Why does this stop running early? - c

Question

I'm actually working on a projecteuler.com problem (#12 specifically) and I thought I had this nailed when I wasn't getting any compile errors.
It runs and gives me several results that appear to be correct, but it's not finishing. I haven't been using C all that long so I am probably overlooking something I'm just not familiar with. Can someone tell me why it's stopping? It is giving me correct triangle numbers up to 12.5M. I would also gladly accept optimization suggestions in the comments.
The results are first, even after a few hours it didn't move on past the first number with 30 factors, which it found rather quickly.
It gives me this, from the code following it:
$ ./euler12
Current= 1
Factors= 1
Current= 3
Factors= 2
Current= 6
Factors= 4
Current= 28
Factors= 6
Current= 36
Factors= 9
Current= 120
Factors= 16
Current= 300
Factors= 18
Current= 528
Factors= 20
Current= 630
Factors= 24
Current= 1008
Factors= 30
where Current gives me the number it got the factors from and obviously then Factors is the number of factors. I doesn't give me any errors and the only warning from -Wall is that I don't actually use the "useless" variable for anything.
#include <stdio.h>
#include <time.h>
/*
Tristen
euler12.c
December 23, 2013
What's the value of the first triangle number to have over 500 divisors?
*/
int main(void)
{
/*---------Variables----------*/
time_t t1 = time(NULL);
int g,l,i,j,k,t,number,val,flag1,flag2;
int h=1,x=0,p=0,n=5000,m=500,m2=600,twitch=0
int answer=0,count=0,useless=0,linlen=0; /*modify n to change size*/
/*----------Arrays------------*/
int numtocheck[n];
int factors[m2];
/*find triangle numbers*/
for(i=0;i<=n+1;i++){
x+=i;
if(x!=0){
numtocheck[i]=x;
}
else{
useless=0;
}
}
/*begin checking for factors*/
while(twitch!=1){
count=0;
for(l=1;l<m2;l++){
factors[l]=0;
}
number=numtocheck[h];
for(j=0;j<=number;j++){
for(k=0;k<=number;k++){
val=j*k;
if(val==number){
flag1=0,flag2=0;
for(g=0;g<m2;g++){
if(factors[g]==j){
flag1=1;
}
else if(factors[g]==k){
flag2=1;
}
else{
useless=0;
}
}
if(flag1==0){
factors[p]=j;
p+=1;
}
else if(flag2==0){
factors[p]=k;
p+=1;
}
else{
useless=0;
}
}
}
}
for(l=0;l<m2;l++){
if(factors[l]!=0){
count+=1;
}
}
if(count>=m){
answer=number;
printf("Current= %d\n",number);
printf("Factors= %d\n",linlen);
twitch=1;
}
else{
if(count>linlen){
linlen=count;
printf("Current= %d\n",number);
printf("Factors= %d\n",linlen);
}
else{
useless=0;
}
}
h+=1;
}
time_t t2 = time(NULL);
t=t2-t1;
printf("Survey says %d\n", answer);
printf("time %d\n", t);
getchar();
return 0;
}

Answer 1

Here are some points to consider (point 5 is the big problem):
1) you forgot a semicolon so this code won't compile, based on this, I'm a little concerned you don't have everything in here verbatim to how you have it...
int h=1,x=0,p=0,n=5000,m=500,m2=600,twitch=0 //<- ; needed
2) as noted in the comments your first for loop overflows the array:
int n=5000;
int numtocheck[n];
/* so numtocheck[] goes from 0 to 4999 */
/*find triangle numbers*/
for(i=0;i<=n+1;i++){ // this loops from 0 to 5001
So this needs to be:
for(i=0;i<n;i++){
3) since you don't initialize the 0th element of numtocheck that means there is garbage data in numtocheck[0]. Same thing later for factors[0], the latter is OK since you over write it, but it's just something to be careful about.
4) You use useless to avoid empty else cases... this is, as you aptly named it, useless. If you don't have anything to do, don't do anything.
For example instead of writing this:
if(count>linlen){
linlen=count;
printf("Current= %d\n",number);
printf("Factors= %d\n",linlen);
}
else{
useless=0;
}
Just remove the else, a if can be paired with an else, it does not need to be.
5) OK, here's the big problem:
if(flag1==0){
factors[p]=j;
p+=1;
}
else if(flag2==0){
factors[p]=k;
p+=1;
}
factors[] is an array of 600 elements, and you access using p a value that's initially 0 but is incremented every time it enters either of these checks. By the time your number is around 2346 p more than overflows the 600 elements in factors[] and you start doing bad things. This causes undefined behavior. On one implantation in Linux this caused a very nice setfault. On a different one in Windows this simply over wrote the values in numtocheck basically making you go into an infinite loop.