KiB
Text
BEGIN # find all primes with strictly decreasing digits #
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PR read "primes.incl.a68" PR # include prime utilities #
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PR read "rows.incl.a68" PR # include array utilities #
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[ 1 : 512 ]INT primes; # there will be at most 512 (2^9) primes #
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INT p count := 0; # number of primes found so far #
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FOR d1 FROM 0 TO 1 DO
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INT n1 = IF d1 = 1 THEN 9 ELSE 0 FI;
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FOR d2 FROM 0 TO 1 DO
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INT n2 = IF d2 = 1 THEN ( n1 * 10 ) + 8 ELSE n1 FI;
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FOR d3 FROM 0 TO 1 DO
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INT n3 = IF d3 = 1 THEN ( n2 * 10 ) + 7 ELSE n2 FI;
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FOR d4 FROM 0 TO 1 DO
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INT n4 = IF d4 = 1 THEN ( n3 * 10 ) + 6 ELSE n3 FI;
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FOR d5 FROM 0 TO 1 DO
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INT n5 = IF d5 = 1 THEN ( n4 * 10 ) + 5 ELSE n4 FI;
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FOR d6 FROM 0 TO 1 DO
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INT n6 = IF d6 = 1 THEN ( n5 * 10 ) + 4 ELSE n5 FI;
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FOR d7 FROM 0 TO 1 DO
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INT n7 = IF d7 = 1 THEN ( n6 * 10 ) + 3 ELSE n6 FI;
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FOR d8 FROM 0 TO 1 DO
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INT n8 = IF d8 = 1 THEN ( n7 * 10 ) + 2 ELSE n7 FI;
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FOR d9 FROM 0 TO 1 DO
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INT n9 = IF d9 = 1 THEN ( n8 * 10 ) + 1 ELSE n8 FI;
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IF n9 > 0 THEN
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IF is probably prime( n9 ) THEN
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# have a prime with strictly descending digits #
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primes[ p count +:= 1 ] := n9
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FI
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FI
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OD
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OD
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OD
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OD
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OD
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OD
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OD
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OD
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OD;
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QUICKSORT primes FROMELEMENT 1 TOELEMENT p count; # sort the primes #
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# display the primes #
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FOR i TO p count DO
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print( ( " ", whole( primes[ i ], -8 ) ) );
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IF i MOD 10 = 0 THEN print( ( newline ) ) FI
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OD
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END
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