$ cat bignum1.cpp

#include <stdio.h>

#include <stdlib.h>

int factorial(int n)

{ int answer=1;

for (int i=1; i<=n; i+=1)

answer*=i;

return answer; }

void main(void)

{ for (int x=1; x<20; x+=1)

printf("%3d! = %d\n", x, factorial(x)); }

$ CC bignum1.cpp

$ a.out

1! = 1

2! = 2

3! = 6

4! = 24

5! = 120

6! = 720

7! = 5040

8! = 40320

9! = 362880

10! = 3628800

11! = 39916800

12! = 479001600

13! = 1932053504

14! = 1278945280

15! = 2004310016

16! = 2004189184

17! = -288522240

18! = -898433024

19! = 109641728

$ cat bignum2.cpp

#include <stdio.h>

#include <stdlib.h>

const int size=100;

struct bignum

{ int digit[size];

bignum(int n)

{ for (int i=0; i<size; i+=1)

{ digit[i]=n%10;

n=n/10; } }

void print(void)

{ int firstdigit=0;

for (int i=size-1; i>=0; i-=1)

if (digit[i]!=0)

{ firstdigit=i;

break; }

for (int i=firstdigit; i>=0; i-=1)

printf("%d", digit[i]); }

void add(bignum b)

{ int carry=0;

for (int i=0; i<size; i+=1)

{ int sum = digit[i] + b.digit[i] + carry;

carry = sum/10;

sum = sum - carry*10;

digit[i] = sum; } }

void multiply_by_int(int n)

{ int carry=0;

for (int i=0; i<size; i+=1)

{ int prod = digit[i] * n + carry;

carry = prod/10;

prod = prod - carry*10;

digit[i] = prod; } } };

bignum factorial(int n)

{ bignum answer(1);

for (int i=1; i<=n; i+=1)

answer.multiply_by_int(i);

return answer; }

void main(void)

{ for (int x=1; x<20; x+=1)

{ printf("%3d! = ", x);

bignum f = factorial(x);

f.print();

printf("\n"); } }

$ CC bignum.cpp

$ a.out

1! = 1

2! = 2

3! = 6

4! = 24

5! = 120

6! = 720

7! = 5040

8! = 40320

9! = 362880

10! = 3628800

11! = 39916800

12! = 479001600

13! = 6227020800

14! = 87178291200

15! = 1307674368000

16! = 20922789888000

17! = 355687428096000

18! = 6402373705728000

19! = 121645100408832000

$

$ cat bignum3.cpp

#include <stdio.h>

#include <stdlib.h>

const int size=100;

struct bignum

{ int digit[size];

bignum(int n);

void print(void);

void add(bignum b);

void multiply_by_int(int n); };

bignum::bignum(int n)

{ for (int i=0; i<size; i+=1)

{ digit[i]=n%10;

n=n/10; } }

void bignum::print(void)

{ int firstdigit=0;

for (int i=size-1; i>=0; i-=1)

if (digit[i]!=0)

{ firstdigit=i;

break; }

for (int i=firstdigit; i>=0; i-=1)

printf("%d", digit[i]); }

void bignum::add(bignum b)

{ int carry=0;

for (int i=0; i<size; i+=1)

{ int sum = digit[i] + b.digit[i] + carry;

carry = sum/10;

sum = sum - carry*10;

digit[i] = sum; } }

void bignum::multiply_by_int(int n)

{ int carry=0;

for (int i=0; i<size; i+=1)

{ int prod = digit[i] * n + carry;

carry = prod/10;

prod = prod - carry*10;

digit[i] = prod; } }

bignum factorial(int n)

{ bignum answer(1);

for (int i=1; i<=n; i+=1)

answer.multiply_by_int(i);

return answer; }

void main(void)

{ for (int x=1; x<20; x+=1)

{ printf("%3d! = ", x);

bignum f = factorial(x);

f.print();

printf("\n"); } }