Long exact numbers in C?

I am making a program that prints the first 100 numbers of Lucas (they are similar to Fibonacci), but the last few numbers don't fit into an unsigned long long int. I have tried using a long double, but it is not accurate and I am getting some differences from what I should be getting.

This is a homework task and my teacher specifically pointed out that we don't need to use any other library other than stdio.h.

I've tried making a method that adds strings as numbers, but that's far beyond experience and I sincerely doubt this is what we should be doing.

With inaccuracies, it looks something like this:

``````#include <stdio.h>

int main()
{
long double firstNumber = 2;
long double secondNumber = 1;
long double thirdNumber;

int i;
for (i = 2; i <= 100; i += 1)
{
thirdNumber = secondNumber + firstNumber;
firstNumber = secondNumber;
secondNumber = thirdNumber;
printf("%Lf, ", thirdNumber);
}

return 0;
}
```

```
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It looks like all you need is an add-on. I see three ways to do this.

• Unless you are prohibited from using libraries, you would use one of the many bigint libraries available.
• Implementation of a row-based accumulator. Basically you are implementing the add method that you learned in 3rd class.
• As a hack, if your highest number is roughly two `unsigned long long ints`

, you can split your number into most significant digits and least significant digits. I would go this route.
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I used below to store really big numbers in an array. Paste the code below with some comments. Hope it helps.

``````#include<stdio.h>
int main()
{
int t;
int a[200]; //array will have the capacity to store 200 digits.
int n,i,j,temp,m,x;

scanf("%d",&t);
while(t--)
{
scanf("%d",&n);
a[0]=1;  //initializes array with only 1 digit, the digit 1.
m=1;    // initializes digit counter

temp = 0; //Initializes carry variable to 0.
for(i=1;i<=n;i++)
{
for(j=0;j<m;j++)
{
x = a[j]*i+temp; //x contains the digit by digit product
a[j]=x%10; //Contains the digit to store in position j
temp = x/10; //Contains the carry value that will be stored on later indexes
}
while(temp>0) //while loop that will store the carry value on array.
{
a[m]=temp%10;
temp = temp/10;
m++; // increments digit counter
}
}
printf("%d",a[i]);
printf("\n");
}
return 0;
}
```

```
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