Consecutive primes with ascending or descending differences: Difference between revisions
Consecutive primes with ascending or descending differences (view source)
Revision as of 13:52, 8 April 2023
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First such sequence (differences in brackets):
322171 (22) 322193 (20) 322213 (16) 322229 (8) 322237 (6) 322243 (4) 322247 (2) 322249
</pre>
=={{header|C}}==
{{trans|Wren}}
More or less.
<syntaxhighlight lang="c">#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>
bool *sieve(int limit) {
int i, p;
limit++;
// True denotes composite, false denotes prime.
bool *c = calloc(limit, sizeof(bool)); // all false by default
c[0] = true;
c[1] = true;
for (i = 4; i < limit; i += 2) c[i] = true;
p = 3; // Start from 3.
while (true) {
int p2 = p * p;
if (p2 >= limit) break;
for (i = p2; i < limit; i += 2 * p) c[i] = true;
while (true) {
p += 2;
if (!c[p]) break;
}
}
return c;
}
void longestSeq(int *primes, int pc, bool asc) {
int i, j, d, pd = 0, lls = 1, lcs = 1;
int longSeqs[25][10] = {{2}};
int lsl[25] = {1};
int currSeq[10] = {2};
const char *dir = asc ? "ascending" : "descending";
for (i = 1; i < pc; ++i) {
d = primes[i] - primes[i-1];
if ((asc && d <= pd) || (!asc && d >= pd)) {
if (lcs > lsl[0]) {
memcpy((void *)longSeqs[0], (void *)currSeq, lcs * sizeof(int));
lsl[0] = lcs;
lls = 1;
} else if (lcs == lsl[0]) {
memcpy((void *)longSeqs[lls], (void *)currSeq, lcs * sizeof(int));
lsl[lls++] = lcs;
}
currSeq[0] = primes[i-1];
currSeq[1] = primes[i];
lcs = 2;
} else {
currSeq[lcs++] = primes[i];
}
pd = d;
}
if (lcs > lsl[0]) {
memcpy((void *)longSeqs[0], (void *)currSeq, lcs * sizeof(int));
lsl[0] = lcs;
lls = 1;
} else if (lcs == lsl[0]) {
memcpy((void *)longSeqs[lls], (void *)currSeq, lcs * sizeof(int));
lsl[lls++] = lcs;
}
printf("Longest run(s) of primes with %s differences is %d:\n", dir, lsl[0]);
for (i = 0; i < lls; ++i) {
int *ls = longSeqs[i];
for (j = 0; j < lsl[i]-1; ++j) printf("%d (%d) ", ls[j], ls[j+1] - ls[j]);
printf("%d\n", ls[lsl[i]-1]);
}
printf("\n");
}
int main() {
const int limit = 999999;
int i, j, pc = 0;
bool *c = sieve(limit);
for (i = 0; i < limit; ++i) {
if (!c[i]) ++pc;
}
int *primes = (int *)malloc(pc * sizeof(int));
for (i = 0, j = 0; i < limit; ++i) {
if (!c[i]) primes[j++] = i;
}
free(c);
printf("For primes < 1 million:\n");
longestSeq(primes, pc, true);
longestSeq(primes, pc, false);
free(primes);
return 0;
}</syntaxhighlight>
{{out}}
<pre>
For primes < 1 million:
Longest run(s) of primes with ascending differences is 8:
128981 (2) 128983 (4) 128987 (6) 128993 (8) 129001 (10) 129011 (12) 129023 (14) 129037
402581 (2) 402583 (4) 402587 (6) 402593 (8) 402601 (12) 402613 (18) 402631 (60) 402691
665111 (2) 665113 (4) 665117 (6) 665123 (8) 665131 (10) 665141 (12) 665153 (24) 665177
Longest run(s) of primes with descending differences is 8:
322171 (22) 322193 (20) 322213 (16) 322229 (8) 322237 (6) 322243 (4) 322247 (2) 322249
752207 (44) 752251 (12) 752263 (10) 752273 (8) 752281 (6) 752287 (4) 752291 (2) 752293
</pre>
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