Since the previous two days of Advent of Code went so well, I decided to up the ante and try different programming languages. Today: C. I haven’t written C since abandoning German Atlantis.
The question is this: given three numbers per line, count the potential triangles. “In a valid triangle, the sum of any two sides must be larger than the remaining side.”
Here goes. Save as `day3.c`, compile using `make day3`, test from the shell using `echo 1 2 3 | ./day3` (not a triangle), save your input in a file called `data` and run using `./day3 < data`.
#include <stdio.h>
#include <stdlib.h>
int read_num () {
int k;
if (scanf("%d", &k) == 1) {
return k;
}
return 0;
}
int * read_triangle() {
static int t[3];
for (int i = 0; i < 3; i++) {
int n = read_num();
if (n == 0) {
if (i == 0) {
return 0;
}
fprintf(stderr, "triangle incomplete, missing %d sides.\n", 3 - i);
abort();
}
t[i] = n;
}
return t;
}
int legal(int *t) {
if (t[0] + t[1] > t[2]
&& t[0] + t[2] > t[1]
&& t[1] + t[2] > t[0]) {
return 1;
}
return 0;
}
int main() {
int n = 0;
int *t;
while ((t = read_triangle()) != 0) {
/* if (legal(t) == 1) { */
/* printf("triangle: [%d %d %d]\n", t[0], t[1], t[2]); */
/* } else { */
/* printf("not a triangle: [%d %d %d]\n", t[0], t[1], t[2]); */
/* } */
n += legal(t);
}
printf("Counted %d triangles.\n", n);
return 0;
}
The setup I had was particularly fortuitous for the second part because the array could just as well hold three triangles instead of just one, and the legal function could return a count instead of just 1 or 0.
In part two, we count vertical triangle candidates. In other words, we read three rows and get three potential triangles. Here’s the example given:
101 301 501 102 302 502 103 303 503 201 401 601 202 402 602 203 403 603
This counts as 6 triangles, because every [101 102 103] is a triangle, [201 202 203] is a triangle, etc.
#include <stdio.h>
#include <stdlib.h>
int read_num () {
int k;
if (scanf("%d", &k) == 1) {
return k;
}
return 0;
}
int * read_three_triangles() {
static int t[9];
for (int i = 0; i < 9; i++) {
int n = read_num();
if (n == 0) {
if (i == 0) {
return 0;
}
fprintf(stderr, "triangle incomplete, missing %d sides.\n", 3 - i);
abort();
}
t[i] = n;
}
return t;
}
int legal(int *t) {
int n = 0;
for (int i = 0; i < 3; i++) {
if (t[i + 0] + t[i + 3] > t[i + 6]
&& t[i + 0] + t[i + 6] > t[i + 3]
&& t[i + 3] + t[i + 6] > t[i + 0]) {
n++;
}
}
return n;
}
int main() {
int n = 0;
int *t;
while ((t = read_three_triangles()) != 0) {
n += legal(t);
}
printf("Counted %d triangles.\n", n);
return 0;
}
All in all I must say C was quite easy to slide back into. The core itself is easy. Luckily this exercise did not require memory management.
#C #Advent of Code #Programming