Find prime n such that reversed n is also prime: Difference between revisions

=={{header|PL/M}}==

{{works with|8080 PL/M Compiler}} ... under CP/M (or an emulator)

<syntaxhighlight lang="plm">

100H: /* FIND PRIMES THAT ARE STILL PRIME WHEN THEIR DIGITS ARE REVERSED */

/* CP/M BDOS SYSTEM CALL */

BDOS: PROCEDURE( FN, ARG ); DECLARE FN BYTE, ARG ADDRESS; GOTO 5; END;

/* I/O ROUTINES */

PR$CHAR: PROCEDURE( C ); DECLARE C BYTE; CALL BDOS( 2, C ); END;

PR$STRING: PROCEDURE( S ); DECLARE S ADDRESS; CALL BDOS( 9, S ); END;

PR$NL: PROCEDURE; CALL PR$CHAR( 0DH ); CALL PR$CHAR( 0AH ); END;

PR$NUMBER: PROCEDURE( N ); /* PRINTS A NUMBER IN THE MINIMUN FIELD WIDTH */

DECLARE N ADDRESS;

DECLARE V ADDRESS, N$STR ( 6 )BYTE, W BYTE;

V = N;

W = LAST( N$STR );

N$STR( W ) = '$';

N$STR( W := W - 1 ) = '0' + ( V MOD 10 );

DO WHILE( ( V := V / 10 ) > 0 );

N$STR( W := W - 1 ) = '0' + ( V MOD 10 );

END;

CALL PR$STRING( .N$STR( W ) );

END PR$NUMBER;

/* RETURNS TRUE IF N IS PRIME, FALSE OTHERWISE, USES TRIAL DIVISION */

IS$PRIME: PROCEDURE( N )BYTE;

DECLARE N ADDRESS;

DECLARE PRIME BYTE;

IF N < 3 THEN PRIME = N = 2;

ELSE IF N MOD 3 = 0 THEN PRIME = N = 3;

ELSE IF N MOD 2 = 0 THEN PRIME = 0;

ELSE DO;

DECLARE ( F, F2, TO$NEXT ) ADDRESS;

PRIME = 1;

F = 5;

F2 = 25;

TO$NEXT = 24; /* NOTE: ( 2N + 1 )^2 - ( 2N - 1 )^2 = 8N */

DO WHILE F2 <= N AND PRIME;

PRIME = N MOD F <> 0;

F = F + 2;

F2 = F2 + TO$NEXT;

TO$NEXT = TO$NEXT + 8;

END;

END;

RETURN PRIME;

END IS$PRIME;

REVERSE: PROCEDURE( N )ADDRESS; /* RETURNS THE REVERSED DIGITS OF N */

DECLARE N ADDRESS;

DECLARE ( R, V ) ADDRESS;

V = N;

R = V MOD 10;

DO WHILE( ( V := V / 10 ) > 0 );

R = ( R * 10 ) + ( V MOD 10 );

END;

RETURN R;

END REVERSE ;

/* FIND THE NUMBERS UP TO 500 */

DECLARE ( I, COUNT ) ADDRESS;

COUNT = 0;

DO I = 1 TO 500;

IF IS$PRIME( I ) THEN DO;

IF IS$PRIME( REVERSE( I ) ) THEN DO;

IF I < 10 THEN CALL PR$CHAR( ' ' );

IF I < 100 THEN CALL PR$CHAR( ' ' );

CALL PR$NUMBER( I );

IF ( COUNT := COUNT + 1 ) MOD 20 = 0 THEN CALL PR$NL;

ELSE CALL PR$CHAR( ' ' );

END;

END;

END;

EOF

</syntaxhighlight>

{{out}}

<pre>

2 3 5 7 11 13 17 31 37 71 73 79 97 101 107 113 131 149 151 157

167 179 181 191 199 311 313 337 347 353 359 373 383 389

</pre>