Prime Numbers

Numbers that cannot be divided evenly by numbers other than 1 or themselves are termed prime. This program lists all prime numbers between 1 and 1000.

Type RUN (NL), and the screen will go blank as the ZX80 figures out the numbers. The program takes a while to finish; press BREAK to list what numbers the computer has found so far.

  10 PRINT "PRIME NUMBERS"
  20 FOR N=1 TO 999
  30 FOR K=3 TO 31
  40 LET Q=N/K
  50 LET R=N-Q*K
  60 IF Q=1 THEN GO TO 200
  70 IF R=0 THEN GO TO 250
  80 IF N=1 AND K=31 THEN GO TO 400
  90 LET K=K+1
  95 NEXT K
 200 PRINT " ";N;
 250 LET N=N+1
 300 NEXT N
 350 STOP
 400 PRINT "1 2";
 410 LET N=N+1
 420 NEXT N
Syntactic Sum: 16764, 4K