Fibonacci numbers with GMP lib
EDIT: solved. ans var needed to be set to 0 in every iteration.That was a mistake I overlooked.A mpz_set_ui(ans,0); at the beginning of each loop solves the problem.Thanks to Oli Charlesworth for his effort!
I'm coding a fibonacci function in C using the GMP library,I use the matrix multiplication algorithm,but for some unexpected reason,I get wrong results(I strongly believe that开发者_Python百科 the algorithm is right).It must have something to do with Gmp structs and inits.Any ideas?
/*********************************************************
Find n th Fibonacci number.Matrix algorithm.
+-- --+ +-- --+
|f(n+1) f(n) | | 1 1 | ^n
| | == | |
|f(n) f(n-1) | | 1 0 |
+-- --+ +-- --+
*********************************************************/
int
fastfib(mpz_t result,int n)
{
int cons[2][2]={{1,1},{1,0}}; //"constant" matrix
mpz_t mat[2][2]; //actual matrix holding fibonacci numbers
mpz_t ans; //holds f(n+1)
mpz_t return_val; //used for calculations
mpz_t temp; //used for calculations as well
//initialize (automatically set to zero)
mpz_init(ans);
mpz_init(return_val);
mpz_init(temp);
mpz_init(mat[0][0]);
mpz_init(mat[0][1]);
mpz_init(mat[1][0]);
mpz_init(mat[1][1]);
//start with n=1
mpz_set_ui(mat[0][0],1);
mpz_set_ui(mat[1][0],1);
mpz_set_ui(mat[0][1],1);
mpz_set_ui(mat[1][1],0);
//some trivial cases
if(n==0){
mpz_set_ui(result,0);
return 0;
}
if(n==1){
mpz_set_ui(result,1);
return 0;
}
if(n==2){
mpz_set_ui(result,1);
return 0;
}
n--;
while(n>1){
//fib[n+1]
//ans=mat[0][0]*cons[0][0]+mat[0][1]*cons[1][0];
mpz_set_ui(ans,0);
mpz_mul_ui(temp,mat[0][0],cons[0][0]);
mpz_add(ans,ans,temp);
mpz_mul_ui(temp,mat[0][1],cons[1][0]);
mpz_add(ans,ans,temp);
//update matrix
mpz_set(mat[1][1],mat[1][0]); //mat[1][1]=mat[1][0];
mpz_set(mat[1][0],mat[0][0]); //mat[0][1]=mat[1][0]=mat[0][0];
mpz_set(mat[0][1],mat[0][0]);
mpz_set(mat[0][0],ans); //mat[0][0]=ans;
n--;
}
//clear vars
mpz_clear(ans);
mpz_clear(return_val);
mpz_clear(temp);
mpz_set(result,mat[0][0]);
return 0;
}
Some debug info
results for n=2 // expected
2 1 // 2 1
1 1 // 1 1
results for n=3
5 2 // 3 2
2 1 // 2 1
results for n=4
12 5 // 5 3
5 2 // 3 2
results for n=5
29 12 // 8 5
12 5 // 5 3
results for n=6
70 29 // 13 8
29 12 // 8 5
70
solved. ans var needed to be set to 0 in every iteration.That was a mistake I overlooked.A mpz_set_ui(ans,0); at the beginning of each loop solves the problem.Thanks to Oli Charlesworth for his effort!
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