Scan of source lines for '0'
0000001  SECTION_HEADING.....  Main.
0000002  COMMENT_BODY........  
0000003  PURPOSE.............  Implied Purpose: This example of using inweb is a whole web in a single short file, to look for twin primes, a classic problem in number theory.
0000004  COMMENT_BODY........  
0000005  HEADING_START.......  @h The conjecture.
0000006  COMMENT_BODY........  It is widely believed that there are an infinite number of twin primes, that
0000007  COMMENT_BODY........  is, prime numbers occurring in pairs different by 2. Twins are known to exist
0000008  COMMENT_BODY........  at least as far out as $10^{388,342}$ (as of 2016), and there are infinitely
0000009  COMMENT_BODY........  many pairs of primes closer together than about 250 (Zhang, 2013; Tao, Maynard,
0000010  COMMENT_BODY........  and many others, 2014).
0000011  COMMENT_BODY........  
0000012  COMMENT_BODY........  This program finds a few small pairs of twins, by the simplest method possible,
0000013  COMMENT_BODY........  and should print output like so:
0000014  BEGIN_CODE..........  = (text)
0000015  TEXT_EXTRACT........  	3 and 5
0000016  TEXT_EXTRACT........  	5 and 7
0000017  TEXT_EXTRACT........  	11 and 13
0000018  TEXT_EXTRACT........  	...
0000019  END_EXTRACT.........  =
0000020  COMMENT_BODY........  
0000021  BEGIN_DEFINITION....  @d RANGE 100 /* the upper limit to the numbers we will consider */
0000022  COMMENT_BODY........  
0000023  BEGIN_CODE..........  =
0000024  C_LIBRARY_INCLUDE...  #include <stdio.h>
0000025  CODE_BODY...........  
0000026  CODE_BODY...........  int main(int argc, char *argv[]) {
0000027  CODE_BODY...........  	for (int i=1; i<RANGE; i++)
0000028  CODE_BODY...........  		@<Test for twin prime at i@>;
0000029  CODE_BODY...........  }
0000030  CODE_BODY...........  
0000031  PARAGRAPH_START.....  @
0000032  MACRO_DEFINITION....  @<Test for twin prime at i@> =
0000033  CODE_BODY...........  	if ((isprime(i)) && (isprime(i+2)))
0000034  CODE_BODY...........  		printf("%d and %d\n", i, i+2);
0000035  CODE_BODY...........  
0000036  HEADING_START.......  @h Primality.
0000037  COMMENT_BODY........  This simple and slow test tries to divide by every whole number at least
0000038  COMMENT_BODY........  2 and up to the square root: if none divide exactly, the number is prime.
0000039  COMMENT_BODY........  A common error with this algorithm is to check where $m^2 < n$, rather
0000040  COMMENT_BODY........  than $m^2 \leq n$, thus wrongly considering 4, 9, 25, 49, ... as prime:
0000041  COMMENT_BODY........  Cambridge folklore has it that this bug occurred on the first computation
0000042  COMMENT_BODY........  of the EDSAC computer on 6 May 1949.
0000043  COMMENT_BODY........  
0000044  BEGIN_DEFINITION....  @d TRUE 1
0000045  BEGIN_DEFINITION....  @d FALSE 0
0000046  COMMENT_BODY........  
0000047  BEGIN_CODE..........  =
0000048  CODE_BODY...........  int isprime(int n) {
0000049  CODE_BODY...........  	if (n <= 1) return FALSE;
0000050  CODE_BODY...........  	for (int m = 2; m*m <= n; m++)
0000051  CODE_BODY...........  		if (n % m == 0)
0000052  CODE_BODY...........  			return FALSE;
0000053  CODE_BODY...........  	return TRUE;
0000054  CODE_BODY...........  }
0000055  CODE_BODY...........