I dont remember if this was already written here...

there are 3 fibonacci functions: fib (fibonacci 2 recursive), fib1
(fibonacci 1 recursive), and f (fibunacci iterative) -------------------------------------------------
#include <stdio.h>
#include <time.h>

unsigned fib(unsigned n){if(n<2)return n; return fib(n-1)+fib(n-2);}

unsigned fibr(unsigned i, unsigned j, unsigned n)
{if(n==0) return i;
return fibr(j,i+j,n-1);
}

unsigned fib1(unsigned n){return fibr(0,1,n);}

unsigned f(unsigned n)
{unsigned r,i,j;
if(n==0) return 0;
for(r=j=i=1,n-=1;n;--n)
{r=j;j+=i;i=r;}
return r;
}


main(void)
{time_t in,fn;
double d;
unsigned i, r, j;

in=time(0);
for(i=0;i<40;++i)
{r=fib(i);
//printf("fibonacciRic(%u)=%u\n", i, r);
}
fn=time(0); d=difftime(fn,in); printf("DeltaTime Fib 2 recursive: %f
\n", d);

in=time(0);
for(j=0;j<1000000;++j)
for(i=0;i<40;++i)
{r=fib1(i);
//printf("fibonacciRic(%u)=%u\n", i, r);
}
fn=time(0); d=difftime(fn,in); printf("DeltaTime Fib 1 recursive: %f
\n", d);

in=time(0);
for(j=0;j<1000000;++j)
for(i=0;i<40;++i)
{r=f(i);
//printf("fibonacciIter(%u)=%u\n", i, r);
}
fn=time(0); d=difftime(fn,in); printf("DeltaTime Fib iterative: %f
\n", d);

return 0;
}
--------------

results

fibonacci

DeltaTime Fib 2 recursive: 4.000000
DeltaTime Fib 1 recursive: 5.000000
DeltaTime Fib iterative: 2.000000

so fib 2 recursive is about 1,000,000 times more slow than the fib 1
recursive

and fib iterative is 2x more fast than the fib 1 recursive...

right?

in assembly I have seen that here j is created and the loop extern
shoul be run, the same the one inner


--- PyGate Linux v1.5.14
* Origin: Dragon's Lair, PyGate NNTP<>Fido Gate (3:633/10)

On Wed, 22 Apr 2026 11:35:16 +0200
Rosario19 <Ros@invalid.invalid> wrote:

I dont remember if this was already written here...

there are 3 fibonacci functions: fib (fibonacci 2 recursive), fib1
(fibonacci 1 recursive), and f (fibunacci iterative) -------------------------------------------------
#include <stdio.h>
#include <time.h>

unsigned fib(unsigned n){if(n<2)return n; return fib(n-1)+fib(n-2);}

unsigned fibr(unsigned i, unsigned j, unsigned n)
{if(n==0) return i;
return fibr(j,i+j,n-1);
}

unsigned fib1(unsigned n){return fibr(0,1,n);}

unsigned f(unsigned n)
{unsigned r,i,j;
if(n==0) return 0;
for(r=j=i=1,n-=1;n;--n)
{r=j;j+=i;i=r;}
return r;
}

main(void)
{time_t in,fn;
double d;
unsigned i, r, j;

in=time(0);
for(i=0;i<40;++i)
{r=fib(i);
//printf("fibonacciRic(%u)=%u\n", i, r);
}
fn=time(0); d=difftime(fn,in); printf("DeltaTime Fib 2 recursive: %f
\n", d);

in=time(0);
for(j=0;j<1000000;++j)
for(i=0;i<40;++i)
{r=fib1(i);
//printf("fibonacciRic(%u)=%u\n", i, r);
}
fn=time(0); d=difftime(fn,in); printf("DeltaTime Fib 1 recursive: %f
\n", d);

in=time(0);
for(j=0;j<1000000;++j)
for(i=0;i<40;++i)
{r=f(i);
//printf("fibonacciIter(%u)=%u\n", i, r);
}
fn=time(0); d=difftime(fn,in); printf("DeltaTime Fib iterative: %f
\n", d);

return 0;
}
--------------

results

fibonacci

DeltaTime Fib 2 recursive: 4.000000
DeltaTime Fib 1 recursive: 5.000000
DeltaTime Fib iterative: 2.000000

so fib 2 recursive is about 1,000,000 times more slow than the fib 1 recursive

and fib iterative is 2x more fast than the fib 1 recursive...

right?

in assembly I have seen that here j is created and the loop extern
shoul be run, the same the one inner

Since you don't use results of calculations and since compiler can see
the bodies of the functions fib(), fibr(), fib1() and f() and can
figure out that they have no side effects, I would expect that good
optimizing compiler will not call this functions at all.


--- PyGate Linux v1.5.14
* Origin: Dragon's Lair, PyGate NNTP<>Fido Gate (3:633/10)

Am 22.04.2026 um 13:12 schrieb Michael S:

Since you don't use results of calculations and since compiler can
see the bodies of the functions fib(), fibr(), fib1() and f() and
can figure out that they have no side effects, I would expect that
good optimizing compiler will not call this functions at all.

With a *very* good compiler you can help the compiler to do that with constexpr. ;-)

--- PyGate Linux v1.5.14
* Origin: Dragon's Lair, PyGate NNTP<>Fido Gate (3:633/10)