I have an array of prime numbers. The length of this array is 1,000,000. This means that my last element primes[999999] should have the 1 millionth prime, and actually it should have the 1,000,001th prime since I left 2 out of the beginning.
When I run my program and have it spit out the last element of the array it spits out the 999,999th prime number. I am in the process of learning C and this is a little homework project for a math class I am taking. I have no idea where to even start in my troubleshooting of this problem. Any help would be greatly appreciated.
Edit: Sorry, for some reason I thought it might be an easy answer. I posted the code below. I have printed out the array of primes under option 1 and all the primes are correct and the result is correct. Any other option and I get an erroneous output.
void createArray( int, unsigned long long * );
unsigned long long root(unsigned long long);
void print( unsigned long long *, int );
int main(void){
unsigned long long *prime = (unsigned long long *)calloc(SIZE6,
sizeof(unsigned long long));
unsigned long long *ptrEmpty = &prime[15];
unsigned long long i;
unsigned long long j;
long int sizeComp;
int flag = 0;
unsigned long option = 0;
int length = 0;
system( "clear" );
do{
flag = 0;
option = -1;
printf( "Welcome to the prime number finder program. Below are a list "
"of prime numbers\nyou can find as well as their average runni"
"ng time on the Loki system of UNO.\n\nOption 1: Return the 10"
"0th prime number. ( Running time: .006s )\nOption 2: Return t"
"he 1000th prime number. ( Running time: .008s )\nOption 3: Re"
"turn the 10000th prime number. ( Running time: .089s )\nOptio"
"n 4: Return the 100000th prime number. ( Running time: 2.188s"
" )\nOption 5: Return the 1000000th prime number. ( Running ti"
"me: 57.156s )\nOption 6: Return the 10000000th prime number. "
"( **CAUTION!** Running time: 35m51.3s )\n\nEnter the number o"
"f the option you would like the program to\nperform (Enter 0 "
"to exit): " );
scanf( "%lu", &option );
switch( option ){
case 0:
flag = 1;
break;
case 1:
createArray( SIZE1, prime );
ptrEmpty = &prime[15];
length = SIZE1;
break;
case 2:
createArray( SIZE2, prime );
ptrEmpty = &prime[15];
length = SIZE2;
break;
case 3:
createArray( SIZE3, prime );
ptrEmpty = &prime[15];
length = SIZE3;
break;
case 4:
createArray( SIZE4, prime );
ptrEmpty = &prime[15];
length = SIZE4;
break;
case 5:
createArray( SIZE5, prime );
ptrEmpty = &prime[15];
length = SIZE5;
break;
case 6:
createArray( SIZE6, prime );
ptrEmpty = &prime[15];
length = SIZE6;
break;
default:
printf( "Please enter one of the available options ( 1 - 7 ) "
"or 0 to exit: " );
flag= 1;
break;
}
if( flag != 1 ){
for( i = START, sizeComp = 15; sizeComp <= length; i += 2 ){
flag = 0;
for( j = 0; prime[j] < root( i ); j++ ){
if( ( i % prime[j] ) == 0 ){
flag = 1;
break;
}
}
if( flag == 0 ){
*ptrEmpty = i;
++sizeComp;
ptrEmpty++;
}
}
printf( "\nThe %dth prime number is %llu.\n\n", length,
prime[ length - 2 ]);
// print( prime, length );
printf( "****************************************"
"****************************************\n\n" );
}
}while( option != 0 );
free( prime );
return 0;
}
void print( unsigned long long *prime, int length ){
int i;
printf( "\n%4d", 2 );
for( i = 0; i < length; i++){
printf( "%4llu ", prime[i] );
}
printf( "\n" );
}
void createArray( int length, unsigned long long *prime ){
unsigned long long *pHolder = NULL;
pHolder = (unsigned long long *)realloc( prime, length *
sizeof( unsigned long long ) );
if(pHolder != NULL){
prime = pHolder;
}else{
free( prime );
printf( "Error reallocating memory.\n" );
exit(1);
}
prime[0] = 3;
prime[1] = 5;
prime[2] = 7;
prime[3] = 11;
prime[4] = 13;
prime[5] = 17;
prime[6] = 19;
prime[7] = 23;
prime[8] = 29;
prime[9] = 31;
prime[10] = 37;
prime[11] = 41;
prime[12] = 43;
prime[13] = 47;
prime[14] = 53;
}
unsigned long long root(unsigned long long a) {
unsigned long long rem = 0;
unsigned long long root = 0;
int i;
for (i = 0; i < 16; i++) {
root <<= 1;
rem <<= 2;
rem += a >> 30;
a <<= 2;
if (root < rem) {
root++;
rem -= root;
root++;
}
}
return (root >> 1);
}
Edit: SIZE1 = 100, SIZE2 = 1000, SIZE3 = 10000, SIZE4 = 100000, SIZE5 = 1000000, SIZE6 = 10000000