Why is the code throwing segmentation fault? - c

Question

The problem is to find the number i<=n, n<=500000 for which the longest collatz series exists.
Collatz series for a number n terminates at 1, and the conditions are
if n is even, next term = n/2
if n is odd, next term = 3*n + 1
Well as a matter of fact, the collatz series always terminates at 1 for all numbers.
Hence any number won't repeat in its collatz series. Using this fact, I have written the following code
LOGIC:
I start a while loop, that goes till n and for each iteration, I store the length of the series for that i.
If i occurs in the series of some n >= r > i, then i terminate the loop and add the length of i to r.
For example, say series of 3 is 3, 10, 5, 16, 8, 4, 2, 1. Now the length corresponding to 2 will already be stored in the series_length array, so I use that value.
Then the for loop next to that, finds the longest series and displays the answer.
The code works fine for n <= 1818 to be precise, but shows segmentation fault onwards (dunno why :(). Please help
CODE :
#include <stdio.h>
int length = 0, series_length[500000], maxlength = 0;
void store_length(int n) {
while(n > 1 && series_length[n] == 0) {
length++;
if(n%2 == 0) {
n = n/2;
}
else {
n = 3*n + 1;
}
}
length += series_length[n];
}
int main() {
int n, i = 1, result;
scanf("%d", &n);
series_length[1] = 1;//redundant statement
while(i <= n) {
store_length(i);
series_length[i] = length;
length = 0;
i++;
}
for(int i = 1;i <= n; i++) {
if(maxlength <= series_length[i]) {
maxlength = series_length[i];
result = i;
}
}
printf("%d %d\n", result, maxlength);
return 0;
}
INPUT-
10
OUTPUT-
9 20 (AS Expected)
INPUT-
100000
OUTPUT-
Segmentation Fault
Expected-
77031 351

Answer 1

Your value for n goes outside the range.
You have a line n = 3*n + 1; in the function store_length
Running this with the gdb with input as 100000 gives
Thread 1 received signal SIGSEGV, Segmentation fault.
0x0000000000401545 in store_length (n=532060) at 29_01.c:6
6 while(n > 1 && series_length[n] == 0) {
(gdb) p n
$1 = 532060

Answer 2

only store it if it fits
... and use it if it already has been computed
avoid global variables
prefer unsigned values
[use descriptive variable names]
#include <stdio.h>
#define THE_SIZE 500000
unsigned series_length[THE_SIZE]= {0,};
unsigned get_length(unsigned val) {
unsigned steps;
for (steps=0; val > 1 ; steps++) {
if (val < THE_SIZE && series_length[val]) { steps += series_length[val]; break; }
if(val %2 ) val = 3*val + 1;
else val /= 2;
}
return steps;
}
int main( int argc, char **argv) {
unsigned top, val , result;
unsigned best,maxlength ;
sscanf(argv[1], "%u", &top);
series_length[1] = 1;//redundant statement
best = maxlength = 0;
for(val=1;val <= top; val++) {
result = get_length(val);
// store it if it fits;
if(val<THE_SIZE) series_length[val] = result;
if (result < maxlength) continue;
best = val; maxlength = result;
}
printf("%u %u\n", best, maxlength);
return 0;
}
Finally, just for fun, make the array smaller
#define THE_SIZE 500
, and the program should give the same result for a given value. (it does)

Answer 3

You get the maximum value 24,648,077,896 with n = 487039.
You must thus use the type long long int for n and you should use an array of 24,648,077,896 integers to avoid a segmentation fault. Unfortunately I never succeeded in allocating a block of 100GB. Your optimization is thus not viable.
Without the array optimization I can scan all 500000 n values in 265ms.
Here is my code:
#include <stdio.h>
int collatz_length(int n) {
int length = 0;
long long int v = (long long int)n;
while (v > 1) {
if ((v&1) == 0)
v = v / 2;
else
v = v*3 + 1;
length++;
}
return length;
}
int main() {
int max_i, max_l = 0;
for (int i = 500000; i > 0; i--) {
int l = collatz_length(i);
if (l > max_l){
max_l = l;
max_i = i;
}
}
printf("i: %d l: %d\n", max_i, max_l);
return 0;
}