C programming segmentation error - c

Question

This is the code for finding the last digit of nth Fibonacci number
#include <stdio.h>
int main() {
long i, j, fib[1000];
fib[0] = 0;
fib[1] = 1;
scanf("%li", &j);
for(i = 2; i != 1000; i++)
{
fib[i] = (fib[i - 2] + fib[i - 1]) % 10;
}
printf("%li", fib[j]);
return 0;
}
It shows segmentation fault. How can I fix it?

Answer 1

The only reason I can see for this not working is that you're inputting a number that is outside of the range 0 <= j <= 999. This is due to the limit on your array variable: long fib[1000].
This can be fixed in one of two ways, depending on what you want:
You can add a check to make sure that the input value j is in range, and ask for another number if it isn't.
You can stop using an array variable, and only use three variables: one to store the current value, and two more to store the two previous values. These are updated as you calculate. A loop is still used with this approach.
#1 is the simplest to implement, as shown here:
while (1)
{
printf("j > ");
scanf(" %li", &j);
if (0 <= j <= 999)
{
break;
}
}
#2 is a bit more complex, but it effectively removes the arbitrary limit that j must be less than 1000 (and changes the limit so that j must be less than LONG_MAX):
// num_cache[0] is the number before the previous number
// num_cache[1] is the previous number to the current number
long num_cache[2] = { 0, 0 };
long current_fib = 1;
for (i = 2; i < j; i++)
{
// Push back the numbers
num_cache[0] = num_cache[1];
num_cache[1] = current_fib;
// Calculate the new number
current_fib = (num_cache[0] + num_cache[1]) % 10;
}
One of those solutions should fix your issue.

Answer 2

It appears that the segmentation fault is occurring due to inadequate checking of input values.
If the input to the program is not a valid number, then the value of j will be unchanged after the call to scanf(). Since this variable is uninitialized, this will result in undefined behaviour when you attempt to access the jth element of the fib[] array.
If the value of j is less than zero or greater than 999, you will be accessing a non-existent member of fib[] when you exit the for() loop. Your code should check that j is valid before continuing.
Here's your code with a few modifications to implement these safeguards and move the "magic number" 1000 to a #defined value.
#include <stdio.h>
#define FIBONACCI_LIMIT 1000L
int main(){
long i, j, fib[FIBONACCI_LIMIT];
fib[0] = 0;
fib[1] = 1;
if (scanf("%li", &j) != 1)
{
fprintf(stderr, "Invalid input\n");
return 1;
}
if (j<0 || j>=FIBONACCI_LIMIT)
{
fprintf(stderr, "Number must be in range 0 <= n < %li\n", FIBONACCI_LIMIT);
return 2;
}
for(i=2; i!=1000; i++)
{
fib[i] = (fib[i-2] + fib[i-1])%10;
}
printf("%li\n", fib[j]);
return 0;
}
The code can be improved by getting rid of the fib[] array altogether, since there is no need to store 1000 values when you only need to calculate one value. Furthermore, the final digits of numbers in the Fibonacci sequence form a repeating pattern of 60 values, so your first step should be to replace j with j % 60. Here is an improved version of the code that will work with any non-negative input capable of fitting into a long integer:
#include <stdio.h>
int main() {
long i, j, t, f0=0, f1=1;
if (scanf("%li", &j) != 1)
{
fprintf(stderr, "Invalid input\n");
return 1;
}
if (j < 0)
{
fprintf(stderr, "Number must be non-negative\n");
return 2;
}
j %= 60;
for (i=0; i<j; i++)
{
t = f0;
f0 = f1;
f1 = (f1 + t) % 10;
}
printf("%li\n", f0);
return 0;
}

Answer 3

You did not show what value of the variable j was entered.
Take into account that the next Fibonacci number is calculated incorrectly in the loop.
If to use the integer type unsigned long long then an object of this type can accommodate only 93 Fibonacci numbers.
The program can look like
#include <stdio.h>
#define N 100
int main(void)
{
while ( 1 )
{
unsigned long long fib[N] = { 0, 1 };
unsigned int n;
printf( "Enter a sequantial number of a fibonacci number (0 - exit): " );
if ( scanf("%u", &n) != 1 || n == 0 ) break;
unsigned int i = 2;
for (; i < N && i <= n && fib[i-1] <= fib[i-2] + fib[i-1]; i++)
{
fib[i] = fib[i - 2] + fib[i - 1];
}
if (n < i)
{
printf("#%u: %llu %llu\n", n, fib[n], fib[n] % 10);
}
else
{
puts("Too big fibonacci number");
}
}
return 0;
}
Its output might look like
Enter a sequantial number of a fibonacci number (0 - exit): 1
#1: 1 1
Enter a sequantial number of a fibonacci number (0 - exit): 2
#2: 1 1
Enter a sequantial number of a fibonacci number (0 - exit): 3
#3: 2 2
Enter a sequantial number of a fibonacci number (0 - exit): 93
#93: 12200160415121876738 8
Enter a sequantial number of a fibonacci number (0 - exit): 94
Too big fibonacci number
Enter a sequantial number of a fibonacci number (0 - exit): 0